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https://github.com/modernish/modernish

Modernish is a library for writing robust, portable, readable, and powerful programs for POSIX-based shells and utilities.
https://github.com/modernish/modernish

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Modernish is a library for writing robust, portable, readable, and powerful programs for POSIX-based shells and utilities.

Lists

README 

        
Releases


For code examples, see



EXAMPLES.md

and



share/doc/modernish/examples




# modernish – harness the shell #



-   *Sick of quoting hell and split/glob pitfalls?*

-   *Tired of brittle shell scripts going haywire and causing damage?*

-   *Mystified by line noise commands like `[`, `[[`, `((` ?*

-   *Is scripting basic things just too hard?*

-   *Ever wish that `find` were a built-in shell loop?*

-   *Do you want your script to work on nearly any shell on any Unix-like OS?*



Modernish is a library for shell script programming which provides features

like safer variable and command expansion, new language constructs for loop

iteration, and much more. Modernish programs are shell programs; the new

constructs are mixed with shell syntax so that the programmer can take

advantage of the best of both.



There is no compiled code to install, as modernish is written entirely in the

shell language. It can be deployed in embedded or multi-user systems in which

new binary executables may not be introduced for security reasons, and is

portable among numerous shell implementations. The installer can also

[bundle](#user-content-appendix-f-bundling-modernish-with-your-script)

a reduced copy of the library with your scripts, so they can run portably with

a known version of modernish without requiring prior installation.



**Join us and help breathe some new life into the shell!** We

are looking for testers, early adopters, and developers to join us.

[Download the latest release](https://github.com/modernish/modernish/releases)

or check out the very latest development code from the master branch.

Read through the documentation below. Play with the example scripts and

write your own. Try to break the library and send reports of breakage.



## Table of contents ##



* [Getting started](#user-content-getting-started)

* [Two basic forms of a modernish program](#user-content-two-basic-forms-of-a-modernish-program)

    * [Simple form](#user-content-simple-form)

    * [Portable form](#user-content-portable-form)

* [Interactive use](#user-content-interactive-use)

* [Non-interactive command line use](#user-content-non-interactive-command-line-use)

    * [Non-interactive usage examples](#user-content-non-interactive-usage-examples)

* [Shell capability detection](#user-content-shell-capability-detection)

* [Names and identifiers](#user-content-names-and-identifiers)

    * [Internal namespace](#user-content-internal-namespace)

    * [Modernish system constants](#user-content-modernish-system-constants)

    * [Control character, whitespace and shell-safe character constants](#user-content-control-character-whitespace-and-shell-safe-character-constants)

* [Reliable emergency halt](#user-content-reliable-emergency-halt)

* [Low-level shell utilities](#user-content-low-level-shell-utilities)

    * [Outputting strings](#user-content-outputting-strings)

    * [Legibility aliases: `not`, `so`, `forever`](#user-content-legibility-aliases-not-so-forever)

    * [Enhanced `exit`](#user-content-enhanced-exit)

    * [`chdir`](#user-content-chdir)

    * [`insubshell`](#user-content-insubshell)

    * [`isset`](#user-content-isset)

    * [`setstatus`](#user-content-setstatus)

* [Testing numbers, strings and files](#user-content-testing-numbers-strings-and-files)

    * [Integer number arithmetic tests and operations](#user-content-integer-number-arithmetic-tests-and-operations)

        * [The arithmetic command `let`](#user-content-the-arithmetic-command-let)

        * [Arithmetic shortcuts](#user-content-arithmetic-shortcuts)

    * [String and file tests](#user-content-string-and-file-tests)

        * [String tests](#user-content-string-tests)

            * [Unary string tests](#user-content-unary-string-tests)

            * [Binary string matching tests](#user-content-binary-string-matching-tests)

            * [Multi-matching option](#user-content-multi-matching-option)

        * [File type tests](#user-content-file-type-tests)

        * [File comparison tests](#user-content-file-comparison-tests)

        * [File status tests](#user-content-file-status-tests)

        * [I/O tests](#user-content-io-tests)

        * [File permission tests](#user-content-file-permission-tests)

* [The stack](#user-content-the-stack)

    * [The shell options stack](#user-content-the-shell-options-stack)

    * [The trap stack](#user-content-the-trap-stack)

* [Modules](#user-content-modules)

    * [`use safe`](#user-content-use-safe)

        * [Why the safe mode?](#user-content-why-the-safe-mode)

        * [How the safe mode works](#user-content-how-the-safe-mode-works)

        * [Important notes for safe mode](#user-content-important-notes-for-safe-mode)

        * [Extra options for the safe mode](#user-content-extra-options-for-the-safe-mode)

    * [`use var/loop`](#user-content-use-varloop)

        * [Simple repeat loop](#user-content-simple-repeat-loop)

        * [BASIC-style arithmetic `for` loop](#user-content-basic-style-arithmetic-for-loop)

        * [C-style arithmetic `for` loop](#user-content-c-style-arithmetic-for-loop)

        * [Enumerative `for`/`select` loop with safe split/glob](#user-content-enumerative-forselect-loop-with-safe-splitglob)

        * [The `find` loop](#user-content-the-find-loop)

            * [Available *options*](#user-content-available-options)

            * [Available *find-expression* operands](#user-content-available-find-expression-operands)

            * [Picking a `find` utility](#user-content-picking-a-find-utility)

            * [Compatibility mode for obsolete `find` utilities](#user-content-compatibility-mode-for-obsolete-find-utilities)

            * [`find` loop usage examples](#user-content-find-loop-usage-examples)

        * [Creating your own loop](#user-content-creating-your-own-loop)

    * [`use var/local`](#user-content-use-varlocal)

        * [Important `var/local` usage notes](#user-content-important-varlocal-usage-notes)

    * [`use var/arith`](#user-content-use-vararith)

        * [Arithmetic operator shortcuts](#user-content-arithmetic-operator-shortcuts)

        * [Arithmetic comparison shortcuts](#user-content-arithmetic-comparison-shortcuts)

    * [`use var/assign`](#user-content-use-varassign)

    * [`use var/readf`](#user-content-use-varreadf)

    * [`use var/shellquote`](#user-content-use-varshellquote)

        * [`shellquote`](#user-content-shellquote)

        * [`shellquoteparams`](#user-content-shellquoteparams)

    * [`use var/stack`](#user-content-use-varstack)

        * [`use var/stack/extra`](#user-content-use-varstackextra)

        * [`use var/stack/trap`](#user-content-use-varstacktrap)

            * [Trap stack compatibility considerations](#user-content-trap-stack-compatibility-considerations)

            * [The new `DIE` pseudosignal](#user-content-the-new-die-pseudosignal)

    * [`use var/string`](#user-content-use-varstring)

        * [`use var/string/touplow`](#user-content-use-varstringtouplow)

        * [`use var/string/trim`](#user-content-use-varstringtrim)

        * [`use var/string/replacein`](#user-content-use-varstringreplacein)

        * [`use var/string/append`](#user-content-use-varstringappend)

    * [`use var/unexport`](#user-content-use-varunexport)

    * [`use var/genoptparser`](#user-content-use-vargenoptparser)

    * [`use sys/base`](#user-content-use-sysbase)

        * [`use sys/base/mktemp`](#user-content-use-sysbasemktemp)

        * [`use sys/base/readlink`](#user-content-use-sysbasereadlink)

        * [`use sys/base/rev`](#user-content-use-sysbaserev)

        * [`use sys/base/seq`](#user-content-use-sysbaseseq)

            * [Differences with GNU and BSD `seq`](#user-content-differences-with-gnu-and-bsd-seq)

        * [`use sys/base/shuf`](#user-content-use-sysbaseshuf)

        * [`use sys/base/tac`](#user-content-use-sysbasetac)

        * [`use sys/base/which`](#user-content-use-sysbasewhich)

        * [`use sys/base/yes`](#user-content-use-sysbaseyes)

    * [`use sys/cmd`](#user-content-use-syscmd)

        * [`use sys/cmd/extern`](#user-content-use-syscmdextern)

        * [`use sys/cmd/harden`](#user-content-use-syscmdharden)

            * [Important note on variable assignments](#user-content-important-note-on-variable-assignments)

            * [Hardening while allowing for broken pipes](#user-content-hardening-while-allowing-for-broken-pipes)

            * [Tracing the execution of hardened commands](#user-content-tracing-the-execution-of-hardened-commands)

            * [Simple tracing of commands](#user-content-simple-tracing-of-commands)

        * [`use sys/cmd/mapr`](#user-content-use-syscmdmapr)

            * [Differences from `mapfile`](#user-content-differences-from-mapfile)

            * [Differences from `xargs`](#user-content-differences-from-xargs)

        * [`use sys/cmd/procsubst`](#user-content-use-syscmdprocsubst)

        * [`use sys/cmd/source`](#user-content-use-syscmdsource)

    * [`use sys/dir`](#user-content-use-sysdir)

        * [`use sys/dir/countfiles`](#user-content-use-sysdircountfiles)

        * [`use sys/dir/mkcd`](#user-content-use-sysdirmkcd)

    * [`use sys/term`](#user-content-use-systerm)

        * [`use sys/term/putr`](#user-content-use-systermputr)

        * [`use sys/term/readkey`](#user-content-use-systermreadkey)

* [Appendix A: List of shell cap IDs](#user-content-appendix-a-list-of-shell-cap-ids)

    * [Capabilities](#user-content-capabilities)

    * [Quirks](#user-content-quirks)

    * [Bugs](#user-content-bugs)

    * [Warning IDs](#user-content-warning-ids)

* [Appendix B: Regression test suite](#user-content-appendix-b-regression-test-suite)

    * [Difference between capability detection and regression tests](#user-content-difference-between-capability-detection-and-regression-tests)

    * [Testing modernish on all your shells](#user-content-testing-modernish-on-all-your-shells)

* [Appendix C: Supported locales](#user-content-appendix-c-supported-locales)

* [Appendix D: Supported shells](#user-content-appendix-d-supported-shells)

* [Appendix E: zsh: integration with native scripts](#user-content-appendix-e-zsh-integration-with-native-scripts)

* [Appendix F: Bundling modernish with your script](#user-content-appendix-f-bundling-modernish-with-your-script)



## Getting started ##



Run `install.sh` and follow instructions, choosing your preferred shell

and install location. After successful installation you can run modernish

shell scripts and write your own. Run `uninstall.sh` to remove modernish.



Both the install and uninstall scripts are interactive by default, but

support fully automated (non-interactive) operation as well. Command

line options are as follows:



`install.sh` [ `-n` ] [ `-s` *shell* ] [ `-f` ] [ `-P` *pathspec* ]

[ `-d` *installroot* ] [ `-D` *prefix* ] [ `-B` *scriptfile* ... ]



* `-n`: non-interactive operation

* `-s`: specify default shell to execute modernish

* `-f`: force unconditional installation on specified shell

* `-P`: specify an alternative [`DEFPATH`](#user-content-modernish-system-constants)

        for the installation (be careful; usually *not* recommended)

* `-d`: specify root directory for installation

* `-D`: extra destination directory prefix (for packagers)

* `-B:` bundle modernish with your scripts (`-D` required, `-n` implied), see

        [Appendix F](#user-content-appendix-f-bundling-modernish-with-your-script)



`uninstall.sh` [ `-n` ] [ `-f` ] [ `-d` *installroot* ]



* `-n`: non-interactive operation

* `-f`: delete `*/modernish` directories even if files left

* `-d`: specify root directory of modernish installation to uninstall



## Two basic forms of a modernish program ##



In the *simple form*, modernish is added to a script written for a specific

shell. In the *portable form*, your script is shell-agnostic and may run on any

[shell that can run modernish](#user-content-appendix-d-supported-shells).



### Simple form ###



The **simplest** way to write a modernish program is to source modernish as a

dot script. For example, if you write for bash:



```sh

#! /bin/bash

. modernish

use safe

use sys/base

...your program starts here...

```



The modernish `use` command load modules with optional functionality. The

`safe` module initialises the [safe mode](#user-content-use-safe).

The `sys/base` module contains modernish versions of certain basic but

non-standardised utilities (e.g. `readlink`, `mktemp`, `which`), guaranteeing

that modernish programs all have a known version at their disposal. There are

many other modules as well. See [Modules](#user-content-modules) for more

information.



The above method makes the program dependent on one particular shell (in this

case, bash). So it is okay to mix and match functionality specific to that

particular shell with modernish functionality.



(On **zsh**, there is a way to integrate modernish with native zsh scripts. See

[Appendix E](#user-content-appendix-e-zsh-integration-with-native-scripts).)



### Portable form ###



The **most portable** way to write a modernish program is to use the special

generic hashbang path for modernish programs. For example:



```sh

#! /usr/bin/env modernish

#! use safe

#! use sys/base

...your program begins here...

```



For portability, it is important there is no space after `env modernish`;

NetBSD and OpenBSD consider trailing spaces part of the name, so `env` will

fail to find modernish.



A program in this form is executed by whatever shell the user who installed

modernish on the local system chose as the default shell. Since you as the

programmer can't know what shell this is (other than the fact that it passed

some rigorous POSIX compliance testing executed by modernish), a program in

this form *must be strictly POSIX compliant* – except, of course, that it

should also make full use of the rich functionality offered by modernish.



Note that modules are loaded in a different way: the `use` commands are part of

hashbang comment (starting with `#!` like the initial hashbang path). Only such

lines that *immediately* follow the initial hashbang path are evaluated; even

an empty line in between causes the rest to be ignored.

This special way of pre-loading modules is needed to make any aliases they

define work reliably on all shells.



## Interactive use ##



Modernish is primarily designed to enhance shell programs/scripts, but also

offers features for use in interactive shells. For instance, the new `repeat`

loop construct from the `var/loop` module can be quite practical to repeat

an action x times, and the `safe` module on interactive shells provides

convenience functions for manipulating, saving and restoring the state of

field splitting and globbing.



To use modernish on your favourite interactive shell, you have to add it to

your `.profile`, `.bashrc` or similar init file.



**Important:** Upon initialising, modernish adapts itself to

other settings, such as the locale. It also removes certain aliases that

may keep modernish from initialising properly. So you have to organise your

`.profile` or similar file in the following order:



* *first*, define general system settings (`PATH`, locale, etc.);

* *then*, `. modernish` and `use` any modules you want;

* *then* define anything that may depend on modernish, and set your aliases.



## Non-interactive command line use ##



After installation, the `modernish` command can be invoked as if it were a

shell, with the standard command line options from other shells (such as

`-c` to specify a command or script directly on the command line), plus some

enhancements. The effect is that the shell chosen at installation time will

be run enhanced with modernish functionality. It is not possible to use

modernish as an interactive shell in this way.



Usage:



1. `modernish` [ `--use=`*module* | *shelloption* ... ]

   [ *scriptfile* ] [ *arguments* ]

2. `modernish` [ `--use=`*module* | *shelloption* ... ]

   `-c` [ *script* [ *me-name* [ *arguments* ] ] ]

3. `modernish --test` [ *testoption* ... ]

4. `modernish` [ `--version` | `--help` ]



In the first form, the script in the file *scriptfile* is

loaded and executed with any *arguments* assigned to the positional parameters.



In the second form, `-c` executes the specified modernish

*script*, optionally with the *me-name* assigned to `$ME` and the

*arguments* assigned to the positional parameters.



The `--use` option pre-loads any given modernish [modules](#user-content-modules)

before executing the script.

The *module* argument to each specified `--use` option is split using

standard shell field splitting. The first field is the module name and any

further fields become arguments to that module's initialisation routine.



Any given short-form or long-form *shelloption*s are

set or unset before executing the script. Both POSIX

[shell options](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_25_03)

and shell-specific options are supported, depending on

[the shell executing modernish](#user-content-appendix-d-supported-shells).

Using the shell option `-e` or `-o errexit` is an error, because modernish

[does not support it](#user-content-use-syscmdharden) and

would break.



The `--test` option runs the regression test suite and exits. This verifies

that the modernish installation is functioning correctly. See

[Appendix B](#user-content-appendix-b-regression-test-suite)

for more information.



The `--version` and `--help` options output the relative information and exit.



### Non-interactive usage examples ###



* Count to 10 using a [basic loop](#user-content-use-varloop):    

  `modernish --use=var/loop -c 'LOOP for i=1 to 10; DO putln "$i"; DONE'`

* Run a [portable-form](#user-content-portable-form)

  modernish program using zsh and enhanced-prompt xtrace:    

  `zsh /usr/local/bin/modernish -o xtrace /path/to/program.sh`



## Shell capability detection ##



Modernish includes a battery of shell feature, quirk and bug detection

tests, each of which is given a special capability ID.

See [Appendix A](#user-content-appendix-a-list-of-shell-cap-ids) for a

list of shell capabilities that modernish currently detects, as well

as further general information on the capability detection framework.



`thisshellhas` is the central function of the capability detection

framework. It not only tests for the presence of shell features/quirks/bugs,

but can also detect specific shell built-in commands, shell reserved words,

shell options (short or long form), and signals.



Modernish itself extensively uses capability detection to adapt itself to the

shell it's running on. This is how it works around shell bugs and takes

advantage of efficient features not all shells have. But any script using

the library can do this in the same way, with the help of this function.



Test results are cached in memory, so repeated checks using `thisshellhas`

are efficient and there is no need to avoid calling it to optimise

performance.



Usage:



`thisshellhas` *item* ...



* If *item* contains only ASCII capital letters A-Z, digits 0-9 or `_`,

  return the result status of the associated modernish

  [capability detection test](#user-content-appendix-a-list-of-shell-cap-ids).

* If *item* is any other ASCII word, check if it is a shell reserved

  word or built-in command on the current shell.

* If *item* is `--` (end-of-options delimiter), disable the recognition of

  operators starting with `-` for subsequent items.

* If *item* starts with `--rw=` or `--kw=`, check if the identifier

  immediately following these characters is a shell reserved word

  (a.k.a. shell keyword).

* If *item* starts with `--bi=`, similarly check for a shell built-in command.

* If *item* starts with `--sig=`, check if the shell knows about a signal

  (usable by `kill`, `trap`, etc.) by the name or number following the `=`.

  If a number \> 128 is given, the remainder of its division by 128 is checked.

  If the signal is found, its canonicalised signal name is left in the

  `REPLY` variable, otherwise `REPLY` is unset. (If multiple `--sig=` items

  are given and all are found, `REPLY` contains only the last one.)

* If *item* is `-o` followed by a separate word, check if this shell has a

  long-form shell option by that name.

* If *item* is any other letter or digit preceded by a single `-`, check if

  this shell has a short-form shell option by that character.

* *item* can also be one of the following two operators.

    * `--cache` runs all external modernish shell capability tests

      that have not yet been run, causing the cache to be complete.

    * `--show` performs a `--cache` and then outputs all the IDs of

      positive results, one per line.



`thisshellhas` continues to process *item*s until one of them produces a

negative result or is found invalid, at which point any further *item*s are

ignored. So the function only returns successfully if all the *item*s

specified were found on the current shell. (To check if either one *item* or

another is present, use separate `thisshellhas` invocations separated by the

`||` shell operator.)



Exit status: 0 if this shell has all the *items* in question; 1 if not; 2 if

an *item* was encountered that is not recognised as a valid identifier.



**Note:** The tests for the presence of reserved words, built-in commands,

shell options, and signals are different from capability detection tests in an

important way: they only check if an item by that name exists on this shell,

and don't verify that it does the same thing as on another shell.



## Names and identifiers ##



All modernish functions require portable variable and shell function names,

that is, ones consisting of ASCII uppercase and lowercase letters, digits,

and the underscore character `_`, and that don't begin with digit. For shell

option names, the constraints are the same except a dash `-` is also

accepted. An invalid identifier is generally treated as a fatal error.



### Internal namespace ###



Function-local variables are not supported by the standard POSIX shell; only

global variables are provided for. Modernish needs a way to store its

internal state without interfering with the program using it. So most of the

modernish functionality uses an internal namespace `_Msh_*` for variables,

functions and aliases. All these names may change at any time without

notice. *Any names starting with `_Msh_` should be considered sacrosanct and

untouchable; modernish programs should never directly use them in any way.*

Of course this is not enforceable, but names starting with `_Msh_` should be

uncommon enough that no unintentional conflict is likely to occur.



### Modernish system constants ###



Modernish provides certain constants (read-only variables) to make life easier.

These include:



* `$MSH_VERSION`: The version of modernish.

* `$MSH_PREFIX`: Installation prefix for this modernish installation (e.g.

  /usr/local).

* `$MSH_MDL`: Main [modules](#user-content-modules) directory.

* `$MSH_AUX`: Main helper scripts directory.

* `$MSH_CONFIG`: Path to modernish user configuration directory.

* `$ME`: Path to the current program. Replacement for `$0`. This is

  necessary if the hashbang path `#!/usr/bin/env modernish` is used, or if

  the program is launched like `sh /path/to/bin/modernish

  /path/to/script.sh`, as these set `$0` to the path to bin/modernish and

  not your program's path.

* `$MSH_SHELL`: Path to the default shell for this modernish installation,

  chosen at install time (e.g. /bin/sh). This is a shell that is known to

  have passed all the modernish tests for fatal bugs. Cross-platform scripts

  should use it instead of hard-coding /bin/sh, because on some operating

  systems (NetBSD, OpenBSD, Solaris) /bin/sh is not POSIX compliant.

* `$SIGPIPESTATUS`: The exit status of a command killed by `SIGPIPE` (a

  broken pipe). For instance, if you use `grep something somefile.txt |

  more` and you quit `more` before `grep` is finished, `grep` is killed by

  `SIGPIPE` and exits with that particular status.

  Hardened commands or functions may need to handle such a `SIGPIPE` exit

  specially to avoid unduly killing the program. The exact value of this

  exit status is shell-specific, so modernish runs a quick test to determine

  it at initialisation time.    

  If `SIGPIPE` was set to ignore by the process that invoked the current

  shell, `$SIGPIPESTATUS` can't be detected and is set to the special value

  99999. See also the description of the

  [`WRN_NOSIGPIPE`](#user-content-warning-ids)

  ID for

  [`thisshellhas`](#user-content-shell-capability-detection).

* `$DEFPATH`: The default system path guaranteed to find compliant POSIX

  utilities, as given by `getconf PATH`.

* `$ERROR`: A guaranteed unset variable that can be used to trigger an

   error that exits the (sub)shell, for instance:

   `: "${4+${ERROR:?excess arguments}}"` (error on 4 or more arguments)



### Control character, whitespace and shell-safe character constants ###



POSIX does not provide for the quoted C-style escape codes commonly used in

bash, ksh and zsh (such as `$'\n'` to represent a newline character),

leaving the standard shell without a convenient way to refer to control

characters. Modernish provides control character constants (read-only

variables) with hexadecimal suffixes `$CC01` .. `$CC1F` and `$CC7F`, as well as `$CCe`,

`$CCa`, `$CCb`, `$CCf`, `$CCn`, `$CCr`, `$CCt`, `$CCv` (corresponding with

`printf` backslash escape codes). This makes it easy to insert control

characters in double-quoted strings.



More convenience constants, handy for use in bracket glob patterns for use

with `case` or modernish `match`:



* `$CONTROLCHARS`: All ASCII control characters.

* `$WHITESPACE`: All ASCII whitespace characters.

* `$ASCIIUPPER`: The ASCII uppercase letters A to Z.

* `$ASCIILOWER`: The ASCII lowercase letters a to z.

* `$ASCIIALNUM`: The ASCII alphanumeric characters 0-9, A-Z and a-z.

* `$SHELLSAFECHARS`: Safe-list for shell-quoting.

* `$ASCIICHARS`: The complete set of ASCII characters (minus NUL).



Usage examples:



```sh

# Use a glob pattern to check against control characters in a string:

	if str match "$var" "*[$CONTROLCHARS]*"; then

		putln "\$var contains at least one control character"

	fi

# Use '!' (not '^') to check for characters *not* part of a particular set:

	if str match "$var" "*[!$ASCIICHARS]*"; then

		putln "\$var contains at least one non-ASCII character" ;;

	fi

# Safely split fields at any whitespace, comma or slash (requires safe mode):

	use safe

	LOOP for --split=$WHITESPACE,/ field in $my_items; DO

		putln "Item: $field"

	DONE

```



## Reliable emergency halt ##



The `die` function reliably halts program execution, even from within

[subshells](#user-content-insubshell), optionally

printing an error message. Note that `die` is meant for an emergency program

halt only, i.e. in situations were continuing would mean the program is in an

inconsistent or undefined state. Shell scripts running in an inconsistent or

undefined state may wreak all sorts of havoc. They are also notoriously

difficult to terminate correctly, especially if the fatal error occurs within

a subshell: `exit` won't work then. That's why `die` is optimised for

killing *all* the program's processes (including subshells and external

commands launched by it) as quickly as possible. It should never be used for

exiting the program normally.



On interactive shells, `die` behaves differently. It does not kill or exit your

shell; instead, it issues `SIGINT` to the shell to abort the execution of your

running command(s), which is equivalent to pressing Ctrl+C.

In addition, if `die` is invoked from a subshell such as a background job, it

kills all processes belonging to that job, but leaves other running jobs alone.



Usage: `die` [ *message* ]



If the [trap stack module](#user-content-use-varstacktrap)

is active, a special

[`DIE` pseudosignal](#user-content-the-new-die-pseudosignal)

can be trapped (using plain old `trap` or

[`pushtrap`](#user-content-the-trap-stack))

to perform emergency cleanup commands upon invoking `die`.



If the `MSH_HAVE_MERCY` variable is set in a script and `die` is invoked

from a subshell, then `die` will only terminate the current subshell and its

subprocesses and will not execute `DIE` traps, allowing the script to resume

execution in the parent process. This is for use in special cases, such as

regression tests, and is strongly discouraged for general use. Modernish

unsets the variable on init so it cannot be inherited from the environment.



## Low-level shell utilities ##



### Outputting strings ###



The POSIX shell lacks a simple, straightforward and portable way to output

arbitrary strings of text, so modernish adds two commands for this.



* `put` prints each argument separated by a space, without a trailing newline.

* `putln` prints each argument, terminating each with a newline character.



There is no processing of options or escape codes. (Modernish constants

[`$CCn`, etc.](#user-content-control-character-whitespace-and-shell-safe-character-constants)

can be used to insert control characters in double-quoted strings. To process escape codes, use

[`printf`](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/printf.html)

instead.)



The `echo` command is notoriously unportable and kind of broken, so is

**deprecated** in favour of `put` and `putln`. Modernish does provide its own

version of `echo`, but it is only activated for

[portable-form](#user-content-portable-form))

scripts. Otherwise, the shell-specific version of `echo` is left intact.

The modernish version of `echo` does not interpret any escape codes

and supports only one option, `-n`, which, like BSD `echo`, suppresses the

final newline. However, unlike BSD `echo`, if `-n` is the only argument, it is

not interpreted as an option and the string `-n` is printed instead. This makes

it safe to output arbitrary data using this version of `echo` as long as it is

given as a single argument (using quoting if needed).



### Legibility aliases: `not`, `so`, `forever` ###



Modernish sets three aliases that can help to make the shell language look

slightly friendlier. Their use is optional.



`not` is a new synonym for `!`. They can be used interchangeably.



`so` is a command that tests if the previous command exited with a status

of zero, so you can test the preceding command's success with `if so` or

`if not so`.



`forever` is a new synonym for `while :;`. This allows simple infinite loops

of the form: `forever do` *stuff*`; done`.



### Enhanced `exit` ###



The `exit` command can be used as normal, but has gained capabilities.



Extended usage: `exit` [ `-u` ] [ *status* [ *message* ] ]



* As per standard, if *status* is not specified, it defaults to the exit

  status of the command executed immediately prior to `exit`.

  Otherwise, it is evaluated as a shell arithmetic expression. If it is

  invalid as such, the shell exits immediately with an arithmetic error.

* Any remaining arguments after *status* are combined, separated by spaces,

  and taken as a *message* to print on exit. The message shown is preceded by

  the name of the current program (`$ME` minus directories). Note that it is

  not possible to skip *status* while specifying a *message*.

* If the `-u` option is given, and the shell function `showusage` is defined,

  that function is run in a subshell before exiting. It is intended to print

  a message showing how the command should be invoked. The `-u` option has no

  effect if the script has not defined a `showusage` function.

* If *status* is non-zero, the *message* and the output of the `showusage`

  function are redirected to standard error.



### `chdir` ###



`chdir` is a robust `cd` replacement for use in scripts.



The [standard `cd` command](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/cd.html)

is designed for interactive shells and appropriate to use there.

However, for scripts, its features create serious pitfalls:



* The `$CDPATH` variable is searched. A script may inherit a user's

  exported `$CDPATH`, so `cd` may change to an unintended directory.

* `cd` cannot be used with arbitrary directory names (such as untrusted user

  input), as some operands have special meanings, even after `--`. POSIX

  specifies that `-` changes directory to `$OLDPWD`. On zsh (even in sh mode

  on zsh \<= 5.7.1), numeric operands such as `+12` or `-345` represent

  directory stack entries. All such paths need escaping by prefixing `./`.

* Symbolic links in directory path components are not resolved by default,

  leaving a potential symlink attack vector.



Thus, robust and portable use of `cd` in scripts is unreasonably difficult.

The modernish `chdir` function calls `cd` in a way that takes care of all

these issues automatically: it disables `$CDPATH` and special operand

meanings, and resolves symbolic links by default.



Usage: `chdir` [ `-f` ] [ `-L` ] [ `-P` ] [ `--` ] *directorypath*



Normally, failure to change the present working directory to *directorypath*

is a fatal error that ends the program. To tolerate failure, add the `-f`

option; in that case, exit status 0 signifies success and exit status 1

signifies failure, and scripts should always check and handle exceptions.



The options `-L` (logical: don't resolve symlinks) and `-P` (physical:

resolve symlinks) are the same as in `cd`, except that `-P` is the default.

Note that on a shell with [`BUG_CDNOLOGIC`](#user-content-bugs) (NetBSD sh),

the `-L` option to `chdir` does nothing.



To use arbitrary directory names (e.g. directory names input by the user or

other untrusted input) always use the `--` separator that signals the end of

options, or paths starting with `-` may be misinterpreted as options.



### `insubshell` ###



The `insubshell` function checks if you're currently running in a

[subshell environment](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_12)

(usually called simply *subshell*).



A *subshell* is a copy of the parent shell that starts out as an exact

duplicate (including non-exported variables, functions, etc.), except for

traps. A new subshell is invoked by constructs like `(`parentheses`)`,

`$(`command substitutions`)`, pipe`|`lines, and `&` (to launch a background

subshell). Upon exiting a subshell, all changes to its state are lost.



This is not to be confused with a newly initialised shell that is

merely a child process of the current shell, which is sometimes

(confusingly and **wrongly**) called a "subshell" as well.

This documentation avoids such a misleading use of the term.



Usage: `insubshell` [ `-p` | `-u` ]



This function returns success (0) if it was called from within a subshell

and non-success (1) if not. One of two options can be given:

* `-p`: Store the process ID (PID) of the current subshell or main shell

  in `REPLY`.

* `-u`: Store an identifier in `REPLY` that is useful for determining if

  you've entered a subshell relative to a previously stored identifier. The

  content and format are unspecified and shell-dependent.



### `isset` ###



`isset` checks if a variable, shell function or option is set, or has

certain attributes. Usage:



* `isset` *varname*: Check if a variable is set.

* `isset -v` *varname*: Id.

* `isset -x` *varname*: Check if variable is exported.

* `isset -r` *varname*: Check if variable is read-only.

* `isset -f` *funcname*: Check if a shell function is set.

* `isset -`*optionletter* (e.g. `isset -C`): Check if shell option is set.

* `isset -o` *optionname*: Check if shell option is set by long name.



Exit status: 0 if the item is set; 1 if not; 2 if the argument is not

recognised as a [valid identifier](#user-content-names-and-identifiers).

Unlike most other modernish commands, `isset` does not treat an invalid

identifier as a fatal error.



When checking a shell option, a nonexistent shell option is not an error,

but returns the same result as an unset shell option. (To check if a shell

option exists, use [`thisshellhas`](#user-content-shell-capability-detection).



Note: just `isset -f` checks if shell option `-f` (a.k.a. `-o noglob`) is

set, but with an extra argument, it checks if a shell function is set.

Similarly, `isset -x` checks if shell option `-x` (a.k.a `-o xtrace`)

is set, but `isset -x` *varname* checks if a variable is exported. If you

use unquoted variable expansions here, make sure they're not empty, or

the shell's empty removal mechanism will cause the wrong thing to be checked

(even in the [safe mode](#user-content-use-safe)).



### `setstatus` ###



`setstatus` manually sets the exit status `$?` to the desired value. The

function exits with the status indicated. This is useful in conditional

constructs if you want to prepare a particular exit status for a subsequent

`exit` or `return` command to inherit under certain circumstances.

The status argument is a parsed as a shell arithmetic expression. A negative

value is treated as a fatal error. The behaviour of values greater than 255

is not standardised and depends on your particular shell.



## Testing numbers, strings and files ##



The `test`/`[` command is the bane of casual shell scripters. Even advanced

shell programmers are frequently caught unaware by one of the many pitfalls

of its arcane, hackish syntax. It attempts to look like shell grammar without

*being* shell grammar, causing myriad problems

([1](http://wiki.bash-hackers.org/commands/classictest),

[2](https://mywiki.wooledge.org/BashPitfalls)).

Its `-a`, `-o`, `(` and `)` operators are *inherently and fatally broken* as

there is no way to reliably distinguish operators from operands, so POSIX

[deprecates their use](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html#tag_20_128_16);

however, most manual pages do not include this essential information, and

even the few that do will not tell you what to do instead.



Ksh, zsh and bash offer a `[[` alternative that fixes many of these problems,

as it is integrated into the shell grammar. Nevertheless, it increases

confusion, as entirely different grammar and quoting rules apply

within `[[`...`]]` than outside it, yet many scripts end up using them

interchangeably. It is also not available on all POSIX shells. (To make

matters worse, Busybox ash has a false-friend `[[` that is just an alias

of `[`, with none of the shell grammar integration!)



Finally, the POSIX `test`/`[` command is incompatible with the modernish

"safe mode" which aims to eliminate most of the need to quote variables.

See [`use safe`](#user-content-use-safe) for more information.



Modernish deprecates `test`/`[` and `[[` completely. Instead, it offers a

comprehensive alternative command design that works with the usual shell

grammar in a safer way while offering various feature enhancements. The

following replacements are available:



### Integer number arithmetic tests and operations ###



To test if a string is a valid number in shell syntax, `str isint` is

available. See [String tests](#user-content-string-tests).



#### The arithmetic command `let` ####

An implementation of `let` as in ksh, bash and zsh is now available to all

POSIX shells. This makes C-style signed integer arithmetic evaluation

available to every

[supported shell](#user-content-appendix-d-supported-shells),

*with the exception of the unary `++` and `--` operators*

(which are a nonstandard shell capability detected by modernish under the ID of

[`ARITHPP`](#user-content-appendix-a-list-of-shell-cap-ids)).



This means `let` should be used for operations and tests, e.g. both

`let "x=5"` and `if let "x==5"; then`... are supported (note: single `=` for

assignment, double `==` for comparison). See POSIX

[2.6.4 Arithmetic Expansion](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04)

for more information on the supported operators.



Multiple expressions are supported, one per argument. The exit status of `let`

is zero (the shell's idea of success/true) if the last expression argument

evaluates to non-zero (the arithmetic idea of true), and 1 otherwise.



It is recommended to adopt the habit to quote each `let` expression with

`"`double quotes`"`, as this consistently makes everything work as expected:

double quotes protect operators that would otherwise be misinterpreted as

shell grammar, while shell expansions starting with `$` continue to work.



#### Arithmetic shortcuts ####

Various handy functions that make common arithmetic operations

and comparisons easier to program are available from the

[`var/arith`](#user-content-use-vararith) module.



### String and file tests ###



The following notes apply to all commands described in the subsections of

this section:

1. "True" is understood to mean exit status 0, and "false" is understood to

   mean a non-zero exit status – specifically 1.

2. Passing *more* than the number of arguments specified for each command

   is a [fatal error](#user-content-reliable-emergency-halt). (If the

   [safe mode](#user-content-use-safe) is not used, excessive arguments

   may be generated accidentally if you forget to quote a variable. The

   test result would have been wrong anyway, so modernish kills the

   program immediately, which makes the problem much easier to trace.)

3. Passing *fewer* than the number of arguments specified to the command is

   assumed to be the result of removal of an empty unquoted expansion.

   Where possible, this is not treated as an error, and an exit status

   corresponding to the omitted argument(s) being empty is returned instead.

   (This helps make the [safe mode](#user-content-use-safe) possible; unlike

   with `test`/`[`, paranoid quoting to avoid empty removal is not needed.)



#### String tests ####

The `str` function offers various operators for tests on strings. For

example, `str in $foo "bar"` tests if the variable `foo` contains "bar".



The `str` function takes unary (one-argument) operators that check a property

of a single word, binary (two-argument) operators that check a word against a

pattern, as well as an option that makes binary operators check multiple words

against a pattern.



##### Unary string tests ####

Usage: `str` *operator* [ *word* ]



The *word* is checked for the property indicated by *operator*; if the result

is true, `str` returns status 0, otherwise it returns status 1.



The available unary string test *operator*s are:



* `empty`: The *word* is empty.

* `isint`: The *word* is a decimal, octal or hexadecimal integer number in

  valid POSIX shell syntax, safe to use with `let`, `$((`...`))` and other

  arithmetic contexts on all POSIX-derived shells. This operator ignores

  leading (but not trailing) spaces and tabs.

* `isvarname`: The *word* is a valid portable shell variable or function name.



If *word* is omitted, it is treated as empty, on the assumption that it is

an unquoted empty variable. Passing more than one argument after the

*operator* is a fatal error.



##### Binary string matching tests #####

Usage: `str` *operator* [ [ *word* ] *pattern* ]



The *word* is compared to the *pattern* according to the *operator*; if it

matches, `str` returns status 0, otherwise it returns status 1.

The available binary matching *operator*s are:



* `eq`: *word* is equal to *pattern*.

* `ne`: *word* is not equal to *pattern*.

* `in`: *word* includes *pattern*.

* `begin`: *word* begins with *pattern*.

* `end`: *word* ends with *pattern*.

* `match`: *word* matches *pattern* as a shell glob pattern

  (as in the shell's native `case` construct).

  A *pattern* that ends in an unescaped backslash is considered invalid

  and causes `str` to return status 2.

* `ematch`: *word* matches *pattern* as a POSIX

  [extended regular expression](http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04).

  An empty *pattern* is a fatal error.

  (In UTF-8 locales, check if

  thisshellhas [WRN_EREMBYTE](#user-content-warning-ids)

  before matching multi-byte characters.)

* `lt`: *word* lexically sorts before (is 'less than') *pattern*.

* `le`: *word* is lexically 'less than or equal to' *pattern*.

* `gt`: *word* lexically sorts after (is 'greater than') *pattern*.

* `ge`: *word* is lexically 'greater than or equal to' *pattern*.



If *word* is omitted, it is treated as empty on the assumption that it is an

unquoted empty variable, and the single remaining argument is assumed to be

the *pattern*. Similarly, if both *word* and *pattern* are omitted, an empty

*word* is matched against an empty *pattern*. Passing more than two

arguments after the *operator* is a fatal error.



##### Multi-matching option #####

Usage: `str -M` *operator* [ [ *word* ... ] *pattern* ]



The `-M` option causes `str` to compare any number of *word*s to the

*pattern*. The available *operator*s are the same as the binary string

matching operators listed above.



All matching *word*s are stored in the `REPLY` variable, separated

by newline characters (`$CCn`) if there is more than one match.

If no *word*s match, `REPLY` is unset.



The exit status returned by `str -M` is as follows:



* If no *word*s match, the exit status is 1.

* If one *word* matches, the exit status is 0.

* If between two and 254 *word*s match, the exit status is the number of matches.

* If 255 or more *word*s match, the exit status is 255.



Usage example: the following matches a given GNU-style long-form command

line option `$1` against a series of available options. To make it possible

for the options to be abbreviated, we check if any of the options begin with

the given argument `$1`.



```sh

if str -M begin --fee --fi --fo --fum --foo --bar --baz --quux "$1"; then

	putln "OK. The given option $1 matched $REPLY"

else

	case $? in

	( 1 )	putln "No such option: $1" >&2 ;;

	( * )	putln "Ambiguous option: $1" "Did you mean:" "$REPLY" >&2 ;;

	esac

fi

```



#### File type tests ####

These avoid the snags with symlinks you get with `[` and `[[`.

By default, symlinks are *not* followed. Add `-L` to operate on files

pointed to by symlinks instead of symlinks themselves (the `-L` makes

no difference if the operands are not symlinks).



These commands all take one argument. If the argument is absent, they return

false. More than one argument is a fatal error. See notes 1-3 in the

[parent section](#user-content-string-and-file-tests).



`is present` *file*: Returns true if the file is present in the file

system (even if it is a broken symlink).



`is -L present` *file*: Returns true if the file is present in the file

system and is not a broken symlink.



`is sym` *file*: Returns true if the file is a symbolic link (symlink).



`is -L sym` *file*: Returns true if the file is a non-broken symlink, i.e.

a symlink that points (either directly or indirectly via other symlinks)

to a non-symlink file that is present in the file system.



`is reg` *file*: Returns true if *file* is a regular data file.



`is -L reg` *file*: Returns true if *file* is either a regular data file

or a symlink pointing (either directly or indirectly via other symlinks)

to a regular data file.



Other commands are available that work exactly like `is reg` and `is -L reg`

but test for other file types. To test for them, replace `reg` with one of:

* `dir` for a directory

* `fifo` for a named pipe (FIFO)

* `socket` for a socket

* `blockspecial` for a block special file

* `charspecial` for a character special file



#### File comparison tests ####

The following notes apply to these commands:

* Symlinks are *not* resolved/followed by default. To operate on files pointed

  to by symlinks, add `-L` before the operator argument, e.g. `is -L newer`.

* Omitting any argument is a fatal error, because no empty argument (removed or

  otherwise) would make sense for these commands.



`is newer` *file1* *file2*: Compares file timestamps, returning true if *file1*

is newer than *file2*. Also returns true if *file1* exists, but *file2* does

not; this is consistent for all shells (unlike `test file1 -nt file2`).



`is older` *file1* *file2*: Compares file timestamps, returning true if *file1*

is older than *file2*. Also returns true if *file1* does not exist, but *file2*

does; this is consistent for all shells (unlike `test file1 -ot file2`).



`is samefile` *file1* *file2*: Returns true if *file1* and *file2* are the same

file (hardlinks).



`is onsamefs` *file1* *file2*: Returns true if *file1* and *file2* are on the

same file system. If any non-regular, non-directory files are specified, their

parent directory is tested instead of the file itself.



#### File status tests ####

These always follow symlinks.



`is nonempty` *file*: Returns true if the *file* exists, is not a broken

symlink, and is not empty. Unlike `[ -s file ]`, this also works

for directories, as long as you have read permission in them.



`is setuid` *file*: Returns true if the *file* has its set-user-ID flag set.



`is setgid` *file*: Returns true if the *file* has its set-group-ID flag set.



#### I/O tests ####

`is onterminal` *FD*: Returns true if file descriptor *FD* is associated

with a terminal. The *FD* may be a non-negative integer number or one of the

special identifiers `stdin`, `stdout` and `stderr` which are equivalent to

0, 1, and 2. For instance, `is onterminal stdout` returns true if commands

that write to standard output (FD 1), such as `putln`, would write to the

terminal, and false if the output is redirected to a file or pipeline.



#### File permission tests ####

Any symlinks given are resolved, as these tests would be meaningless

for a symlink itself.



`can read` *file*: True if the file's permission bits indicate that you can read

the file - i.e., if an `r` bit is set and applies to your user.



`can write` *file*: True if the file's permission bits indicate that you can

write to the file: for non-directories, if a `w` bit is set and applies to your

user; for directories, both `w` and `x`.



`can exec` *file*: True if the file's type and permission bits indicate that

you can execute the file: for regular files, if an `x` bit is set and applies

to your user; for other file types, never.



`can traverse` *file*: True if the file is a directory and its permission bits

indicate that a path can traverse through it to reach its subdirectories: for

directories, if an `x` bit is set and applies to your user; for other file

types, never.



## The stack ##



In modernish, every variable and shell option gets its own stack. Arbitrary

values/states can be pushed onto the stack and popped off it in reverse

order. For variables, both the value and the set/unset state is (re)stored.



Usage:



* `push` [ `--key=`*value* ] *item* [ *item* ... ]

* `pop` [ `--keepstatus` ] [ `--key=`*value* ] *item* [ *item* ... ]



where *item* is a valid portable variable name, a short-form shell option

(dash plus letter), or a long-form shell option (`-o` followed by an option

name, as two arguments).



Before pushing or popping anything, both functions check if all the given

arguments are valid and `pop` checks all items have a non-empty stack. This

allows pushing and popping groups of items with a check for the integrity of

the entire group. `pop` exits with status 0 if all items were popped

successfully, and with status 1 if one or more of the given items could not

be popped (and no action was taken at all).



The `--key=` option is an advanced feature that can help different modules

or functions to use the same variable stack safely. If a key is given to

`push`, then for each *item*, the given key *value* is stored along with the

variable's value for that position in the stack. Subsequently, restoring

that value with `pop` will only succeed if the key option with the same key

value is given to the `pop` invocation. Similarly, popping a keyless value

only succeeds if no key is given to `pop`. If there is any key mismatch, no

changes are made and `pop` returns status 2.  Note that this is

a robustness/convenience feature, not a security feature; the keys are not

hidden in any way.



If the `--keepstatus` option is given, `pop` will exit with the

exit status of the command executed immediately prior to calling `pop`. This

can avoid the need for awkward workarounds when restoring variables or shell

options at the end of a function. However, note that this makes failure to pop

(stack empty or key mismatch) a fatal error that kills the program, as `pop`

no longer has a way to communicate this through its exit status.



### The shell options stack ###



`push` and `pop` allow saving and restoring the state of any shell option

available to the `set` builtin. The precise shell options supported

(other than the ones guaranteed by POSIX) depend on

[the shell modernish is running on](#user-content-appendix-d-supported-shells).

To facilitate portability, nonexistent shell options are treated as unset.



Long-form shell options are matched to their equivalent short-form shell

options, if they exist. For instance, on all POSIX shells, `-f` is

equivalent to `-o noglob`, and `push -o noglob` followed by `pop -f` works

correctly. This also works for shell-specific short & long option

equivalents.



On shells with a dynamic `no` option name prefix, that is on ksh, zsh and

yash (where, for example, `noglob` is the opposite of `glob`), the `no`

prefix is ignored, so something like `push -o glob` followed by `pop -o

noglob` does the right thing. But this depends on the shell and should never

be used in portable scripts.



### The trap stack ###



Modernish can also make traps stack-based, so that each

program component or library module can set its own trap commands

without interfering with others. This functionality is provided

by the [`var/stack/trap`](#user-content-use-varstacktrap) module.



## Modules ##



As modularity is one of modernish's

[design principles](https://github.com/modernish/modernish/blob/master/share/doc/modernish/DESIGN.md),

much of its essential functionality is provided in the form of loadable

modules, so the core library is kept lean. Modules are organised

hierarchically, with names such as `safe`, `var/loop` and `sys/cmd/harden`. The

`use` command loads and initialises a module or a combined directory of modules.



Internally, modules exist in files with the name extension `.mm` in

subdirectories of `lib/modernish/mdl` – for example, the module

`var/stack/trap` corresponds to the file `lib/modernish/mdl/var/stack/trap.mm`.



Usage:

* `use` *modulename* [ *argument* ... ]

* `use` [ `-q` | `-e` ] *modulename*

* `use -l`



The first form loads and initialises a module. All arguments, including the

module name, are passed on to the dot script unmodified, so modules know

their own name and can implement option parsing to influence their

initialisation. See also

[Two basic forms of a modernish program](#user-content-two-basic-forms-of-a-modernish-program)

for information on how to use modules in portable-form scripts.



In the second form, the `-q` option queries if a module is loaded, and the `-e`

option queries if a module exists. `use` returns status 0 for yes, 1 for no,

and 2 if the module name is invalid.



The `-l` option lists all currently loaded modules in the order in which

they were originally loaded. Just add `| sort` for alphabetical order.



If a directory of modules, such as `sys/cmd` or even just `sys`, is given as the

*modulename*, then all the modules in that directory and any subdirectories are

loaded recursively. In this case, passing extra arguments is a fatal error.



If a module file `X.mm` exists along with a directory `X`, resolving to the

same *modulename*, then `use` will load the `X.mm` module file without

automatically loading any modules in the `X` directory, because it is expected

that `X.mm` handles the submodules in `X` manually. (This is currently the case

for `var/loop` which auto-loads submodules containing loop types on first use).



The complete `lib/modernish/mdl` directory path, which depends on where

modernish is installed, is stored in the system constant `$MSH_MDL`.



The following subchapters document the modules that come with modernish.



### `use safe` ###



The `safe` module sets the 'safe mode' for the shell. It removes most of the

need to quote variables, parameter expansions, command substitutions, or glob

patterns. It uses shell settings and modernish library functionality to secure

and demystify split and glob mechanisms. This creates a new and safer way of

shell script programming, essentially building a new shell language dialect

while still running on all POSIX-compliant shells.



#### Why the safe mode? ####

One of the most common headaches with shell scripting is caused by a

fundamental flaw in the shell as a scripting language: *constantly

active field splitting* (a.k.a. word splitting) *and pathname expansion*

(a.k.a. globbing). To cope with this situation, it is hammered into

programmers of shell scripts to be absolutely paranoid about properly

[quoting](https://mywiki.wooledge.org/Quotes) nearly everything, including

variable and parameter expansions, command substitutions, and patterns passed

to commands like `find`.



These mechanisms were designed for interactive command line usage, where they

do come in very handy. But when the shell language is used as a programming

language, splitting and globbing often ends up being applied unexpectedly to

unquoted expansions and command substitutions, helping cause thousands of

buggy, brittle, or outright dangerous shell scripts.



One could blame the programmer for forgetting to quote an expansion properly,

*or* one could blame a pitfall-ridden scripting language design where hammering

punctilious and counterintuitive habits into casual shell script programmers is

necessary. Modernish does the latter, then fixes it.



#### How the safe mode works ####

Every POSIX shell comes with a little-used ability to disable global field

splitting and pathname expansion: `IFS=''; set -f`. An empty `IFS` variable

disables split; the `-f` (or `-o noglob`) shell option disables pathname

expansion. The safe mode sets these, and two others (see below).



The reason these safer settings are hardly ever used is that they are not

practical to use with the standard shell language. For instance, `for

textfile in *.txt`, or `for item in $(some command)` which both (!)

field-splits *and* pathname-expands the output of a command, all break.



However, that is where modernish comes in. It introduces several powerful

new [loop constructs](#user-content-use-varloop), as well as arbitrary code

blocks with [local settings](#user-content-use-varlocal), each of which

has straightforward, intuitive operators for safely applying field splitting

*or* pathname expansion – to specific command arguments only. By default,

they are *not both* applied to the arguments, which is much safer. And your

script code as a whole is kept safe from them at all times.



With global field splitting and pathname expansion removed, a third issue

still affects the safe mode: the shell's *empty removal* mechanism. If the

value of an unquoted expansion like `$var` is empty, it will not expand to

an empty argument, but will be removed altogether, as if it were never

there. This behaviour cannot be disabled.



Thankfully, the vast majority of shell and Un*x commands order their arguments

in a way that is actually designed with empty removal in mind, making it a

good thing. For instance, when doing `ls $option some_dir`, if `$option` is

`-l` the listing will be long-format and if is empty it will be removed, which

is the desired behaviour. (An empty argument there would cause an error.)



However, one command that is used in almost all shell scripts, `test`/`[`,

is *completely unable to cope with empty removal* due to its idiosyncratic

and counterintuitive syntax. Potentially empty operands come before options,

so operands removed as empty expansions cause errors or, worse, false

positives. Thus, the safe mode does *not* remove the need for paranoid

quoting of expansions used with `test`/`[` commands. Modernish fixes

this issue by *deprecating `test`/`[` completely* and offering

[a safe command design](#user-content-testing-numbers-strings-and-files)

to use instead, which correctly deals with empty removal.



With the 'safe mode' shell settings, plus the safe, explicit and readable

split and glob operators and `test`/`[` replacements, the only quoting

requirements left are:

1. a very occasional need to stop empty removal from happening;

2. to quote `"$@"` and `"$*"` until shell bugs are fixed (see notes below).



In addition to the above, the safe mode also sets these shell options:

* `set -C` (`set -o noclobber`) to prevent accidentally overwriting files using

  output redirection. To force overwrite, use `>|` instead of `>`.

* `set -u` (`set -o nounset`) to make it an error to use unset (that is,

  uninitialised) variables by default. You'll notice this will catch many

  typos before they cause you hard-to-trace problems. To bypass the check

  for a specific variable, use `${var-}` instead of `$var` (be careful).



#### Important notes for safe mode ####

* The safe mode is *not* compatible with existing conventional shell scripts,

  written in what we could now call the 'legacy mode'. Essentially, the safe

  mode is a new way of shell script programming. That is why it is not enabled

  by default, but activated by loading the `safe` module. *It is highly

  recommended that new modernish scripts start out with `use safe`.*

* The shell applies entirely different quoting rules to string matching glob

  patterns within `case` constructs. The safe mode changes nothing here.

* Due to [shell bugs](#user-content-bugs) ID'ed as `BUG_PP_*`, the positional

  parameters expansions `$@` and `$*` should still *always* be quoted. As of

  late 2018, these bugs have been fixed in the latest or upcoming release

  versions of all

  [supported shells](#user-content-appendix-d-supported-shells).

  But, until buggy versions fall out of use

  and modernish no longer supports any `BUG_PP_*` shell bugs, quoting `"$@"`

  and `"$*"` remains mandatory even in safe mode (unless you know with

  certainty that your script will be used on a shell with none of these bugs).

* The behaviour of `"$*"` changes in safe mode. It uses the first character

  of `$IFS` as the separator for combining all positional parameters into

  one string. Since `IFS` is emptied in safe mode, there is no separator,

  so it will string them together unseparated. You can use something like

  [`push IFS; IFS=' '; var="$*"; pop IFS`](#user-content-the-stack)

  or [`LOCAL IFS=' '; BEGIN var="$*"; END`](#user-content-use-varlocal)

  to use the space character as a separator.

  (If you're outputting the positional parameters, note that the

  [`put`](#user-content-outputting-strings)

  command always separates its arguments by spaces, so you can

  safely pass it multiple arguments with `"$@"` instead.)



#### Extra options for the safe mode ####

Usage: `use safe` [ `-k` | `-K` ] [ `-i` ]



The `-k` and `-K` module options install an extra handler that

[reliably kills the program](#user-content-reliable-emergency-halt)

if it tries to execute a command that is not found, on shells that have the

ability to catch and handle 'command not found' errors (currently bash, yash,

and zsh). This helps catch typos, forgetting to load a module, etc., and stops

your program from continuing in an inconsistent state and potentially causing

damage. The `MSH_NOT_FOUND_OK` variable may be set to temporarily disable this

check. The uppercase `-K` module option aborts the program on shells that

cannot handle 'command not found' errors (so should not be used for portable

scripts), whereas the lowercase `-k` variant is ignored on such shells.



If the `-i` option is given, or the shell is interactive, two extra one-letter

functions are loaded, `s` and `g`. These are pre-command modifiers for use when

split and glob are globally disabled; they allow running a single command with

local split and glob applied to that command's arguments only. They also have

some options designed to manipulate, examine, save, restore, and generally

experiment with the global split and glob state on interactive shells. Type

`s --help` and `g --help` for more information. In general, the safe mode is

designed for scripts and is not recommended for interactive shells.



### `use var/loop` ###



The `var/loop` module provides an innovative, robust and extensible

shell loop construct. Several powerful loop types are provided, while

advanced shell programmers may find it easy and fun to

[create their own](#user-content-creating-your-own-loop).

This construct is also ideal for the

[safe mode](#user-content-use-safe):

the `for`, `select` and `find` loop types allow you to selectively

apply field splitting and/or pathname expansion to specific arguments

without subjecting a single line of your code to them.



The basic form is a bit different from native shell loops. Note the caps:    

`LOOP` *looptype* *arguments*; `DO`    

      *your commands here*    

`DONE`



The familiar `do`...`done` block syntax cannot be used because the shell

will not allow modernish to add its own functionality to it. The

`DO`...`DONE` block does behave in the same way as `do`...`done`: you can

append redirections at the end, pipe commands into a loop, etc. as usual.

The `break` and `continue` shell builtin commands also work as normal.



**Remember:** *using lowercase `do`...`done` with modernish `LOOP` will

cause the shell to throw a misleading syntax error.* So will using uppercase

`DO`...`DONE` with the shell's native loops. To help you remember to use the

uppercase variants for modernish loops, the `LOOP` keyword itself is also in

capitals.



Loops exist in submodules of `var/loop` named after the loop type; for

instance, the `find` loop lives in the `var/loop/find` module. However, the

core `var/loop` module will automatically load a loop type's module when

that loop is first used, so `use`-ing individual loop submodules at your

script's startup time is optional.



The `LOOP` block internally uses file descriptor 8 to do

[its thing](#user-content-creating-your-own-loop).

If your script happens to use FD 8 for other purposes, you should

know that FD 8 is made local to each loop block, and always appears

initially closed within `DO`...`DONE`.



#### Simple repeat loop ####

This simply iterates the loop the number of times indicated. Before the first

iteration, the argument is evaluated as a shell integer arithmetic expression

as in [`let`](#user-content-integer-number-arithmetic-tests-and-operations)

and its value used as the number of iterations.



```sh

LOOP repeat 3; DO

	putln "This line is repeated 3 times."

DONE

```



#### BASIC-style arithmetic `for` loop ####

This is a slightly enhanced version of the

[`FOR` loop in BASIC](https://en.wikipedia.org/wiki/BASIC#Origin).

It is more versatile than the `repeat` loop but still very easy to use.



`LOOP for` *varname*`=`*initial* to *limit* [ `step` *increment* ]; DO    

      *some commands*    

`DONE`



To count from 1 to 20 in steps of 2:



```sh

LOOP for i=1 to 20 step 2; DO

	putln "$i"

DONE

```



Note the *varname*`=`*initial* needs to be one argument as in a shell

assignment (so no spaces around the `=`).



If "`step` *increment*" is omitted, *increment* defaults to 1 if *limit* is

equal to or greater than *initial*, or to -1 if *limit* is less than

*initial* (so counting backwards 'just works').



Technically precise description: On entry, the *initial*, *limit* and

*increment* values are evaluated once as shell arithmetic expressions as in

[`let`](#user-content-integer-number-arithmetic-tests-and-operations),

the value of *initial* is assigned to *varname*, and the loop iterates.

Before every subsequent iteration, the value of *increment* (as determined

on the first iteration) is added to the value of *varname*, then the *limit*

expression is re-evaluated; as long as the current value of *varname* is

less (if *increment* is non-negative) or greater (if *increment* is

negative) than or equal to the current value of *limit*, the loop reiterates.



#### C-style arithmetic `for` loop ####

A C-style for loop akin to `for (( ))` in ksh93, bash and zsh is now

available on all POSIX-compliant shells, with a slightly different syntax.

The one loop argument contains three arithmetic expressions (as in

[`let`](#user-content-integer-number-arithmetic-tests-and-operations)),

separated by semicolons within that argument. The first is only evaluated

before the first iteration, so is typically used to assign an initial value.

The second is evaluated before each iteration to check whether to continue

the loop, so it typically contains some comparison operator. The third is

evaluated before the second and further iterations, and typically increases

or decreases a value. For example, to count from 1 to 10:



```sh

LOOP for "i=1; i<=10; i+=1"; DO

	putln "$i"

DONE

```



However, using complex expressions allows doing much more powerful things.

Any or all of the three expressions may also be left empty (with their

separating `;` character remaining). If the second expression is empty, it

defaults to 1, creating an infinite loop.



(Note that `++i` and `i++` can only be used on shells with

[`ARITHPP`](#user-content-appendix-a-list-of-shell-cap-ids),

but `i+=1` or `i=i+1` can be used on all POSIX-compliant shells.)



#### Enumerative `for`/`select` loop with safe split/glob ####

The enumarative `for` and `select` loop types mirror those already present in

native shell implementations. However, the modernish versions provide safe

field splitting and globbing (pathname expansion) functionality that can be

used without globally enabling split or glob for any of your code – ideal

for the [safe](#user-content-use-safe) mode. They also add a unique operator

for processing text in fixed-size slices. The `select` loop type brings

`select` functionality to all POSIX shells and not just ksh, zsh and bash.



Usage:



`LOOP` [ `for` | `select` ] [ *operators* ] *varname* `in` *argument* ... `;`

`DO` *commands* `;` `DONE`



Simple usage example:



```sh

LOOP select --glob textfile in *.txt; DO

	putln "You chose text file $textfile."

DONE

```



If the loop type is `for`, the loop iterates once for each *argument*, storing

it in the variable named *varname*.



If the loop type is `select`, the loop presents before each iteration a

numbered menu that allows the user to select one of the *argument*s. The prompt

from the `PS3` variable is displayed and a reply read from standard input. The

literal reply is stored in the `REPLY` variable. If the reply was a number

corresponding to an *argument* in the menu, that *argument* is stored in the

variable named *varname*. Then the loop iterates. If the user enters ^D (end of

file), `REPLY` is cleared and the loop breaks with an exit status of 1. (To

break the menu loop under other conditions, use the `break` command.)



The following operators are supported. Note that the split and glob

operators are only for use in the [safe mode](#user-content-use-safe).

* One of `--split` or `--split=`*characters*. This operator safely applies

  the shell's field splitting mechanism to the *argument*s given. The simple

  `--split` operator applies the shell's default field splitting by space,

  tab, and newline. If you supply one or more of your own *characters* to

  split by, each of these characters will be taken as a field separator if

  it is whitespace, or field terminator if it is non-whitespace. (Note that

  shells with [`QRK_IFSFINAL`](#user-content-quirks) treat both whitespace and

  non-whitespace characters as separators.)

* One of `--glob` or `--fglob`. These operators safely apply shell pathname

  expansion (globbing) to the *argument*s given. Each *argument* is taken as

  a pattern, whether or not it contains any wildcard characters. For any

  resulting pathname that starts with `-` or `+` or is identical to `!` or

  `(`, `./` is prefixed to keep various commands from misparsing it as an

  option or operand. Non-matching patterns are treated as follows:

    * `--glob`: Any non-matching patterns are quietly removed. If none match,

      the loop will not iterate but break with exit status 103.

    * `--fglob`: All patterns must match. Any nonexistent path terminates the

      program. Use this if your program would not work after a non-match.

* `--base=`*string*. This operator prefixes the given *string* to each of the

  *arguments*, after first applying field splitting and/or pathname expansion

  if specified.

  If `--glob` or `--fglob` are given, then the *string* is used as a base

  directory path for pathname expansion, without expanding any wildcard

  characters in that base directory path itself.

  If such base directory can't be entered, then if `--glob` was given, the loop

  breaks with status 98, or if `--fglob` was given, the program terminates.

* One of `--slice` or `--slice=`*number*. This operator divides the

  *argument*s in slices of up to *number* characters. The default slice size

  is 1 character, allowing for easy character-by-character processing.

  (Note that shells with [`WRN_MULTIBYTE`](#user-content-warning-ids) will

  not slice multi-byte characters correctly.)



If multiple operators are given, their mechanisms are applied in the

following order: split, glob, base, slice.



#### The `find` loop ####

This powerful loop type turns your local POSIX-compliant

[`find` utility](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/find.html)

into a shell loop, safely integrating both `find`

and `xargs` functionality into the POSIX shell. The infamous

[pitfalls and limitations](https://dwheeler.com/essays/filenames-in-shell.html#find)

of using `find` and `xargs` as external commands are gone, as all

the results from `find` are readily available to your main shell

script. Any "dangerous" characters in file names (including

whitespace and even newlines) "just work", especially if the

[safe mode](#user-content-use-safe)

is also active. This gives you the flexibility to use either the `find`

expression syntax, or shell commands (including your own shell functions), or

some combination of both, to decide whether and how to handle each file found.



Usage:



`LOOP find` [ *options* ] *varname* [ `in` *path* ... ]

[ *find-expression* ] `;` `DO` *commands* `;` `DONE`



`LOOP find` [ *options* ] `--xargs`[`=`*arrayname*] [ `in` *path* ... ]

[ *find-expression* ] `;` `DO` *commands* `;` `DONE`



The loop recursively walks down the directory tree for each *path* given.

For each file encountered, it uses the *find-expression* to decide

whether to iterate the loop with the path to the file stored in the

variable referenced by *varname*. The *find-expression* is a standard

[`find`](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/find.html)

utility expression except as described below.



Any number of paths to search may be specified after the `in` keyword.

By default, a nonexistent path is a [fatal error](#user-content-reliable-emergency-halt).

The entire `in` clause may be omitted, in which case it defaults to `in .`

so the current working directory will be searched. Any argument that starts

with a `-`, or is identical to `!` or `(`, indicates the end of the *path*s

and the beginning of the *find-expression*; if you need to explicitly

specify a path with such a name, prefix `./` to it.



Except for syntax errors, any errors or warnings issued by `find` are

considered non-fatal and will cause the exit status of the loop to be

non-zero, so your script has the opportunity to handle the exception.



##### Available *options* #####

* Any single-letter options supported by your local `find` utility. Note that

  [POSIX specifies](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/find.html)

  `-H` and `-L` only, so portable scripts should only use these.

  Options that require arguments (`-f` on BSD `find`) are not supported.

* `--xargs`. This operator is specified **instead** of the *varname*; it is a

  syntax error to have both. Instead of one iteration per found item, as many

  items as possible per iteration are stored into the positional parameters

  (PPs), so your program can access them in the usual way (using `"$@"` and

  friends). Note that `--xargs` therefore overwrites the current PPs (however,

  a shell function or [`LOCAL`](#user-content-use-varlocal) block will give

  you local PPs). Modernish clears the PPs upon completion of the loop, but if

  the loop is exited prematurely (such as by `break`), the last chunk survives.

    * On shells with the `KSHARRAY`

      [capability](#user-content-appendix-a-list-of-shell-cap-ids), an

      extra variant is available: `--xargs=`*arrayname* which uses the named

      array instead of the PPs. It otherwise works identically.

* `--try`. If this option is specified, then if one of the primaries used in

  the *find-expression* is not supported by either the `find` utility used by

  the loop or by modernish itself, `LOOP find` will not throw a

  [fatal error](#user-content-reliable-emergency-halt)

  but will instead quietly abort the loop without iterating it, set the loop's

  exit status to 128, and leave the invalid primary in the `REPLY` variable.

  (Expression errors other than 'unknown primary' remain fatal errors.)

* One of `--split` or `--split=`*characters*. This operator, which is only

  accepted in the [safe mode](#user-content-use-safe), safely applies the

  shell's field splitting mechanism to the *path* name(s) given *(but **not**

  to any patterns in the *find-expression*, which are passed on to the `find`

  utility as given)*. The simple `--split` operator applies the shell's default

  field splitting by space, tab, and newline. Alternatively, you can supply

  one or more *characters* to split by. If any pathname resulting from the

  split starts with `-` or `+` or is identical to `!` or `(`, `./` is prefixed.

* One of `--glob` or `--fglob`. These operators are only accepted in the

  [safe mode](#user-content-use-safe). They safely apply shell pathname

  expansion (globbing) to the *path* name(s) given *(but **not** to any

  patterns in the *find-expression*, which are passed on to the `find` utility

  as given)*. All *path* names are taken as patterns, whether or not they

  contain any wildcard characters. If any pathname resulting from the

  expansion start with `-` or `+` or is identical to `!` or `(`, `./` is

  prefixed. Non-matching patterns are treated as follows:

    * `--glob`: Any pattern not matching an existing path will output a

      warning to standard error and set the loop's exit status to 103 upon

      normal completion, even if other existing paths are processed

      successfully. If none match, the loop will not iterate.

    * `--fglob`: Any pattern not matching an existing path is a fatal error.

* `--base=`*basedirectory*. This operator prefixes the given *basedirectory*

  to each of the *path* names (and thus to each path found by `find`), after

  first applying field splitting and/or pathname expansion if specified.

  If `--glob` or `--fglob` are given, then wildcard characters are only

  expanded in the *path* names and not in the prefixed *basedirectory*.

  If the *basedirectory* can't be entered, then either the loop breaks with

  status 98, or if `--fglob` was given, the program terminates.



##### Available *find-expression* operands #####

`LOOP find` can use all expression operands supported by your local `find`

utility; see its manual page. However, portable scripts should use only

[operands specified by POSIX](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/find.html#tag_20_47_05)

along with the modernish additions described below.



The modernish `-iterate` expression primary evaluates as true and causes the

loop to iterate, executing your *commands* for each matching file. It may be

used any number of times in the *find-expression* to start a corresponding

series of loop iterations. If it is not given, the loop acts as if the entire

*find-expression* is enclosed in parentheses with `-iterate` appended. If the

entire *find-expression* is omitted, it defaults to `-iterate`.



The modernish `-ask` primary asks confirmation of the user. The text of the

prompt may be specified in one optional argument (which cannot start with `-`

or be equal to `!` or `(`). Any occurrences of the characters `{}` within the

prompt text are replaced with the current pathname. If not specified, the

default prompt is: `"{}"?` If the answer is affirmative (`y` or `Y` in the

POSIX locale), `-ask` yields true, otherwise false. This can be used to make

any part of the expression conditional upon user input, and (unlike commands in

the shell loop body) is capable of influencing directory traversal mid-run.



The standard `-exec` and `-ok` primaries are integrated into the main shell

environment. When used with `LOOP find`, they can call a shell builtin command

or your own shell function directly in the main shell (no subshell). Its exit

status is used in the `find` expression as a true/false value capable of

influencing directory traversal (for example, when combined with `-prune`),

just as if it were an external command -exec'ed with the standard utility.



Some familiar, easy-to-use but non-standard `find` operands from GNU and/or

BSD may be used with `LOOP find` on all systems. Before invoking the `find`

utility, modernish translates them internally to portable equivalents.

The following expression operands are made portable:



* The `-or`, `-and` and `-not` operators: same as `-o`, `-a`, `!`.

* The `-true` and `-false` primaries, which always yield true/false.

* The BSD-style `-depth` *n* primary, e.g. `-depth +4` yields true on depth

  greater than 4 (minimum 5), `-depth -4` yields true on depth less than 4

  (maximum 3), and `-depth 4` yields true on a depth of exactly 4.

* The GNU-style `-mindepth` and `-maxdepth` global options.

  Unlike BSD `-depth`, these GNU-isms are pseudo-primaries that

  always yield true and affect the entire `LOOP find` operation.



Expression primaries that write output (`-print` and friends) may be used for

debugging or logging the loop. Their output is redirected to standard error.



##### Picking a `find` utility #####

Upon initialisation, the `var/loop/find` module searches for a POSIX-compliant

`find` utility under various names in `$DEFPATH` and then in `$PATH`. To see a

trace of the full command lines of utility invocations when the loop runs, set

the `_loop_DEBUG` variable to any value.



For debugging or system-specific usage, it is possible to use a certain `find`

utility in preference to any others on the system. To do this, add an argument

to a `use var/loop/find` command before the first use of the loop. For example:



* `use var/loop/find bsdfind` (prefer utility by this name)

* `use var/loop/find /opt/local/bin` (look for a utility here first)

* `use var/loop/find /opt/local/bin/gfind`  (try this one first)



##### Compatibility mode for obsolete `find` utilities #####

Some systems come with obsolete or broken `find` utilities that don't fully

support `-exec ... {} +` aggregating functionality as specified by POSIX.

Normally, this is a fatal error, but passing the `-b`/`-B` option to the

`use` command, e.g. `use var/loop/find -b`, enables a compatibility mode

that tolerates this defect. If no compliant `find` is found, then an obsolete

or broken `find` is used as a last resort, a warning is printed to standard

error, and the variable `_loop_find_broken` is set. The `-B` option is

equivalent to `-b` but does not print a warning. Loop performance may suffer as

modernish adapts to using older `exec ... {} \;` which is very inefficient.



Scripts using this compatibility mode should handle their logic using shell

code in the loop body as much as possible (after `DO`) and use only simple

`find` expressions (before `DO`), as obsolete utilities are often buggy and

breakage is likely if complex expressions or advanced features are used.



##### `find` loop usage examples #####

Simple example script: without the safe mode, the `*.txt` pattern

must be quoted to prevent it from being expanded by the shell.



```sh

. modernish

use var/loop

LOOP find TextFile in ~/Documents -name '*.txt'

DO

	putln "Found my text file: $TextFile"

DONE

```



Example script with [safe mode](#user-content-use-safe): the `--glob` option

expands the patterns of the `in` clause, but *not* the expression – so it

is not necessary to quote any pattern.



```sh

. modernish

use safe

use var/loop

LOOP find --glob lsProg in /*bin /*/*bin -type f -name ls*

DO

	putln "This command may list something: $lsProg"

DONE

```



Example use of the modernish `-ask` primary: ask the user if they want to

descend into each directory found. The shell loop body could skip unwanted

results, but cannot physically influence directory traversal, so skipping large

directories would take long. A `find` expression can prevent directory

traversal using the standard `-prune` primary, which can be combined with

`-ask`, so that unwanted directories never iterate the loop in the first place.



```sh

. modernish

use safe

use var/loop

LOOP find file in ~/Documents \

	-type d \( -ask 'Descend into "{}" directory?' -or -prune \) \

	-or -iterate

DO

	put "File found: "

	ls -li $file

DONE

```



#### Creating your own loop ####

The modernish loop construct is extensible. To define a new loop type, you

only need to define a shell function called `_loopgen_`*type* where *type*

is the loop type. This function, called the *loop iteration generator*, is

expected to output lines of text to file descriptor 8, containing properly

[shell-quoted](#user-content-use-varshellquote)

iteration commands for the shell to run, one line per iteration.



The internal commands expanded from `LOOP`, `DO` and `DONE` (which are

defined as aliases) launch that loop iteration generator function in the

background with [safe](#user-content-use-safe) mode enabled, while causing

the main shell to read lines from that background process through a pipe,

`eval`ing each line as a command before iterating the loop. As long as that

iteration command finishes with an exit status of zero, the loop keeps

iterating. If it has a nonzero exit status or if there are no more commands

to read, iteration terminates and execution continues beyond the loop.



Instead of the normal [internal namespace](#user-content-internal-namespace)

which is considered off-limits for modernish scripts, `var/loop` and its

submodules use a `_loop_*` internal namespace for variables, which is also

for use by user-implemented loop iteration generator functions.



The above is just the general principle. For the details, study the comments

and the code in `lib/modernish/mdl/var/loop.mm` and the loop generators in

`lib/modernish/mdl/var/loop/*.mm`.



### `use var/local` ###



This module defines a new `LOCAL`...`BEGIN`...`END` shell code block

construct with local variables, local positional parameters and local shell

options. The local positional parameters can be filled using safe field

splitting and pathname expansion operators similar to those in the `LOOP`

construct described [above](#user-content-use-varloop).



Usage: `LOCAL` [ *localitem* | *operator* ... ] [ `--` [ *word* ... ] ] `;`

`BEGIN` *commands* `;` `END`



The *commands* are executed once, with the specified *localitem*s applied.

Each *localitem* can be:

* A variable name with or without a `=` immediately followed by a value.

  This renders that variable local to the block, initially either unsetting

  it or assigning the value, which may be empty.

* A shell option letter immediately preceded by a `-` or `+` sign. This

  locally turns that shell option on or off, respectively. This follows the

  counterintuitive syntax of `set`. Long-form shell options like `-o`

  *optionname* and `+o` *optionname* are also supported. It depends on the

  shell what options are supported. Specifying a nonexistent option is a

  fatal error. Use [`thisshellhas`](#user-content-shell-capability-detection) to check

  for a non-POSIX option's existence on the current shell before using it.



Modernish implements `LOCAL` blocks as one-time shell functions that use

[the stack](#user-content-the-stack)

to save and restore variables and settings. So the `return` command exits the

block, causing the global variables and settings to be restored and resuming

execution at the point immediately following `END`. Like any shell function, a

`LOCAL` block exits with the exit status of the last command executed within

it, or with the status passed on by or given as an argument to `return`.



The positional parameters (`$@`, `$1`, etc.) are always local to the block, but

a copy is inherited from outside the block by default. Any changes to the

positional parameters made within the block will be discarded upon exiting it.



However, if a double-dash `--` argument is given in the `LOCAL` command line,

the positional parameters outside the block are ignored and the set of *word*s

after `--` (which may be empty) becomes the positional parameters instead.



These *word*s can be modified prior to entering the `LOCAL` block using the

following *operator*s. The safe glob and split operators are only accepted in

the [safe mode](#user-content-use-safe). The operators are:



* One of `--split` or `--split=`*characters*. This operator safely applies

  the shell's field splitting mechanism to the *word*s given. The simple

  `--split` operator applies the shell's default field splitting by space,

  tab, and newline. If you supply one or more of your own *characters* to

  split by, each of these characters will be taken as a field separator if

  it is whitespace, or field terminator if it is non-whitespace. (Note that

  shells with [`QRK_IFSFINAL`](#user-content-quirks) treat both whitespace and

  non-whitespace characters as separators.)

* One of `--glob` or `--fglob`. These operators safely apply shell pathname

  expansion (globbing) to the *word*s given. Each *word* is taken as a pattern,

  whether or not it contains any wildcard characters. For any resulting

  pathname that starts with `-` or `+` or is identical to `!` or `(`, `./`

  is prefixed to keep various commands from misparsing it as an option

  or operand. Non-matching patterns are treated as follows:

    * `--glob`: Any non-matching patterns are quietly removed.

    * `--fglob`: All patterns must match. Any nonexistent path terminates the

      program. Use this if your program would not work after a non-match.

* `--base=`*string*. This operator prefixes the given *string* to each of the

  *word*s, after first applying field splitting and/or pathname expansion

  if specified.

  If `--glob` or `--fglob` are given, then the *string* is used as a base

  directory path for pathname expansion, without expanding any wildcard

  characters in that base directory path itself.

  If such base directory can't be entered, then if `--glob` was given, all

  *word*s are removed, or if `--fglob` was given, the program terminates.

* One of `--slice` or `--slice=`*number*. This operator divides the

  *word*s in slices of up to *number* characters. The default slice size

  is 1 character, allowing for easy character-by-character processing.

  (Note that shells with [`WRN_MULTIBYTE`](#user-content-warning-ids) will

  not slice multi-byte characters correctly.)



If multiple operators are given, their mechanisms are applied in the

following order: split, glob, base, slice.



#### Important `var/local` usage notes ####

* Due to the limitations of aliases and shell reserved words, `LOCAL` has

  to use its own `BEGIN`...`END` block instead of the shell's `do`...`done`.

  Using the latter results in a misleading shell syntax error.

* `LOCAL` blocks do **not** mix well with use of the shell capability

  [`LOCALVARS`](#user-content-user-content-capabilities)

  (shell-native functionality for local variables), especially not on shells

  with `QRK_LOCALUNS` or `QRK_LOCALUNS2`. Using both with the same variables

  causes unpredictable behaviour, depending on the shell.

* **Warning!** Never use `break` or `continue` within a `LOCAL` block to

  resume or break from enclosing loops outside the block! Shells with

  [`QRK_BCDANGER`](#user-content-quirks) allow this, preventing `END` from

  restoring the global settings and corrupting the stack; shells without

  this quirk will throw an error if you try this. A proper way to do what

  you want is to exit the block with a nonzero status using something like

  `return 1`, then append something like `|| break` or `|| continue` to

  `END`. Note that this caveat only applies when crossing `BEGIN`...`END`

  boundaries. Using `continue` and `break` to continue or break loops

  entirely *within* the block is fine.



### `use var/arith` ###



These shortcut functions are alternatives for using

[`let`](#user-content-the-arithmetic-command-let).



#### Arithmetic operator shortcuts ####

`inc`, `dec`, `mult`, `div`, `mod`: simple integer arithmetic shortcuts. The first

argument is a variable name. The optional second argument is an

arithmetic expression, but a sane default value is assumed (1 for inc

and dec, 2 for mult and div, 256 for mod). For instance, `inc X` is

equivalent to `X=$((X+1))` and `mult X Y-2` is equivalent to `X=$((X*(Y-2)))`.



`ndiv` is like `div` but with correct rounding down for negative numbers.

Standard shell integer division simply chops off any digits after the

decimal point, which has the effect of rounding down for positive numbers

and rounding up for negative numbers. `ndiv` consistently rounds down.



#### Arithmetic comparison shortcuts ####

These have the same name as their `test`/`[` option equivalents. Unlike

with `test`, the arguments are shell integer arith expressions, which can be

anything from simple numbers to complex expressions. As with `$(( ))`,

variable names are expanded to their values even without the `$`.



    Function:         Returns successfully if:

    eq    the two expressions evaluate to the same number

    ne    the two expressions evaluate to different numbers

    lt    the 1st expr evaluates to a smaller number than the 2nd

    le    the 1st expr eval's to smaller than or equal to the 2nd

    gt    the 1st expr evaluates to a greater number than the 2nd

    ge    the 1st expr eval's to greater than or equal to the 2nd



### `use var/assign` ###



This module is provided to solve a common POSIX shell language annoyance: in a

normal shell variable assignment, only literal variable names are accepted, so

it is impossible to use a variable whose name is stored in another variable.

The only way around this is to use `eval` which is too difficult to use safely.

Instead, you can now use the `assign` command.



Usage: `assign` [ [ `+r` ] *variable*`=`*value* ... ] | [ `-r` *variable*`=`*variable2* ... ] ...



`assign` safely processes assignment-arguments in the same form as customarily

given to the `readonly` and `export` commands, but it only assigns *value*s to

*variable*s without setting any attributes. Each argument is grammatically an

ordinary shell word, so any part or all of it may result from an expansion. The

absence of a `=` character in any argument is a fatal error. The text preceding

the first `=` is taken as the variable name in which to store the *value*; an

invalid *variable* name is a fatal error. No whitespace is accepted before the

`=` and any whitespace after the `=` is part of the *value* to be assigned.



The `-r` (reference) option causes the part to the right of the `=` to be

taken as a second variable name *variable2*, and its value is assigned to

*variable* instead. `+r` turns this option back off.



**Examples:** Each of the lines below assigns the value 'hello world' to the

variable `greeting`.



```sh

var=greeting; assign $var='hello world'

var=greeting; assign "$var=hello world"

tag='greeting=hello world'; assign "$tag"

var=greeting; gvar=myinput; myinput='hello world'; assign -r $var=$gvar

```



### `use var/readf` ###



`readf` reads arbitrary data from standard input into a variable until end

of file, converting it into a format suitable for passing to the

[`printf`](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/printf.html)

utility. For example, `readf var bar` will copy foo to

bar. Thus, `readf` allows storing both text and binary files into shell

variables in a textual format suitable for manipulation with standard shell

facilities.



All non-printable, non-ASCII characters are converted to `printf` octal or

one-letter escape codes, except newlines. Not encoding newline characters

allows for better processing by line-based utilities such as `grep`, `sed`,

`awk`, etc. However, if the file ends in a newline, that final newline is

encoded to `\n` to protect it from being stripped by command substitutions.



Usage: `readf` [ `-h` ] *varname*



The `-h` option disables conversion of high-byte characters (accented letters,

non-Latin scripts). Do not use for binary files; this is only guaranteed to

work for text files in an encoding compatible with the current locale.



Caveats:

* Best for small-ish files. The encoded file is stored in memory (a shell

  variable). For a binary file, encoding in `printf` format typically

  about doubles the size, though it could be up to four times as large.

* If the shell executing your program does not have `printf` as a builtin

  command, the external `printf` command will fail if the encoded file

  size exceeds the maximum length of arguments to external commands

  (`getconf ARG_MAX` will obtain this limit for your system). Shell builtin

  commands do not have this limit. Check for a `printf` builtin using

  [`thisshellhas`](#user-content-shell-capability-detection) if you need to be sure,

  and always [`harden`](#user-content-use-syscmdharden)

  `printf`!



### `use var/shellquote` ###



This module provides an efficient, fast, safe and portable shell-quoting

algorithm for quoting arbitrary data in such a way that the quoted values are

safe to pass to the shell for parsing as string literals. This is essential

for any context where the shell must grammatically parse untrusted input,

such as when supplying arbitrary values to `trap` or `eval`.



The shell-quoting algorithm is optimised to minimise exponential growth when

quoting repeatedly. By default, it also ensures that quoted strings are

always one single printable line, making them safe for terminal output and

processing by line-oriented utilities.



#### `shellquote` ####

Usage: `shellquote` [ `-f`|`+f`|`-P`|`+P` ] *varname*[`=`*value*] ...



The values of the variables specified by name are shell-quoted and stored

back into those variables.

Repeating a variable name will add another level of shell-quoting.

If a `=` plus a *value* (which may be empty) is appended to the *varname*,

that value is shell-quoted and assigned to the variable.



Options modify the algorithm for variable names following them, as follows:



* By default, newlines and any control characters are converted into

  [`${CC*}`](#user-content-control-character-whitespace-and-shell-safe-character-constants)

  expansions and quoted with double quotes, ensuring that the quoted string

  consists of a single line of printable text. The `-P` option forces pure

  POSIX quoted strings that may span multiple lines; `+P` turns this back off.



* By default, a value is only quoted if it contains characters not present

  in `$SHELLSAFECHARS`. The `-f` option forces unconditional quoting,

  disabling optimisations that may leave shell-safe characters unquoted;

  `+f` turns this back off.



`shellquote` will [die](#user-content-reliable-emergency-halt) if you

attempt to quote an unset variable (because there is no value to quote).



#### `shellquoteparams` ####

The `shellquoteparams` command shell-quotes the current positional

parameters in place using the default quoting method of `shellquote`. No

options are supported and any attempt to add arguments results in a syntax

error.



### `use var/stack` ###



Modules that extend [the stack](#user-content-the-stack).



#### `use var/stack/extra` ####

This module contains stack query and maintenance functions.



If you only need one or two of these functions, they can also be loaded as

individual submodules of `var/stack/extra`.



For the four functions below, *item* can be:

* a valid portable variable name

* a short-form shell option: dash plus letter

* a long-form shell option: `-o` followed by an option name (two arguments)

* `--trap=`*SIGNAME* to refer to the trap stack for the indicated signal

  (as set by `pushtrap` from [`var/stack/trap`](#user-content-use-varstacktrap))



`stackempty` [ `--key=`*value* ] [ `--force` ] *item*: Tests if the stack

for an item is empty. Returns status 0 if it is, 1 if it is not. The key

feature works as in [`pop`](#user-content-the-stack): by default, a key

mismatch is considered equivalent to an empty stack. If `--force` is given,

this function ignores keys altogether.



`clearstack` [ `--key=`*value* ] [ `--force` ] *item* [ *item* ... ]:

Clears one or more stacks, discarding all items on it.

If (part of) the stack is keyed or a `--key` is given, only clears until a

key mismatch is encountered. The `--force` option overrides this and always

clears the entire stack (be careful, e.g. don't use within

[`LOCAL` ... `BEGIN` ... `END`](#user-content-use-varlocal)).

Returns status 0 on success, 1 if that stack was already empty, 2 if

there was nothing to clear due to a key mismatch.



`stacksize` [ `--silent` | `--quiet` ] *item*: Leaves the size of a stack in

the `REPLY` variable and, if option `--silent` or `--quiet` is not given,

writes it to standard output.

The size of the complete stack is returned, even if some values are keyed.



`printstack` [ `--quote` ] *item*: Outputs a stack's content.

Option `--quote` shell-quotes each stack value before printing it, allowing

for parsing multi-line or otherwise complicated values.

Column 1 to 7 of the output contain the number of the item (down to 0).

If the item is set, column 8 and 9 contain a colon and a space, and

if the value is non-empty or quoted, column 10 and up contain the value.

Sets of values that were pushed with a key are started with a special

line containing `--- key: `*value*. A subsequent set pushed with no key is

started with a line containing `--- (key off)`.

Returns status 0 on success, 1 if that stack is empty.



#### `use var/stack/trap` ####

This module provides `pushtrap` and `poptrap`. These functions integrate

with the [main modernish stack](#user-content-the-stack)

to make traps stack-based, so that each

program component or library module can set its own trap commands without

interfering with others.



This module also provides a new

[`DIE` pseudosignal](#user-content-the-new-die-pseudosignal)

that allows pushing traps to execute when

[`die`](#user-content-reliable-emergency-halt)

is called.



Note an important difference between the trap stack and stacks for variables

and shell options: pushing traps does not save them for restoring later, but

adds them alongside other traps on the same signal. All pushed traps are

active at the same time and are executed from last-pushed to first-pushed

when the respective signal is triggered. Traps cannot be pushed and popped

using `push` and `pop` but use dedicated commands as follows.



Usage:



* `pushtrap` [ `--key=`*value* ] [ `--nosubshell` ] [ `--` ] *command* *sigspec* [ *sigspec* ... ]

* `poptrap` [ `--key=`*value* ] [ `-R` ] [ `--` ] *sigspec* [ *sigspec* ... ]



`pushtrap` works like regular `trap`, with the following exceptions:



* Adds traps for a signal without overwriting previous ones.

* An invalid signal is a fatal error. When using non-standard signals, check if

  [`thisshellhas --sig=`*yoursignal*](#user-content-shell-capability-detection)

  before using it.

* Unlike regular traps, a stack-based trap does not cause a signal to be

  ignored. Setting one will cause it to be executed upon the shell receiving

  that signal, but after the stack traps complete execution, modernish re-sends

  the signal to the main shell, causing it to behave as if no trap were set

  (unless a regular POSIX trap is also active).

  Thus, `pushtrap` does not accept an empty *command* as it would be pointless.

* Each stack trap is executed in a new

  [subshell](#user-content-insubshell)

  to keep it from interfering

  with others. This means a stack trap cannot change variables except within

  its own environment, and `exit` will only exit the trap and not the program.

  The `--nosubshell` option overrides this behaviour, causing that particular

  trap to be executed in the main shell environment instead. This is not

  recommended if not absolutely needed, as you have to be extra careful to

  avoid exiting the shell or otherwise interfere with other stack traps.

  This option cannot be used with

  [`DIE` traps](#user-content-the-new-die-pseudosignal).

* Each stack trap is executed with `$?` initially set to the exit status

  that was active at the time the signal was triggered.

* Stack traps do not have access to the positional parameters.

* `pushtrap` stores current `$IFS` (field splitting) and `$-` (shell options)

  along with the pushed trap. Within the subshell executing each stack trap,

  modernish restores `IFS` and the shell options `f` (`noglob`), `u`

  (`nounset`) and `C` (`noclobber`) to the values in effect during the

  corresponding `pushtrap`. This is to avoid unexpected effects in case a trap

  is triggered while temporary settings are in effect.

  The `--nosubshell` option disables this functionality for the trap pushed.

* The `--key` option applies the keying functionality inherited from

  [plain `push`](#user-content-the-stack) to the trap stack.

  It works the same way, so the description is not repeated here.



`poptrap` takes just signal names or numbers as arguments. It takes the

last-pushed trap for each signal off the stack. By default, it discards

the trap commands. If the `-R` option is given, it stores commands to

restore those traps into the `REPLY` variable, in a format suitable for

re-entry into the shell. Again, the `--key` option works as in

[plain `pop`](#user-content-the-stack).



With the sole exception of

[`DIE` traps](#user-content-the-new-die-pseudosignal),

all stack-based traps, like native shell traps, are reset upon entering a

[subshell](#user-content-insubshell).

However, commands for printing traps will print the traps for

the parent shell, until another `trap`, `pushtrap` or `poptrap` command is

invoked, at which point all memory of the parent shell's traps is erased.



##### Trap stack compatibility considerations #####

Modernish tries hard to avoid incompatibilities with existing trap practice.

To that end, it intercepts the regular POSIX `trap` command using an alias,

reimplementing and interfacing it with the shell's builtin trap facility

so that plain old regular traps play nicely with the trap stack. You should

not notice any changes in the POSIX `trap` command's behaviour, except for

the following:



* The regular `trap` command does not overwrite stack traps (but still

  overwrites existing regular traps).

* Unlike zsh's native trap command, signal names are case insensitive.

* Unlike dash's native trap command, signal names may have the `SIG` prefix;

  that prefix is quietly accepted and discarded.

* Setting an empty trap action to ignore a signal only works fully (passing

  the ignoring on to child processes) if there are no stack traps associated

  with the signal; otherwise, an empty trap action merely suppresses the

  signal's default action for the current process – e.g., after executing

  the stack traps, it keeps the shell from exiting.

* The `trap` command with no arguments, which prints the traps that are set

  in a format suitable for re-entry into the shell, now also prints the

  stack traps as `pushtrap` commands. (`bash` users might notice the `SIG`

  prefix is not included in the signal names written.)

* The bash/yash-style `-p` option, including its yash-style `--print`

  equivalent, is now supported on all shells. If further arguments are

  given after that option, they are taken as signal specifications and

  only the commands to recreate the traps for those signals are printed.

* Saving the traps to a variable using command substitution (as in:

  `var=$(trap)`) now works on every

  [shell supported by modernish](#user-content-appendix-d-supported-shells),

  including (d)ash, mksh and zsh which don't support this natively.

* To reset (unset) a trap, the modernish `trap` command accepts both

  [valid POSIX syntax](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_28_03)

  and legacy bash/(d)ash/zsh syntax, like `trap INT` to unset a `SIGINT`

  trap (which only works if the `trap` command is given exactly one

  argument). Note that this is for compatibility with existing scripts only.

* Bypassing the `trap` alias to set a trap using the shell builtin command

  will cause an inconsistent state. This may be repaired with a simple `trap`

  command; as modernish prints the traps, it will quietly detect ones it

  doesn't yet know about and make them work nicely with the trap stack.



POSIX traps for each signal are always executed after that signal's stack-based

traps; this means they should not rely on modernish modules that use the trap

stack to clean up after themselves on exit, as those cleanups would already

have been done.



##### The new `DIE` pseudosignal #####

The `var/stack/trap` module adds new `DIE` pseudosignal whose traps are

executed upon invoking [`die`](#user-content-reliable-emergency-halt).

This allows for emergency cleanup operations upon fatal program failure,

as `EXIT` traps cannot be executed after `die` is invoked.



* On non-interactive shells (as well as

  [subshells](#user-content-insubshell)

  of interactive shells), `DIE` is its own pseudosignal with its own trap

  stack and POSIX trap. In order to kill the malfunctioning program as quickly

  as possible (hopefully before it has a chance to delete all your data), `die`

  doesn't wait for those traps to complete before killing the program. Instead,

  it executes each `DIE` trap simultaneously as a background job, then gathers

  the process IDs of the main shell and all its subprocesses, sending `SIGKILL`

  to all of them except any `DIE` trap processes. Unlike other traps, `DIE`

  traps are inherited by and survive in subshell processes, and `pushtrap` may

  add to them within the subshell. Whatever shell process invokes `die` will

  fork all `DIE` trap actions before being `SIGKILL`ed itself. (Note that any

  `DIE` traps pushed or set within a subshell will still be forgotten upon

  exiting the subshell.)

* On an interactive shell (*not* including its

  [subshells](#user-content-insubshell)),

  `DIE` is simply an alias for `INT`, and `INT` traps

  (both POSIX and stack) are cleared out after executing them once. This is

  because `die` uses `SIGINT` for command interruption on interactive shells, and

  it would not make sense to execute emergency cleanup commands repeatedly. As

  a side effect of this special handling, `INT` traps on interactive shells do

  not have access to the positional parameters and cannot return from functions.



### `use var/string` ###



String comparison and manipulation functions.



#### `use var/string/touplow` ####

`toupper` and `tolower`: convert case in variables.



Usage:

* `toupper` *varname* [ *varname* ... ]

* `tolower` *varname* [ *varname* ... ]



Arguments are taken as variable names (note: they should be given without

the `$`) and case is converted in the contents of the specified variables,

without reading input or writing output.



`toupper` and `tolower` try hard to use the fastest available method on the

particular shell your program is running on. They use built-in shell

functionality where available and working correctly, otherwise they fall back

on running an external utility.



Which external utility is chosen depends on whether the current locale uses

the Unicode UTF-8 character set or not. For non-UTF-8 locales, modernish

assumes the POSIX/C locale and `tr` is always used. For UTF-8 locales,

modernish tries hard to find a way to correctly convert case even for

non-Latin alphabets. A few shells have this functionality built in with

`typeset`. The rest need an external utility. Modernish initialisation

tries `tr`, `awk`, GNU `awk` and GNU `sed` before giving up and setting

the variable `MSH_2UP2LOW_NOUTF8`. If `isset MSH_2UP2LOW_NOUTF8`, it

means modernish is in a UTF-8 locale but has not found a way to convert

case for non-ASCII characters, so `toupper` and `tolower` will convert

only ASCII characters and leave any other characters in the string alone.



#### `use var/string/trim` ####

`trim`: strip whitespace from the beginning and end of a variable's value.

Whitespace is defined by the `[:space:]` character class. In the POSIX

locale, this is tab, newline, vertical tab, form feed, carriage return, and

space, but in other locales it may be different.

(On shells with [`BUG_NOCHCLASS`](#user-content-bugs),

[`$WHITESPACE`](#user-content-control-character-whitespace-and-shell-safe-character-constants)

is used to define whitespace instead.) Optionally, a string of literal

characters can be provided in the second argument. Any characters appearing

in that string will then be trimmed instead of whitespace.

Usage: `trim` *varname* [ *characters* ]



#### `use var/string/replacein` ####

`replacein`: Replace leading, `-t`railing or `-a`ll occurrences of a string by

another string in a variable.    

Usage: `replacein` [ `-t` | `-a` ] *varname* *oldstring* *newstring*



#### `use var/string/append` ####

`append` and `prepend`: Append or prepend zero or more strings to a

variable, separated by a string of zero or more characters, avoiding the

hairy problem of dangling separators.

Usage: `append`|`prepend` [ `--sep=`*separator* ] [ `-Q` ] *varname* [ *string* ... ]    

If the separator is not specified, it defaults to a space character.

If the `-Q` option is given, each *string* is

[shell-quoted](#user-content-use-varshellquote)

before appending or prepending.



### `use var/unexport` ###



The `unexport` function clears the "export" bit of a variable, conserving

its value, and/or assigns values to variables without setting the export

bit. This works even if `set -a` (allexport) is active, allowing an "export

all variables, except these" way of working.



Usage is like `export`, with the caveat that variable assignment arguments

containing non-shell-safe characters or expansions must be quoted as

appropriate, unlike in some specific shell implementations of `export`.

(To get rid of that headache, [`use safe`](#user-content-use-safe).)



Unlike `export`, `unexport` does not work for read-only variables.



### `use var/genoptparser` ###



As the `getopts` builtin is not portable when used in functions, this module

provides a command that generates modernish code to parse options for your

shell function in a standards-compliant manner. The generated parser

supports short-form (one-character) options which can be stacked/combined.



Usage:

`generateoptionparser` [ `-o` ] [ `-f` *func* ] [ `-v` *varprefix* ]

[ `-n` *options* ] [ -a *options* ] [ *varname* ]



* `-o`: Write parser to standard output.

* `-f`: Function name to prefix to error messages. Default: none.

* `-v`: Variable name prefix for options. Default: `opt_`.

* `-n`: String of options that do not take arguments.

* `-a`: String of options that require arguments.

* *varname*: Store parser in specified variable. Default: `REPLY`.



At least one of `-n` and `-a` is required. All other arguments are optional.

Option characters must be valid components of portable variable names, so

they must be ASCII upper- or lowercase letters, digits, or the underscore.



`generateoptionparser` stores the generated parser code in a variable: either

`REPLY` or the *varname* specified as the first non-option argument. This makes

it possible to generate and use the parser on the fly with a command like

`eval "$REPLY"` immediately following the `generateoptionparser` invocation.



For better efficiency and readability, it will often be preferable to insert

the option parser code directly into your shell function instead. The `-o`

option writes the parser code to standard output, so it can be redirected to

a file, inserted into your editor, etc.



Parsed options are shifted out of the positional parameters while setting or

unsetting corresponding variables, until a non-option argument, a `--`

end-of-options delimiter argument, or the end of arguments is encountered.

Unlike with `getopts`, no additional `shift` command is required.



Each specified option gets a corresponding variable with a name consisting

of the *varprefix* (default: `opt_`) plus the option character. If an option

is not passed to your function, the parser unsets its variable; otherwise it

sets it to either the empty value or its option-argument if it requires one.

Thus, your function can check if any option `x` was given using

[`isset`](#user-content-isset),

for example, `if isset opt_x; then`...



### `use sys/base` ###



Some very common and essential utilities are not specified by POSIX, differ

widely among systems, and are not always available. For instance, the

`which` and `readlink` commands have incompatible options on various GNU and

BSD variants and may be absent on other Unix-like systems. The `sys/base`

module provides a complete re-implementation of such non-standard but basic

utilities, written as modernish shell functions. Using the modernish version

of these utilities can help a script to be fully portable. These versions

also have various enhancements over the GNU and BSD originals, some of which

are made possible by their integration into the modernish shell environment.



#### `use sys/base/mktemp` ####

A cross-platform shell implementation of `mktemp` that aims to be just as

safe as native `mktemp`(1) implementations, while avoiding the problem of

having various mutually incompatible versions and adding several unique

features of its own.



Creates one or more unique temporary files, directories or named pipes,

atomically (i.e. avoiding race conditions) and with safe permissions.

The path name(s) are stored in `REPLY` and optionally written to stdout.



Usage: `mktemp` [ `-dFsQCt` ] [ *template* ... ]



* `-d`: Create a directory instead of a regular file.

* `-F`: Create a FIFO (named pipe) instead of a regular file.

* `-s`: Silent. Store output in `$REPLY`, don't write any output or message.

* `-Q`: Shell-quote each unit of output. Separate by spaces, not newlines.

* `-C`: Automated cleanup.

        [Pushes a trap](#user-content-the-trap-stack)

        to remove the files

        on exit. On an interactive shell, that's all this option does. On a

        non-interactive shell, the following applies: Clean up on receiving

        `SIGPIPE` and `SIGTERM` as well. On receiving `SIGINT`, clean up if the

        option was given at least twice, otherwise notify the user of files

        left. On the invocation of

        [`die`](#user-content-reliable-emergency-halt),

        clean up if the option was given at least three times, otherwise notify

        the user of files left.

* `-t`: Prefix one temporary files directory to all the *template*s:

        `$XDG_RUNTIME_DIR` or `$TMPDIR` if set, or `/tmp`. The *template*s

        may not contain any slashes. If the template has neither any trailing

        `X`es nor a trailing dot, a dot is added before the random suffix.



The template defaults to “`/tmp/temp.`”. An suffix of random shell-safe ASCII

characters is added to the template to create the file. For compatibility with

other `mktemp` implementations, any optional trailing `X` characters in the

template are removed. The length of the suffix will be equal to the amount of

`X`es removed, or 10, whichever is more. The longer the random suffix, the

higher the security of using `mktemp` in a shared directory such as `tmp`.



Since `/tmp` is a world-writable directory shared by other users, for best

security it is recommended to create a private subdirectory using `mktemp -d`

and work within that.



Option `-C` cannot be used without option `-s` when in a

[subshell](#user-content-insubshell).

Modernish will detect this and treat it as a

fatal error. The reason is that a typical command substitution like

`tmpfile=$(mktemp -C)`

is incompatible with auto-cleanup, as the cleanup EXIT trap would be

triggered not upon exiting the program but upon exiting the command

substitution subshell that just ran `mktemp`, thereby immediately undoing

the creation of the file. Instead, do something like:

`mktemp -sC; tmpfile=$REPLY`



This module depends on the trap stack to do auto-cleanup (the `-C` option),

so it will automatically `use var/stack/trap` on initialisation.



#### `use sys/base/readlink` ####

`readlink` reads the target of a symbolic link, robustly handling strange

filenames such as those containing newline characters. It stores the result

in the `REPLY` variable and optionally writes it on standard output.



Usage: `readlink` [ `-nsefmQ` ] *path* [ *path* ... ]



* `-n`: If writing output, don't add a trailing newline.

  This does not remove the separating newlines if multiple *path*s are given.

* `-s`: *S*ilent operation: don't write output, only store it in `REPLY`.

* `-e`, `-f`, `-m`: Canonicalise. Convert each *path* found into a canonical

  and absolute path that can be used starting from any working directory.

  Relative *path*s are resolved starting from the present working directory.

  Double slashes are removed. Any special pathname components

  `.` and `..` are resolved. All symlinks encountered are

  followed, but a *path* does not need to contain any symlinks. UNC network

  paths (as on Cygwin) are supported. These options differ as follows:

    * `-e`: All pathname components must exist to produce a result.

    * `-f`: All but the last pathname component must exist to produce a result.

    * `-m`: No pathname component needs to exist; this always produces a result.

      Nonexistent pathname components are simulated as regular directories.

* `-Q`: Shell-*q*uote each unit of output. Separate by spaces instead

  of newlines. This generates a list of arguments in shell syntax,

  guaranteed to be suitable for safe parsing by the shell, even if the

  resulting pathnames should contain strange characters such as spaces or

  newlines and other control characters.



The exit status of `readlink` is 0 on success and 1 if the *path* either is

not a symlink, or could not be canonicalised according to the option given.



#### `use sys/base/rev` ####

`rev` copies the specified files to the standard output, reversing the order

of characters in every line. If no files are specified, the standard input

is read.



Usage: like `rev` on Linux and BSD, which is like `cat` except that `-` is

a filename and does not denote standard input. No options are supported.



#### `use sys/base/seq` ####

A cross-platform implementation of `seq` that is more powerful and versatile

than native GNU and BSD `seq`(1) implementations. The core is written in

`bc`, the POSIX arbitrary-precision calculator language. That means this

`seq` inherits the capacity to handle numbers with a precision and size only

limited by computer memory, as well as the ability to handle input numbers

in any base from 1 to 16 and produce output in any base 1 and up.



Usage: `seq` [ `-w` ] [ `-L` ] [ `-f` *format* ] [ `-s` *string* ] [ `-S` *scale* ]

[ `-B` *base* ] [ `-b` *base* ] [ *first* [ *incr* ] ] *last*



`seq` prints a sequence of arbitrary-precision floating point numbers, one

per line, from *first* (default 1), to as near *last* as possible, in increments of

*incr* (default 1). If *first* is larger than *last*, the default *incr* is -1.

An *incr* of zero is treated as a fatal error.



* `-w`: Equalise width by padding with leading zeros. The longest of the

	*first*, *incr* or *last* arguments is taken as the length that each

	output number should be padded to.

* `-L`: Use the current locale's radix point in the output instead of the

        full stop (`.`).

* `-f`: `printf`-style floating-point format. The format string is passed on

        (with an added `\n`) to `awk`'s builtin `printf` function. Because

        of that, the `-f` option can only be used if the output base is 10.

        Note that `awk`'s floating point precision is limited, so very

        large or long numbers will be rounded.

* `-s`: Instead of writing one number per line, write all numbers on one

        line separated by *string* and terminated by a newline character.

* `-S`: Explicitly set the scale (number of digits after the

        [radix point](https://en.wikipedia.org/wiki/Radix_point)).

	Defaults to the largest number of digits after the radix point

	among the *first*, *incr* or *last* arguments.

* `-B`: Set input and output base from 1 to 16. Defaults to 10.

* `-b`: Set arbitrary output base from 1. Defaults to input base.

        See the `bc`(1) manual for more information on the output format

        for bases greater than 16.



The `-S`, `-B` and `-b` options take shell integer numbers as operands. This

means a leading `0X` or `0x` denotes a hexadecimal number and a leading `0`

denotes an octal number.



For portability reasons, modernish `seq` uses a full stop (`.`) for the

[radix point](https://en.wikipedia.org/wiki/Radix_point), regardless of the

system locale. This applies both to command arguments and to output.

The `-L` option causes `seq` to use the current locale's radix point

character for output only.



##### Differences with GNU and BSD `seq` #####

The `-S`, `-B` and `-b` options are modernish innovations.

The `-w`, `-f` and `-s` options are inspired by GNU and BSD `seq`.

The following differences apply:



* Like GNU and unlike BSD, the separator specified by the `-s` option

  is not appended to the final number and there is no `-t` option to

  add a terminator character.

* Like GNU and unlike BSD, the `-s` option-argument is taken as literal

  characters and is not parsed for backslash escape codes like `\n`.

* Unlike GNU and like BSD, the output radix point defaults to a full stop,

  regardless of the current locale.

* Unlike GNU and like BSD, if *incr* is not specified,

  it defaults to -1 if *first* > *last*, 1 otherwise.

  For example, `seq 5 1` counts backwards from 5 to 1, and

  specifying `seq 5 -1 1` as with GNU is not needed.

* Unlike GNU and like BSD, an *incr* of zero is not accepted.

  To output the same number or string infinite times, use

  [`yes`](#user-content-use-sysbaseyes) instead.

* Unlike both GNU and BSD, the `-f` option accepts any format specifiers

  accepted by `awk`'s `printf()` function.



The `sys/base/seq` module depends on, and automatically loads,

[`var/string/touplow`](#user-content-use-varstringtouplow).



#### `use sys/base/shuf` ####

Shuffle lines of text.

A portable reimplementation of a commonly used GNU utility.



Usage:



* `shuf` [ `-n` *max* ] [ `-r` *rfile* ] *file*

* `shuf` [ `-n` *max* ] [ `-r` *rfile* ] `-i` *low*`-`*high*

* `shuf` [ `-n` *max* ] [ `-r` *rfile* ] `-e` *argument* ...



By default, `shuf` reads lines of text from standard input, or from *file*

(the *file* `-` signifies standard input).

It writes the input lines to standard output in random order.



* `-i`: Use sequence of non-negative integers *low* through *high* as input.

* `-e`: Instead of reading input, use the *argument*s as lines of input.

* `-n`: Output a maximum of *max* lines.

* `-r`: Use *rfile* as the source of random bytes. Defaults to `/dev/urandom`.



Differences with GNU `shuf`:



* Long option names are not supported.

* The `-o`/`--output-file` option is not supported; use output redirection.

  Safely shuffling files in-place is not supported; use a temporary file.

* `--random-source=`*file* is changed to `-r` *file*.

* The `-z`/`--zero-terminated` option is not supported.



#### `use sys/base/tac` ####

`tac` (the reverse of `cat`) is a cross-platform reimplementation of the GNU

`tac` utility, with some extra features.



Usage: `tac` [ `-rbBP` ] [ `-S` *separator* ] *file* [ *file* ... ]



`tac` outputs the *file*s in reverse order of lines/records.

If *file* is `-` or is not given, `tac` reads from standard input.



* `-s`: Specify the record (line) separator. Default: linefeed.

* `-r`: Interpret the record separator as an

  [extended regular expression](http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04).

  This allows using separators that may vary. Each separator is preserved

  in the output as it is in the input.

* `-b`: Assume the separator comes before each record in the input, and also

  output the separator before each record. Cannot be combined with `-B`.

* `-B`: Assume the separator comes after each record in the input, but output

  the separator before each record. Cannot be combined with `-b`.

* `-P`: Paragraph mode: output text last paragraph first. Input paragraphs

  are separated from each other by at least two linefeeds. Cannot be combined

  with any other option.



Differences between GNU `tac` and modernish `tac`:

* The `-B` and `-P` options were added.

* The `-r` option interprets the record separator as an extended regular

  expression. This is an incompatibility with GNU `tac` unless expressions

  are used that are valid as both basic and extended regular expressions.

* In UTF-8 locales, multi-byte characters are recognised and reversed

  correctly.



#### `use sys/base/which` ####

The modernish `which` utility finds external programs and reports their

absolute paths, offering several unique options for reporting, formatting

and robust processing. The default operation is similar to GNU `which`.



Usage: `which` [ `-apqsnQ1f` ] [ `-P` *number* ] *program* [ *program* ... ]



By default, `which` finds the first available path to each given *program*.

If *program* is itself a path name (contains a slash), only that path's base

directory is searched; if it is a simple command name, the current `$PATH`

is searched. Any relative paths found are converted to absolute paths.

Symbolic links are not followed. The first path found for each *program* is

written to standard output (one per line), and a warning is written to

standard error for every *program* not found. The exit status is 0 (success)

if all *program*s were found, 1 otherwise.



`which` also leaves its output in the `REPLY` variable. This may be useful

if you run `which` in the main shell environment. The `REPLY` value will

*not* survive a command substitution

[subshell](#user-content-insubshell)

as in `ls_path=$(which ls)`.



The following options modify the default behaviour described above:



* `-a`: List *a*ll *program*s that can be found in the directories searched,

  instead of just the first one. This is useful for finding duplicate

  commands that the shell would not normally find when searching its `$PATH`.

* `-p`: Search in [`$DEFPATH`](#user-content-modernish-system-constants)

  (the default standard utility `PATH` provided by the operating system)

  instead of in the user's `$PATH`, which is vulnerable to manipulation.

* `-q`: Be *q*uiet: suppress all warnings.

* `-s`: *S*ilent operation: don't write output, only store it in the `REPLY`

  variable. Suppress warnings except, if you run `which -s` in a subshell,

  a warning that the `REPLY` variable will not survive the subshell.

* `-n`: When writing to standard output, do *n*ot write a final *n*ewline.

* `-Q`: Shell-*q*uote each unit of output. Separate by spaces instead

  of newlines. This generates a one-line list of arguments in shell syntax,

  guaranteed to be suitable for safe parsing by the shell, even if the

  resulting pathnames should contain strange characters such as spaces or

  newlines and other control characters.

* `-1` (one): Output the results for at most *one* of the arguments in

  descending order of preference: once a search succeeds, ignore

  the rest. Suppress warnings except a subshell warning for `-s`.

  This is useful for finding a command that can exist under

  several names, for example:

  `which -f -1 gnutar gtar tar`    

  This option modifies which's exit status behaviour: `which -1`

  returns successfully if at least one command was found.

* `-f`: Throw a [*f*atal error](#user-content-reliable-emergency-halt)

  in cases where `which` would otherwise return status 1 (non-success).

* `-P`: Strip the indicated number of *p*athname elements from the output,

  starting from the right.

  `-P1`: strip `/program`;

  `-P2`: strip `/*/program`,

  etc. This is useful for determining the installation root directory for

  an installed package.

* `--help`: Show brief usage information.



#### `use sys/base/yes` ####

`yes` very quickly outputs infinite lines of text, each consisting of its

space-separated arguments, until terminated by a signal or by a failure to

write output. If no argument is given, the default line is `y`. No options

are supported.



This infinite-output command is useful for piping into commands that need an

indefinite input data stream, or to automate a command requiring interactive

confirmation.



Modernish `yes` is like GNU `yes` in that it outputs all its arguments,

whereas BSD `yes` only outputs the first. It can output multiple gigabytes

per second on modern systems.



### `use sys/cmd` ###



Modules in this category contain functions for enhancing the invocation of

commands.



#### `use sys/cmd/extern` ####

`extern` is like `command` but always runs an external command, without

having to know or determine its location. This provides an easy way to

bypass a builtin, alias or function. It does the same `$PATH` search

the shell normally does when running an external command. For instance, to

guarantee running external `printf` just do: `extern printf ...`



Usage: `extern` [ `-p` ] [ `-v` ] [ `-u` *varname* ... ]

[ *varname*`=`*value* ... ] *command* [ *argument* ... ]



* `-p`: The *command*, as well as any commands it further invokes, are searched in

  [`$DEFPATH`](#user-content-modernish-system-constants)

  (the default standard utility `PATH` provided by the operating system)

  instead of in the user's `$PATH`, which is vulnerable to manipulation.

  * `extern -p` is much more reliable than the shell's builtin

    [`command -p`](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html#tag_20_22_04)

    because: (a) many existing shell installations use a wrong search path for

    `command -p`; (b) `command -p` does not export the default `PATH`, so

    something like `command -p sudo cp foo /bin/bar` searches only `sudo` in

    the secure default path and not `cp`.

* `-v`: don't execute *command* but show the full path name of the command that

  would have been executed. Any extra *argument*s are taken as more command

  paths to show, one per line. `extern` exits with status 0 if all the commands

  were found, 1 otherwise. This option can be combined with `-p`.

* `-u`: Temporary export override. Unset the given variable in the

  environment of the command executed, even if it is currently exported. Can

  be specified multiple times.

* *varname*`=`*value* assignment-arguments: These variables/values are

  temporarily exported to the environment during the execution of the command.

  * This is provided because assignments *preceding* `extern` cause unwanted,

    shell-dependent side effects, as `extern` is a shell function. Be

    sure to provide assignment-arguments *following* `extern` instead.

  * Assignment-arguments after a `--` end-of-options delimiter are not parsed;

    this allows *command*s containing a `=` sign to be executed.



#### `use sys/cmd/harden` ####

The `harden` function allows implementing emergency halt on error

for any external commands and shell builtin utilities. It is

modernish's replacement for `set -e` a.k.a. `set -o errexit` (which is

[fundamentally](https://lists.gnu.org/archive/html/bug-bash/2012-12/msg00093.html)

[flawed](http://mywiki.wooledge.org/BashFAQ/105),

not supported and will break the library).

It depends on, and auto-loads, the `sys/cmd/extern` module.



`harden` sets a shell function with the same name as the command hardened,

so it can be used transparently. This function hardens the given command by

checking its exit status against values indicating error or system failure.

Exactly what exit statuses signify an error or failure depends on the

command in question; this should be looked up in the

[POSIX specification](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/contents.html)

(under "Utilities") or in the command's `man` page or other documentation.



If the command fails, the function installed by `harden` calls `die`, so it

will reliably halt program execution, even if the failure occurred within a

[subshell](#user-content-insubshell).



Usage:



`harden` [ `-f` *funcname* ] [ `-[cSpXtPE]` ] [ `-e` *testexpr* ]

[ *var*`=`*value* ... ] [ `-u` *var* ... ] *command_name_or_path*

[ *command_argument* ... ]



The `-f` option hardens the command as the shell function *funcname* instead

of defaulting to *command_name_or_path* as the function name. (If the latter

is a path, that's always an invalid function name, so the use of `-f` is

mandatory.) If *command_name_or_path* is itself a shell function, that

function is bypassed and the builtin or external command by that name is

hardened instead. If no such command is found, `harden` dies with the message

that hardening shell functions is not supported. (Instead, you should invoke

`die` directly from your shell function upon detecting a fatal error.)



The `-c` option causes *command_name_or_path* to be hardened and run

immediately instead of setting a shell function for later use. This option

is meant for commands that run once; it is not efficient for repeated use.

It cannot be used together with the `-f` option.



The `-S` option allows specifying several possible names/paths for a

command. It causes the *command_name_or_path* to be split by comma and

interpreted as multiple names or paths to search. The first name or path

found is used. Requires `-f`.



The `-e` option, which defaults to `>0`, indicates the exit statuses

corresponding to a fatal error. It depends on the command what these are;

consult the POSIX spec and the manual pages.

The status test expression *testexpr*, argument

to the `-e` option, is like a shell arithmetic

expression, with the binary operators `==` `!=` `<=` `>=` `<` `>` turned

into unary operators referring to the exit status of the command in

question. Assignment operators are disallowed. Everything else is the same,

including `&&` (logical and) and `||` (logical or) and parentheses.

Note that the expression needs to be quoted as the characters used in it

clash with shell grammar tokens.



The `-X` option causes `harden` to always search for and harden an external

command, even if a built-in command by that name exists.



The `-E` option causes the hardening function to consider it a fatal error

if the hardened command writes anything to the standard error stream. This

option allows hardening commands (such as

[`bc`](pubs.opengroup.org/onlinepubs/9699919799/utilities/bc.html#tag_20_09_14))

where you can't rely on the exit status to detect an error. The text written

to standard error is passed on as part of the error message printed by

`die`. Note that:

* Intercepting standard error necessitates that the command be executed from a

  [subshell](#user-content-insubshell).

  This means any builtins or shell functions hardened with `-E` cannot

  influence the calling shell (e.g. `harden -E cd` renders `cd` ineffective).

* `-E` does not disable exit status checks; by default, any exit status greater

  than zero is still considered a fatal error as well. If your command does not

  even reliably return a 0 status upon success, then you may want to add `-e

  '>125'`, limiting the exit status check to reserved values indicating errors

  launching the command and signals caught.



The `-p` option causes `harden` to search for commands using the

system default path (as obtained with `getconf PATH`) as opposed to the

current `$PATH`. This ensures that you're using a known-good external

command that came with your operating system. By default, the system-default

PATH search only applies to the command itself, and not to any commands that

the command may search for in turn. But if the `-p` option is specified at

least twice, the command is run in a subshell with `PATH` exported as the

default path, which is equivalent to adding a `PATH=$DEFPATH` assignment

argument (see [below](#user-content-important-note-on-variable-assignments)).



Examples:



```sh

harden make                           # simple check for status > 0

harden -f tar '/usr/local/bin/gnutar' # id.; be sure to use this 'tar' version

harden -e '> 1' grep                  # for grep, status > 1 means error

harden -e '==1 || >2' gzip            # 1 and >2 are errors, but 2 isn't (see manual)

```



##### Important note on variable assignments #####

As far as the shell is concerned, hardened commands are shell functions and

not external or builtin commands. This essentially changes one behaviour of

the shell: variable assignments preceding the command will not be local to

the command as usual, but *will persist* after the command completes.

(POSIX technically makes that behaviour

[optional](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_01)

but all current shells behave the same in POSIX mode.)



For example, this means that something like



```sh

harden -e '>1' grep

# [...]

LC_ALL=C grep regex some_ascii_file.txt

```



should never be done, because the meant-to-be-temporary `LC_ALL` locale

assignment will persist and is likely to cause problems further on.



To solve this problem, `harden` supports adding these assignments as

part of the hardening command, so instead of the above you do:



```sh

harden -e '>1' LC_ALL=C grep

# [...]

grep regex some_ascii_file.txt

```



With the `-u` option, `harden` also supports unsetting variables for the

duration of a command, e.g.:



```sh

harden -e '>1' -u LC_ALL grep

```



The `-u` option may be specified multiple times.

It causes the hardened command to be invoked from a

[subshell](#user-content-insubshell)

with the specified variables unset.



##### Hardening while allowing for broken pipes #####

If you're piping a command's output into another command that may close

the pipe before the first command is finished, you can use the `-P` option

to allow for this:



```sh

harden -e '==1 || >2' -P gzip		# also tolerate gzip being killed by SIGPIPE

gzip -dc file.txt.gz | head -n 10	# show first 10 lines of decompressed file

```



`head` will close the pipe of `gzip` input after ten lines; the operating

system kernel then kills `gzip` with the PIPE signal before it's finished,

causing a particular exit status that is greater than 128. This exit status

would normally make `harden` kill your entire program, which in the example

above is clearly not the desired behaviour. If the exit status caused by a

broken pipe were known, you could specifically allow for that exit status in

the status expression. The trouble is that this exit status varies depending

on the shell and the operating system. The `-p` option was made to solve

this problem: it automatically detects and whitelists the correct exit

status corresponding to `SIGPIPE` termination on the current system.



Tolerating `SIGPIPE` is an option and not the default, because in many

contexts it may be entirely unexpected and a symptom of a severe error if a

command is killed by a broken pipe. It is up to the programmer to decide

which commands should expect `SIGPIPE` and which shouldn't.



*Tip:* It could happen that the same command should expect `SIGPIPE` in one

context but not another. You can create two hardened versions of the same

command, one that tolerates `SIGPIPE` and one that doesn't. For example:



```sh

harden -f hardGrep -e '>1' grep		# hardGrep does not tolerate being aborted

harden -f pipeGrep -e '>1' -P grep	# pipeGrep for use in pipes that may break

```



*Note:* If `SIGPIPE` was set to ignore by the process invoking the current

shell, the `-p` option has no effect, because no process or subprocess of

the current shell can ever be killed by `SIGPIPE`. However, this may cause

various other problems and you may want to refuse to let your program run

under that condition.

[`thisshellhas WRN_NOSIGPIPE`](#user-content-warning-ids) can help

you easily detect that condition so your program can make a decision. See

the [`WRN_NOSIGPIPE` description](#user-content-warning-ids) for more information.



##### Tracing the execution of hardened commands #####

The `-t` option will trace command output. Each execution of a command

hardened with `-t` causes the command line to be output to standard

error, in the following format:



    [functionname]> commandline



where `functionname` is the name of the shell function used to harden the

command and `commandline` is the actual command executed. The

`commandline` is properly shell-quoted in a format suitable for re-entry

into the shell; however, command lines longer than 512 bytes will be

truncated and the unquoted string ` (TRUNCATED)` will be appended to the

trace. If standard error is on a terminal that supports ANSI colours,

the tracing output will be colourised.



The `-t` option was added to `harden` because the commands that you harden

are often the same ones you would be particularly interested in tracing. The

advantage of using `harden -t` over the shell's builtin tracing facility

(`set -x` or `set -o xtrace`) is that the output is a *lot* less noisy,

especially when using a shell library such as modernish.



*Note:* Internally, `-t` uses the shell file descriptor 9, redirecting it to

standard error (using `exec 9>&2`). This allows tracing to continue to work

normally even for commands that redirect standard error to a file (which is

another enhancement over `set -x` on most shells). However, this does mean

`harden -t` conflicts with any other use of the file descriptor 9 in your

shell program.



If file descriptor 9 is already open before `harden` is called, `harden`

does not attempt to override this. This means tracing may be redirected

elsewhere by doing something like `exec 9>trace.out` before calling

`harden`. (Note that redirecting FD 9 on the `harden` command itself will

*not* work as it won't survive the run of the command.)



##### Simple tracing of commands #####

Sometimes you just want to trace the execution of some specific commands as

in `harden -t` (see above) without actually hardening them against command

errors; you might prefer to do your own error handling. `trace` makes this

easy. It is modernish's replacement or complement for `set -x` a.k.a. `set

-o xtrace`.

Unlike `harden -t`, it can also trace shell functions.



**Usage 1:** `trace` [ `-f` *funcname* ] [ `-[cSpXE]` ]

[ *var*`=`*value* ... ] [ `-u` *var* ... ] *command_name_or_path*

[ *command_argument* ... ]



For non-function commands, `trace` acts as a shortcut for

`harden -t -P -e '>125 && !=255'` *command_name_or_path*.

Any further options and arguments are passed on to `harden` as given. The

result is that the indicated command is automatically traced upon execution.

A bonus is that you still get minimal hardening against fatal system errors.

Errors in the traced command itself are ignored, but your program is

immediately halted with an informative error message if the traced command:



- cannot be found (exit status 127);

- was found but cannot be executed (exit status 126);

- was killed by a signal other than `SIGPIPE` (exit status > 128, except

  the shell-specific exit status for `SIGPIPE`, and except 255 which is

  used by some utilities, such as `ssh` and `rsync`, to return an error).



*Note:* The caveat for command-local variable assignments for `harden` also

applies to `trace`. See

[Important note on variable assignments](#user-content-important-note-on-variable-assignments)

above.



**Usage 2:** [ `#!` ] `trace -f` *funcname*



If no further arguments are given, `trace -f` will trace the shell

function *funcname* without applying further hardening (except against

nonexistence). `trace -f` can be used to trace the execution of modernish

library functions as well as your own script's functions. The trace output

for shell functions shows an extra `()` following the function name.



Internally, this involves setting an alias under the function's name, so

the limitations of the shell's alias expansion mechanism apply: only

function calls that the shell had not yet parsed before calling `trace -f`

will be traced. So you should use `trace -f` at the beginning of your

script, before defining your own functions. To facilitate this, `trace -f`

does not check that the function *funcname* exists while setting up

tracing, but only when attempting to execute the traced function.



In [portable-form](#user-content-portable-form)

modernish scripts, `trace -f` should be used as a hashbang command to be

compatible with alias expansion on all shells. Only the `trace -f` form

may be used that way. For example:



```sh

#! /usr/bin/env modernish

#! use safe -k

#! use sys/cmd/harden

#! trace -f push

#! trace -f pop

...your program begins here...

```



#### `use sys/cmd/mapr` ####

`mapr` (map records) is an alternative to `xargs` that shares features with the

`mapfile` command in bash 4.x. It is fully integrated into your script's main

shell environment, so it can call your shell functions as well as builtin and

external utilities.

It depends on, and auto-loads, the `sys/cmd/procsubst` module.



Usage: `mapr` [ `-d` *delimiter* | `-P` ] [ `-s` *count* ] [ `-n` *number* ]

[ `-m` *length* ] [ `-c` *quantum* ] *callback*



`mapr` reads delimited records from the standard input, invoking the specified

*callback* command once or repeatedly as needed, with batches of input records

as arguments. The *callback* may consist of multiple arguments. By default, an

input record is one line of text.



Options:



* `-d` *delimiter*: Use the single character *delimiter* to delimit input records,

  instead of the newline character. A `NUL` (0) character and multi-byte

  characters are not supported.

* `-P`: Paragraph mode. Input records are delimited by sequences consisting of

  a newline plus one or more blank lines, and leading or trailing blank lines

  will not result in empty records at the beginning or end of the input. Cannot

  be used together with `-d`.

* `-s` *count*: Skip and discard the first *count* records read.

* `-n` *number*: Stop processing after passing a total of *number* records to

  invocation(s) of *callback*. If `-n` is not supplied or *number* is 0, all

  records are passed, except those skipped using `-s`.

* `-m` *length*: Set the maximum argument length in bytes of each *callback*

  command call, including the *callback* command argument(s) and the current

  batch of up to *quantum* input records. The length of each argument is

  increased by 1 to account for the terminating null byte. The default

  maximum length depends on constraints set by the operating system for

  invoking external commands. If *length* is 0, this limit is disabled.

* `-c` *quantum*: Pass at most *quantum* arguments at a time to each call to

  *callback*. If `-c` is not supplied or if *quantum* is 0, the number of

  arguments per invocation is not limited except by `-m`; whichever limit is

  reached first applies.



Arguments:



* *callback*: Call the *callback* command with the collected arguments each

  time *quantum* lines are read. The callback command may be a shell function or

  any other kind of command, and is executed from the same shell environment

  that invoked `mapr`. If the callback command exits or returns with status

  255 or is interrupted by the `SIGPIPE` signal, `mapr` will not process any

  further batches but immediately exit with the status of the callback

  command. If it exits with another exit status 126 or greater, a

  [fatal error](#user-content-reliable-emergency-halt)

  is thrown. Otherwise, `mapr` exits with the status of the last-executed

  callback command.

* *argument*:  If there are extra arguments supplied on the mapr command line,

  they will be added before the collected arguments on each invocation on the

  callback command.



##### Differences from `mapfile` #####

`mapr` was inspired by the bash 4.x builtin command `mapfile` a.k.a.

`readarray`, and uses similar options, but there are important differences.



* `mapr` passes all the records as arguments to the callback command.

* `mapr` does not support assigning records directly to an array. Instead,

  all handling is done through the callback command (which could be a shell

  function that assigns its arguments to an array.)

* The callback command is specified directly instead of with a `-C` option,

  and it may consist of several arguments (as with `xargs`).

* The record separator itself is never included in the arguments passed

  to the callback command (so there is no `-t` option to remove it).

* `mapr` supports paragraph mode.

* If the callback command exits with status 255, processing is aborted.



##### Differences from `xargs` #####

`mapr` shares important characteristics with

[`xargs`](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/xargs.html)

while avoiding its myriad pitfalls.



* Instead of being an external utility, `mapr` is fully integrated into the

  shell. The callback command can be a shell function or builtin, which can

  directly modify the shell environment.

* `mapr` is line-oriented by default, so it is safe to use for input

  arguments that contain spaces or tabs.

* `mapr` does not parse or modify the input arguments in any way, e.g. it

  does not process and remove quotes from them like `xargs` does.

* `mapr` supports paragraph mode.



#### `use sys/cmd/procsubst` ####

This module provides a portable

[process substitution](https://en.wikipedia.org/wiki/Process_substitution)

construct, the advantage being that this is not limited to bash, ksh or zsh

but works on all POSIX shells capable of running modernish. It is not

possible for modernish to introduce the original ksh syntax into other

shells. Instead, this module provides a `%` command for use within a

`$(`command substitution`)`.



The `%` command takes one simple command as its arguments, executes it in

the background, and writes a file name from which to read its output. So

if `%` is used within a command substitution as intended, that file name

is passed on to the invoking command as an argument.



The `%` command supports one option, `-o`. If that option is given, then it is

expected that, instead of reading input, the invoking command writes output to

the file name passed on to it, so that the command invoked by `% -o` can read

that data from its standard input.



Example syntax comparison:



ksh/bash/zshmodernish



diff -u <(ls) <(ls -a)



diff -u $(% ls) $(% ls -a)



IFS=' ' read -r user vsz args < <(ps -o 'user= vsz= args=' -p $$)



IFS=' ' read -r user vsz args < $(% ps -o 'user= vsz= args=' -p $$)



{ some commands; } > >(tee stdout.log) 2> >(tee stderr.log)


(both `tee` commands write terminal output to standard output)



{ some commands; } > $(% -o tee stdout.log) 2> $(% -o tee stderr.log)


(both `tee` commands write terminal output to standard error)



Unlike the bash/ksh/zsh version, modernish process substitution only works

with simple commands. This includes shell function calls, but not aliases or

anything involving shell grammar or reserved words (such as redirections,

pipelines, loops, etc.). To use such complex commands, enclose them in a shell

function and call that function from the process substitution.



Also note that anything that a command invoked by the `% -o` writes to its

standard output is redirected to standard error. The main shell environment's

standard output is not available because the command substitution subsumes it.



#### `use sys/cmd/source` ####

The `source` command sources a dot script like the `.` command, but

additionally supports passing arguments to sourced scripts like you would

pass them to a function. It mostly mimics the behaviour of the `source`

command built in to bash and zsh.



If a filename without a directory path is given, then, unlike the `.`

command, `source` looks for the dot script in the current directory by

default, as well as searching `$PATH`.



It is a fatal error to attempt to source a directory, a file with no read

permission, or a nonexistent file.



### `use sys/dir` ###



Functions for working with directories.



#### `use sys/dir/countfiles` ####

`countfiles`: Count the files in a directory using nothing but shell

functionality, so without external commands. (It's amazing how many pitfalls

this has, so a library function is needed to do it robustly.)



Usage: `countfiles` [ `-s` ] *directory* [ *globpattern* ... ]



Count the number of files in a directory, storing the number in `REPLY`

and (unless `-s` is given) printing it to standard output.

If any *globpattern*s are given, only count the files matching them.



#### `use sys/dir/mkcd` ####

The `mkcd` function makes one or more directories, then, upon success,

change into the last-mentioned one. `mkcd` inherits `mkdir`'s usage, so

options depend on your system's `mkdir`; only the

[POSIX options](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/mkdir.html#tag_20_79_04)

are guaranteed.

When `mkcd` is run from a script, it uses `cd -P` to change the working

directory, resolving any symlinks in the present working directory path.



### `use sys/term` ###



Utilities for working with the terminal.



#### `use sys/term/putr` ####

This module provides commands to efficiently output a string repeatedly.



Usage:



* `putr` [ *number* | `-` ] *string*

* `putrln` [ *number* | `-` ] *string*



Output the *string* *number* times. When using `putrln`, add a newline at

the end.



If a `-` is given instead of a *number*, then the total length of the output

is the line length of the terminal divided by the length of the *string*,

rounded down.



Note that, unlike with `put` and `putln`, only a single *string*

argument is accepted.



Example: `putrln - '='` prints a full terminal line of equals signs.



#### `use sys/term/readkey` ####

`readkey`: read a single character from the keyboard without echoing back to

the terminal. Buffering is done so that multiple waiting characters are read

one at a time.



Usage: `readkey` [ `-E` *ERE* ] [ `-t` *timeout* ] [ `-r` ] [ *varname* ]



`-E`: Only accept characters that match the extended regular expression

*ERE* (the type of RE used by `grep -E`/`egrep`). `readkey` will silently

ignore input not matching the ERE and wait for input matching it.



`-t`: Specify a *timeout* in seconds (one significant digit after the

decimal point). After the timeout expires, no character is read and

`readkey` returns status 1.



`-r`: Raw mode. Disables INTR (Ctrl+C), QUIT, and SUSP (Ctrl+Z) processing

as well as translation of carriage return (13) to linefeed (10).



The character read is stored into the variable referenced by *varname*,

which defaults to `REPLY` if not specified.



This module depends on the trap stack to save and restore the terminal state

if the program is stopped while reading a key, so it will automatically

`use var/stack/trap` on initialisation.



---



## Appendix A: List of shell cap IDs ##



This appendix lists all the shell

[capabilities](#user-content-capabilities),

[quirks](#user-content-quirks), and

[bugs](#user-content-bugs)

that modernish can detect in the current shell, so that modernish scripts

can easily query the results of these tests and decide what to do. Certain

[problematic system conditions](#user-content-warning-ids)

are also detected this way and listed here.



The all-caps IDs below are all usable with the

[`thisshellhas`](#user-content-shell-capability-detection)

function. This makes it easy for a cross-platform modernish script to

be aware of relevant conditions and decide what to do.



Each detection test has its own little test script in the

`lib/modernish/cap` directory. These tests are executed on demand, the

first time the capability or bug in question is queried using

`thisshellhas`. See `README.md` in that directory for further information.

The test scripts also document themselves in the comments.



### Capabilities ###



Modernish currently identifies and supports the following non-standard

shell capabilities:



* `ADDASSIGN`: Add a string to a variable using additive assignment,

  e.g. *VAR*`+=`*string*

* `ANONFUNC`: zsh anonymous functions (basically the native zsh equivalent

  of modernish's var/local module)

* `ARITHCMD`: standalone arithmetic evaluation using a command like

  `((`*expression*`))`.

* `ARITHFOR`: ksh93/C-style arithmetic `for` loops of the form

  `for ((`*exp1*`; `*exp2*`; `*exp3*`)) do `*commands*`; done`.

* `ARITHPP`: support for the `++` and `--` unary operators in shell arithmetic.

* `CESCQUOT`: Quoting with C-style escapes, like `$'\n'` for newline.

* `DBLBRACKET`: The ksh88-style `[[` double-bracket command `]]`,

  implemented as a reserved word, integrated into the main shell grammar,

  and with a different grammar applying within the double brackets.

  (ksh93, mksh, bash, zsh, yash >= 2.48)

* `DBLBRACKETERE`: `DBLBRACKET` plus the `=~` binary operator to match a

  string against an extended regular expression.

* `DBLBRACKETV`: `DBLBRACKET` plus the `-v` unary operator to test if a

  variable is set. Named variables only. (Testing positional parameters

  (like `[[ -v 1 ]]`) does not work on bash or ksh93; check `$#` instead.)

* `DOTARG`: Dot scripts support arguments.

* `HERESTR`: Here-strings, an abbreviated kind of here-document.

* `KSH88FUNC`: define ksh88-style shell functions with the `function` keyword,

  supporting dynamically scoped local variables with the `typeset` builtin.

  (mksh, bash, zsh, yash, et al)

* `KSH93FUNC`: the same, but with static scoping for local variables. (ksh93 only)

  See Q28 at the [ksh93 FAQ](http://kornshell.com/doc/faq.html) for an explanation

  of the difference.

* `KSHARRAY`: ksh93-style arrays. Supported on bash, zsh (under `emulate sh`),

  mksh, and ksh93.

* `LEPIPEMAIN`: execute last element of a pipe in the main shell, so that

  things like *somecommand* `| read` *somevariable* work. (zsh, AT&T ksh,

  bash 4.2+)

* `LINENO`: the `$LINENO` variable contains the current shell script line

  number.

* `LOCALVARS`: the `local` command creates dynamically scoped local variables

  within functions defined using standard POSIX syntax.

* `NONFORKSUBSH`: as a performance optimisation,

  [subshells](#user-content-insubshell) are

  implemented without forking a new process, so they share a PID with the main

  shell. (AT&T ksh93; it has [many bugs](https://github.com/att/ast/issues/480)

  related to this, but there's a nice workaround: `ulimit -t unlimited` forces

  a subshell to fork, making those bugs disappear! See also `BUG_FNSUBSH`.)

* `PRINTFV`: The shell's `printf` builtin has the `-v` option to print to a variable,

  which avoids forking a command substitution subshell.

* `PROCREDIR`: the shell natively supports `<(`process redirection`)`,

  a special kind of redirection that connects standard input (or standard

  output) to a background process running your command(s).

  This exists on yash.

  Note this is **not** combined with a redirection like `< <(`*command*`)`.

  Contrast with bash/ksh/zsh's `PROCSUBST` where this `<(`syntax`)`

  substitutes a file name.

* `PROCSUBST`: the shell natively supports `<(`process substitution`)`,

  a special kind of command substitution that substitutes a file name,

  connecting it to a background process running your command(s).

  This exists on ksh93 and zsh.

  (Bash has it too, but its POSIX mode turns it off, so modernish can't use it.)

  Note this is usually combined with a redirection, like `< <(`*command*`)`.

  Contrast this with yash's `PROCREDIR` where the same `<(`syntax`)`

  is itself a redirection.

* `PSREPLACE`: Search and replace strings in variables using special parameter

  substitutions with a syntax vaguely resembling sed.

* `RANDOM`: the `$RANDOM` pseudorandom generator.

  Modernish seeds it if detected. The variable is then set it to read-only

  whether the generator is detected or not, in order to block it from losing

  its special properties by being unset or overwritten, and to stop it being

  used if there is no generator. This is because some of modernish depends

  on `RANDOM` either working properly or being unset.    

  (The use case for non-readonly `RANDOM` is setting a known seed to get

  reproducible pseudorandom sequences. To get that in a modernish script,

  use `awk`'s `srand(yourseed)` and `int(rand()*32768)`.)

* `ROFUNC`: Set functions to read-only with `readonly -f`. (bash, yash)

* `TESTERE`: The regular `test`/`[` builtin command supports the `=~` binary

  operator to match a string against an extended regular expression.

* `TESTO`: The `test`/`[` builtin supports the `-o` unary operator to check if 

  a shell option is set.

* `TRAPPRSUBSH`: The ability to obtain a list of the current shell's native

  traps from a command substitution subshell, for example: `var=$(trap)`,

  as long as no new traps have been set within that command substitution.

  Note that the `var/stack/trap` module transparently reimplements this

  feature on shells without this native capability.

* `TRAPZERR`: This feature ID is detected if the `ERR` trap is an alias for

  the `ZERR` trap. According to the zsh manual, this is the case for zsh on

  most systems, i.e. those that don't have a `SIGERR` signal. (The

  [trap stack](#user-content-the-trap-stack)

  uses this feature test.)

* `VARPREFIX`: Expansions of type `${!`*prefix*`@}` and `${!`*prefix*`*}` yield

  all names of set variables beginning with `prefix` in the same way and with

  the same quoting effects as `$@` and `$*`, respectively.

  This includes the name *prefix* itself, unless the shell has `BUG_VARPREFIX`.

  (bash; AT&T ksh93)



### Quirks ###



Modernish currently identifies and supports the following shell quirks:



* `QRK_32BIT`: mksh: the shell only has 32-bit arithmetic. Since every modern

  system these days supports 64-bit long integers even on 32-bit kernels, we

  can now count this as a quirk.

* `QRK_ANDORBG`: On zsh, the `&` operator takes the last simple command

  as the background job and not an entire AND-OR list (if any).

  In other words, `a && b || c &` is interpreted as

  `a && b || { c & }` and not `{ a && b || c; } &`.

* `QRK_ARITHEMPT`: In yash, with POSIX mode turned off, a set but empty

  variable yields an empty string when used in an arithmetic expression,

  instead of 0. For example, `foo=''; echo $((foo))` outputs an empty line.

* `QRK_ARITHWHSP`: In [yash](https://osdn.jp/ticket/browse.php?group_id=3863&tid=36002)

  and FreeBSD /bin/sh, trailing whitespace from variables is not trimmed in arithmetic

  expansion, causing the shell to exit with an 'invalid number' error. POSIX is silent

  on the issue. The modernish `isint` function (to determine if a string is a valid

  integer number in shell syntax) is `QRK_ARITHWHSP` compatible, tolerating only

  leading whitespace.

* `QRK_BCDANGER`: `break` and `continue` can affect non-enclosing loops,

  even across shell function barriers (zsh, Busybox ash; older versions

  of bash, dash and yash). (This is especially dangerous when using

  [var/local](#user-content-use-varlocal)

  which internally uses a temporary shell function to try to protect against

  breaking out of the block without restoring global parameters and settings.)

* `QRK_EMPTPPFLD`: Unquoted `$@` and `$*` do not discard empty fields.

  [POSIX says](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05_02)

  for both unquoted `$@` and unquoted `$*` that empty positional parameters

  *may* be discarded from the expansion. AFAIK, just one shell (yash)

  doesn't.

* `QRK_EMPTPPWRD`: [POSIX says](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05_02)

  that empty `"$@"` generates zero fields but empty `''` or `""` or

  `"$emptyvariable"` generates one empty field. But it leaves unspecified

  whether something like `"$@$emptyvariable"` generates zero fields or one

  field. Zsh, pdksh/mksh and (d)ash generate one field, as seems logical.

  But bash, AT&T ksh and yash generate zero fields, which we consider a

  quirk. (See also BUG_PP_01)

* `QRK_EVALNOOPT`: `eval` does not parse options, not even `--`, which makes it

  incompatible with other shells: on the one hand, (d)ash does not accept   

  `eval -- "$command"` whereas on other shells this is necessary if the command

  starts with a `-`, or the command would be interpreted as an option to `eval`.

  A simple workaround is to prefix arbitrary commands with a space.

  [Both situations are POSIX compliant](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_19_16),

  but since they are incompatible without a workaround,the minority situation

  is labeled here as a QuiRK.

* `QRK_EXECFNBI`: In pdksh and zsh, `exec` looks up shell functions and

  builtins before external commands, and if it finds one it does the

  equivalent of running the function or builtin followed by `exit`. This

  is probably a bug in POSIX terms; `exec` is supposed to launch a

  program that overlays the current shell, implying the program launched by

  `exec` is always external to the shell. However, since the

  [POSIX language](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_20)

  is rather

  [vague and possibly incorrect](https://www.mail-archive.com/[email protected]/msg01437.html),

  this is labeled as a shell quirk instead of a shell bug.

* `QRK_FNRDREXIT`: On FreeBSD sh and NetBSD sh, an error in a redirection

  attached to a function call causes the shell to exit. This affects

  redirections of all functions, including modernish library functions

  as well as functions set by [`harden`](#user-content-use-syscmdharden).

* `QRK_GLOBDOTS`: Pathname expansion of `.*` matches the pseudonames `.` and

  `..` so that, e.g., `cp -pr .* backup/` cannot be used to copy all your

  hidden files. (bash \< 5.2, (d)ash, AT&T ksh != 93u+m, yash)

* `QRK_HDPARQUOT`: Double **quot**es within certain **par**ameter substitutions in

  **h**ere-**d**ocuments aren't removed (FreeBSD sh; bosh). For instance, if

  `var` is set, `${var+"x"}` in a here-document yields `"x"`, not `x`.

  [POSIX considers it undefined](https://www.mail-archive.com/[email protected]/msg01626.html)

  to use double quotes there, so they should be avoided for a script to be

  fully POSIX compatible.

  (Note this quirk does **not** apply for substitutions that remove patterns,

  such as `${var#"$x"}` and `${var%"$x"}`; those are defined by POSIX

  and double quotes are fine to use.)

  (Note 2: single quotes produce widely varying behaviour and should never

  be used within any form of parameter substitution in a here-document.)

* `QRK_IFSFINAL`: in field splitting, a final non-whitespace `IFS` delimiter

  character is counted as an empty field (yash \< 2.42, zsh, pdksh). This is a QRK

  (quirk), not a BUG, because POSIX is ambiguous on this.

* `QRK_LOCALINH`: On a shell with `LOCALVARS`, local variables, when declared

  without assigning a value, inherit the state of their global namesake, if

  any. (dash, FreeBSD sh)

* `QRK_LOCALSET`: On a shell with `LOCALVARS`, local variables are immediately set

  to the empty value upon being declared, instead of being initially without

  a value. (zsh)

* `QRK_LOCALSET2`: Like `QRK_LOCALSET`, but *only* if the variable by the

  same name in the global/parent scope is unset. If the global variable is

  set, then the local variable starts out unset. (bash 2 and 3)

* `QRK_LOCALUNS`: On a shell with `LOCALVARS`, local variables lose their local

  status when unset. Since the variable name reverts to global, this means that

  *`unset` will not necessarily unset the variable!* (yash, pdksh/mksh. Note:

  this is actually a behaviour of `typeset`, to which modernish aliases `local`

  on these shells.)

* `QRK_LOCALUNS2`: This is a more treacherous version of `QRK_LOCALUNS` that

  is unique to bash. The `unset` command works as expected when used on a local

  variable in the same scope that variable was declared in, **however**, it

  makes local variables global again if they are unset in a subscope of that

  local scope, such as a function called by the function where it is local.

  (Note: since `QRK_LOCALUNS2` is a special case of `QRK_LOCALUNS`, modernish

  will not detect both.)

  On bash \>= 5.0, modernish eliminates this quirk upon initialisation

  by setting `shopt -s localvar_unset`.

* `QRK_OPTABBR`: Long-form shell option names can be abbreviated down to a

  length where the abbreviation is not redundant with other long-form option

  names. (ksh93, yash)

* `QRK_OPTCASE`: Long-form shell option names are case-insensitive. (yash, zsh)

* `QRK_OPTDASH`: Long-form shell option names ignore the `-`. (ksh93, yash)

* `QRK_OPTNOPRFX`: Long-form shell option names use a dynamic `no` prefix for

  all options (including POSIX ones). For instance, `glob` is the opposite

  of `noglob`, and `nonotify` is the opposite of `notify`. (ksh93, yash, zsh)

* `QRK_OPTULINE`: Long-form shell option names ignore the `_`. (yash, zsh)

* `QRK_PPIPEMAIN`: On zsh \<= 5.5.1, in all elements of a pipeline, parameter

  expansions are evaluated in the current environment (with any changes they

  make surviving the pipeline), though the commands themselves of every

  element but the last are executed in a subshell. For instance, given unset

  or empty `v`, in the pipeline `cmd1 ${v:=foo} | cmd2`, the assignment to

  `v` survives, though `cmd1` itself is executed in a subshell.

* `QRK_SPCBIXP`: Variable assignments directly preceding

  [special builtin commands](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_14)

  are exported, and persist as exported. (bash; yash)

* `QRK_UNSETF`: If 'unset' is invoked without any option flag (-v or -f), and

  no variable by the given name exists but a function does, the shell unsets

  the function. (bash)



### Bugs ###



Modernish currently identifies and supports the following shell bugs:



* `BUG_ALIASCSHD`: A spurious syntax error occurs if a here-document

  containing a command substitution is used within two aliases that define a

  block. The syntax error reporting a missing `}` occurs because the alias

  terminating the block is not correctly expanded. This bug affects

  [`var/local`](#user-content-use-varlocal) and

  [`var/loop`](#user-content-use-varloop)

  as they define blocks this way. Workaround: make a shell function that

  handles the here-document and call that shell function from the block/loop

  instead. Bug found on: dash \<= 0.5.10.2; Busybox ash \<= 1.31.1.

* `BUG_ALIASPOSX`: Running any command "foo" in POSIX mode like

  `POSIXLY_CORRECT=y foo` will globally disable alias expansion on a

  non-interactive shell (killing modernish), unless POSIX mode is globally

  enabled. Bug found on bash 4.2 through 5.0.

  **Note:** on bash versions with this bug, modernish automatically enables

  POSIX mode to avoid triggering it. A side effect is that process substitution

  (`PROCSUBST`) isn't available.

* `BUG_ARITHINIT`: Using unset or empty variables (dash <= 0.5.9.1 on macOS)

  or unset variables (yash <= 2.44) in arithmetic expressions causes the

  shell to exit, instead of taking them as a value of zero.

* `BUG_ARITHLNNO`: The shell supports `$LINENO`, but the variable is

   considered unset in arithmetic contexts, like `$(( LINENO > 0 ))`.

   This makes it error out under `set -u` and default to zero otherwise.

   Workaround: use shell expansion like `$(( $LINENO > 0 ))`. (FreeBSD sh)

* `BUG_ARITHNAN`: The case-insensitive special floating point constants

   `Inf` and `NaN` are recognised in arithmetic evaluation, overriding any

   variables with the names `Inf`, `NaN`, `INF`, `nan`, etc. (AT&T ksh93;

   zsh 5.6 - 5.8)

* `BUG_ARITHSPLT`: Unquoted `$((`arithmetic expressions`))` are not

  subject to field splitting as expected. (zsh, mksh<=R49)

* `BUG_ASGNCC01`: if `IFS` contains a `$CC01` (`^A`) character, unquoted expansions in

  shell assignments discard that character (if present). Found on: bash 4.0-4.3

* `BUG_ASGNLOCAL`: If you have a function-local variable (see `LOCALVARS`)

  with the same name as a global variable, and within the function you run a

  shell builtin command preceded by a temporary variable assignment, then

  the global variable is unset. (zsh \<= 5.7.1)

* `BUG_BRACQUOT`: shell quoting within bracket patterns has no effect (zsh < 5.3;

  ksh93) This bug means the `-` retains it special meaning of 'character

  range', and an initial `!` (and, on some shells, `^`) retains the meaning of

  negation, even in quoted strings within bracket patterns, including quoted

  variables.

* `BUG_CASEEMPT`: An empty `case` list on a single line, as in `case x in esac`,

  is a syntax error. (AT&T ksh93)

* `BUG_CASELIT`: If a `case` pattern doesn't match as a pattern, it's tried

  again as a literal string, even if the pattern isn't quoted. This can

  result in false positives when a pattern doesn't match itself, like with

  bracket patterns. This contravenes POSIX and breaks use cases such as

  input validation. (AT&T ksh93) Note: modernish `match` works around this.

* `BUG_CASEPAREN`: `case` patterns without an opening parenthesis

  (i.e. with only an unbalanced closing parenthesis) are misparsed

  as a syntax error within command substitutions of the form `$( )`.

  Workaround: include the opening parenthesis. Found on: bash 3.2

* `BUG_CASESTAT`: The `case` conditional construct prematurely clobbers the

  exit status `$?`. (found in zsh \< 5.3, Busybox ash \<= 1.25.0, dash \<

  0.5.9.1)

* `BUG_CDNOLOGIC`: The `cd` built-in command lacks the POSIX-specified `-L`

  option and does not support logical traversal; it always acts as if the `-P`

  (physical traversal) option was passed. This also renders the `-L` option

  to modernish [`chdir`](#user-content-chdir) ineffective. (NetBSD sh)

* `BUG_CDPCANON`: `cd -P` (and hence also modernish

  [`chdir`](#user-content-chdir)) does not correctly canonicalise/normalise a

  directory path that starts with three or more slashses; it reduces these to

  two initial slashes instead of one in `$PWD`. (zsh \<= 5.7.1)

* `BUG_CMDEXEC`: using `command exec` (to open a file descriptor, using

  `command` to avoid exiting the shell on failure) within a function causes

  bash \<= 4.0 to fail to restore the global positional parameters when

  leaving that function. It also renders bash \<=4.0 prone to hanging.

* `BUG_CMDEXPAN`: if the `command` command results from an expansion, it acts

  like `command -v`, showing the path of the command instead of executing it.

  For example: `v=command; "$v" ls` or `set -- command ls; "$@"` don't work.

  (AT&T ksh93)

* `BUG_CMDOPTEXP`: the `command` builtin does not recognise options if they

  result from expansions. For instance, you cannot conditionally store `-p`

  in a variable like `defaultpath` and then do `command $defaultpath

  someCommand`. (found in zsh \< 5.3)

* `BUG_CMDPV`: `command -pv` does not find builtins ({pd,m}ksh), does not

  accept the -p and -v options together (zsh \< 5.3) or ignores the `-p`

  option altogether (bash 3.2); in any case, it's not usable to find commands

  in the default system PATH.

* `BUG_CMDSETPP`: using `command set --` has no effect; it does not set the

  positional parameters. For compat, use `set` without `command`. (mksh \<= R57)

* `BUG_CMDSPASGN`: preceding a

  [special builtin](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_14)

  with `command` does not stop preceding invocation-local variable

  assignments from becoming global. (AT&T ksh93)

* `BUG_CMDSPEXIT`: preceding a

  [special builtin](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_14)

  (other than `eval`, `exec`, `return` or `exit`)

  with `command` does not always stop

  it from exiting the shell if the builtin encounters error.

  (bash \<= 4.0; zsh \<= 5.2; mksh; ksh93)

* `BUG_CSNHDBKSL`: Backslashes within non-expanding here-documents within

  command substitutions are incorrectly expanded to perform newline joining,

  as opposed to left intact. (bash \<= 4.4)

* `BUG_CSUBBTQUOT`: A spurious syntax erorr is thrown when using double

  quotes within a backtick-style command substitution that is itself within

  double quotes. (AT&T ksh93 \< 93u+m 2022-05-20)

* `BUG_CSUBLNCONT`: Backslash line continuation is not processed correctly

  within modern-form `$(`command substitutions`)`.

  (AT&T ksh93 \< 93u+m 2022-05-21)

* `BUG_CSUBRMLF`: A bug affecting the stripping of final linefeeds from

  command substitutions. If a command substitution does not produce any

  output to substitute **and** is concatenated in a string or here-document,

  then the shell removes any concurrent linefeeds occurring directly before

  the command substitution in that string or here-document.

  (dash \<= 0.5.10.2, Busybox ash, FreeBSD sh)

* `BUG_CSUBSTDO`: If standard output (file descriptor 1) is closed before

  entering a command substitution, and any other file descriptors are

  redirected within the command substitution, commands such as `echo` or

  `putln` will not work within the command substitution, acting as if standard

  output is still closed (AT&T ksh93 \<= AJM 93u+ 2012-08-01). Workaround: see

  [`cap/BUG_CSUBSTDO.t`](https://github.com/modernish/modernish/blob/master/lib/modernish/cap/BUG_CSUBSTDO.t).

* `BUG_DEVTTY`: the shell can't redirect output to `/dev/tty` if

  `set -C`/`set -o noclobber` (part of [safe mode](#user-content-use-safe))

  is active. Workaround: use `>| /dev/tty` instead of `> /dev/tty`.

  Bug found on: bash on certain systems (at least QNX and Interix).

* `BUG_DOLRCSUB`: parsing problem where, inside a command substitution of

  the form `$(...)`, the sequence `$$'...'` is treated as `$'...'` (i.e. as

  a use of CESCQUOT), and `$$"..."` as `$"..."` (bash-specific translatable

  string). (Found in bash up to 4.4)

* `BUG_DQGLOB`: *glob*bing is not properly deactivated within

  *d*ouble-*q*uoted strings. Within double quotes, a `*` or `?` immediately

  following a backslash is interpreted as a globbing character. This applies

  to both pathname expansion and pattern matching in `case`. Found in: dash.

  (The bug is not triggered when using modernish

  [`match`](#user-content-string-tests).)

* `BUG_EXPORTUNS`: Setting the export flag on an otherwise unset variable

  causes a set and empty environment variable to be exported, though the

  variable continues to be considered unset within the current shell.

  (FreeBSD sh \< 13.0)

* `BUG_FNSUBSH`: Function definitions within subshells (including command

  substitutions) are ignored if a function by the same name exists in the

  main shell, so the wrong function is executed. `unset -f` is also silently

  ignored. ksh93 (all current versions as of November 2018) has this bug.

  It only applies to non-forked subshells. See `NONFORKSUBSH`.

* `BUG_FORLOCAL`: a `for` loop in a function makes the iteration variable

  local to the function, so it won't survive the execution of the function.

  Found on: yash. This is intentional and documented behaviour on yash in

  non-POSIX mode, but in POSIX terms it's a bug, so we mark it as such.

* `BUG_GETOPTSMA`: The `getopts` builtin leaves a `:` instead of a `?` in

  the specified option variable if a given option that requires an argument

  lacks an argument, and the option string does not start with a `:`. (zsh)

* `BUG_HDOCBKSL`: Line continuation using *b*ac*ksl*ashes in expanding

  *h*ere-*doc*uments is handled incorrectly. (zsh up to 5.4.2)

* `BUG_HDOCMASK`: Here-documents (and here-strings, see `HERESTRING`) use

  temporary files. This fails if the current `umask` setting disallows the

  user to read, so the here-document can't read from the shell's temporary

  file. Workaround: ensure user-readable `umask` when using here-documents.

  (bash, mksh, zsh)

* `BUG_IFSCC01PP`: If `IFS` contains a `$CC01` (`^A`) control character, the

  expansion `"$@"` (even quoted) is gravely corrupted. *Since many modernish

  functions use this to loop through the positional parameters, this breaks

  the library.* (Found in bash \< 4.4)

* `BUG_IFSGLOBC`: In glob pattern matching (such as in `case` and `[[`), if a

  wildcard character is part of `IFS`, it is matched literally instead of as a

  matching character. This applies to glob characters `*`, `?`, `[` and `]`.

  *Since nearly all modernish functions use `case` for argument validation and

  other purposes, nearly every modernish function breaks on shells with this

  bug if `IFS` contains any of these three characters!*

  (Found in bash \< 4.4)

* `BUG_IFSGLOBP`: In pathname expansion (filename globbing), if a

  wildcard character is part of `IFS`, it is matched literally instead of as a

  matching character. This applies to glob characters `*`, `?`, `[` and `]`.

  (Bug found in bash, all versions up to at least 4.4)

* `BUG_IFSGLOBS`: in glob pattern matching (as in `case` or parameter

  substitution with `#` and `%`), if `IFS` starts with `?` or `*` and the

  `"$*"` parameter expansion inserts any `IFS` separator characters, those

  characters are erroneously interpreted as wildcards when quoted "$*" is

  used as the glob pattern. (AT&T ksh93)

* `BUG_IFSISSET`: AT&T ksh93 (2011/2012 versions): `${IFS+s}` always yields `s`

  even if `IFS` is unset. This applies to `IFS` only.

* `BUG_ISSETLOOP`: AT&T ksh93: Expansions like `${var+set}`

  remain static when used within a `for`, `while` or

  `until` loop; the expansions don't change along with the state of the

  variable, so they cannot be used to check whether a variable is set

  within a loop if the state of that variable may change

  in the course of the loop.

* `BUG_KBGPID`: AT&T ksh93: If a single command ending in `&` (i.e. a background

  job) is enclosed in a `{` braces`; }` block with an I/O redirection, the `$!`

  special parameter is not set to the background job's PID.

* `BUG_KUNSETIFS`: AT&T ksh93: Unsetting `IFS` fails to activate default field

  splitting if the following conditions are met: 1. `IFS` is set and empty

  (i.e. split is disabled) in the main shell, and at least one expansion has

  been processed with that setting; 2. The code is currently executing in a

  non-forked subshell (see [`NONFORKSUBSH`](#user-content-capabilities)).

* `BUG_LNNONEG`: `$LINENO` becomes wildly inaccurate, even negative, when

  dotting/sourcing scripts. Bug found on: dash with LINENO support compiled in.

* `BUG_LOOPRET1`: If a `return` command is given with a status argument within

  the set of conditional commands in a `while` or `until` loop (i.e., between

  `while`/`until` and `do`), the status argument is ignored and the function

  returns with status 0 instead of the specified status.

  Found on: dash \<= 0.5.8; zsh \<= 5.2

* `BUG_LOOPRET2`: If a `return` command is given without a status argument

  within the set of conditional commands in a `while` or `until` loop (i.e.,

  between `while`/`until` and `do`), the exit status passed down from the

  previous command is ignored and the function returns with status 0 instead.

  Found on: dash \<= 0.5.10.2; AT&T ksh93; zsh \<= 5.2

* `BUG_LOOPRET3`: If a `return` command is given within the set of conditional

  commands in a `while` or `until` loop (i.e., between `while`/`until` and

  `do`), *and* the return status (either the status argument to `return` or the

  exit status passed down from the previous command by `return` without a

  status argument) is non-zero, *and* the conditional command list itself yields

  false (for `while`) or true (for `until`), *and* the whole construct is

  executed in a dot script sourced from another script, then too many levels of

  loop are broken out of, causing **program flow corruption** or premature exit.

  Found on: zsh \<= 5.7.1

* `BUG_MULTIBIFS`: We're on a UTF-8 locale and the shell supports UTF-8

  characters in general (i.e. we don't have `WRN_MULTIBYTE`) – however, using

  multi-byte characters as `IFS` field delimiters still doesn't work. For

  example, `"$*"` joins positional parameters on the first byte of `IFS`

  instead of the first character. (ksh93, mksh, FreeBSD sh, Busybox ash)

* `BUG_NOCHCLASS`: POSIX-mandated character `[:`classes`:]` within bracket

  `[`expressions`]` are not supported in glob patterns. (mksh)

* `BUG_NOEXPRO`: Cannot export read-only variables. (zsh <= 5.7.1 in sh mode)

* `BUG_OPTNOLOG`: on dash, setting `-o nolog` causes `$-` to wreak havoc:

  trying to expand `$-` silently aborts parsing of an entire argument,

  so e.g. `"one,$-,two"` yields `"one,"`. (Same applies to `-o debug`.)

* `BUG_PP_01`: [POSIX says](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05_02)

  that empty `"$@"` generates zero fields but empty `''` or `""` or

  `"$emptyvariable"` generates one empty field. This means concatenating

  `"$@"` with one or more other, separately quoted, empty strings (like

  `"$@""$emptyvariable"`) should still produce one empty field. But on

  bash 3.x, this erroneously produces zero fields. (See also QRK_EMPTPPWRD)

* `BUG_PP_02`: Like `BUG_PP_01`, but with unquoted `$@` and only

  with `"$emptyvariable"$@`, not `$@"$emptyvariable"`.

  (mksh \<= R50f; FreeBSD sh \<= 10.3)

* `BUG_PP_03`: When `IFS` is unset or empty (zsh 5.3.x) or empty (mksh \<= R50),

  assigning `var=$*` only assigns the first field, failing to join and

  discarding the rest of the fields. Workaround: `var="$*"`

  (POSIX leaves `var=$@`, etc. undefined, so we don't test for those.)

* `BUG_PP_03A`: When `IFS` is unset, assignments like `var=$*`

  incorrectly remove leading and trailing spaces (but not tabs or

  newlines) from the result. Workaround: quote the expansion. Found on:

  bash 4.3 and 4.4.

* `BUG_PP_03B`: When `IFS` is unset, assignments like `var=${var+$*}`,

  etc. incorrectly remove leading and trailing spaces (but not tabs or

  newlines) from the result. Workaround: quote the expansion. Found on:

  bash 4.3 and 4.4.

* `BUG_PP_03C`: When `IFS` is unset, assigning `var=${var-$*}` only assigns

  the first field, failing to join and discarding the rest of the fields.

  (zsh 5.3, 5.3.1) Workaround: `var=${var-"$*"}`

* `BUG_PP_04A`: Like BUG_PP_03A, but for conditional assignments within

  parameter substitutions, as in `: ${var=$*}` or `: ${var:=$*}`.

  Workaround: quote either `$*` within the expansion or the expansion

  itself. (bash \<= 4.4)

* `BUG_PP_04E`: When assigning the positional parameters ($*) to a variable

  using a conditional assignment within a parameter substitution, e.g.

  `: ${var:=$*}`, the fields are always joined and separated by spaces,

  except if `IFS` is set and empty. Workaround as in BUG_PP_04A.

  (bash 4.3)

* `BUG_PP_04_S`: When `IFS` is null (empty), the result of a substitution

  like `${var=$*}` is incorrectly field-split on spaces.

  The assignment itself succeeds normally.

  Found on: bash 4.2, 4.3

* `BUG_PP_05`: [POSIX says](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05_02)

  that empty `$@` and `$*` generate zero fields, but with null `IFS`, empty

  unquoted `$@` and `$*` yield one empty field. Found on: dash 0.5.9

  and 0.5.9.1; Busybox ash.

* `BUG_PP_06A`: [POSIX says](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05_02)

  that unquoted `$@` and `$*` initially generate as many fields as there are

  positional parameters, and then (because `$@` or `$*` is unquoted) each field is

  split further according to `IFS`. With this bug, the latter step is not

  done if `IFS` is unset (i.e. default split). Found on: zsh \< 5.4

* `BUG_PP_07`: unquoted `$*` and `$@` (including in substitutions like

  `${1+$@}` or `${var-$*}`) do not perform default field splitting if

  `IFS` is unset. Found on: zsh (up to 5.3.1) in sh mode

* `BUG_PP_07A`: When `IFS` is unset, unquoted `$*` undergoes word splitting

  as if `IFS=' '`, and not the expected `IFS=" ${CCt}${CCn}"`.

  Found on: bash 4.4

* `BUG_PP_08`: When `IFS` is empty, unquoted `$@` and `$*` do not generate

  one field for each positional parameter as expected, but instead join

  them into a single field without a separator. Found on: yash \< 2.44

  and dash \< 0.5.9 and Busybox ash \< 1.27.0

* `BUG_PP_08B`: When `IFS` is empty, unquoted `$*` within a substitution (e.g.

  `${1+$*}` or `${var-$*}`) does not generate one field for each positional

  parameter as expected, but instead joins them into a single field without

  a separator. Found on: bash 3 and 4

* `BUG_PP_09`: When `IFS` is non-empty but does not contain a space,

  unquoted `$*` within a substitution (e.g. `${1+$*}` or `${var-$*}`) does

  not generate one field for each positional parameter as expected,

  but instead joins them into a single field separated by spaces

  (even though, as said, `IFS` does not contain a space).

  Found on: bash 4.3

* `BUG_PP_10`: When `IFS` is null (empty), assigning `var=$*` removes any

  `$CC01` (^A) and `$CC7F` (DEL) characters. (bash 3, 4)

* `BUG_PP_10A`: When `IFS` is non-empty, assigning `var=$*` prefixes each

  `$CC01` (^A) and `$CC7F` (DEL) character with a `$CC01` character. (bash 4.4)

* `BUG_PP_1ARG`: When `IFS` is empty on bash <= 4.3 (i.e. field

  splitting is off), `${1+"$@"}` or `"${1+$@}"` is counted as a single

  argument instead of each positional parameter as separate arguments.

  This also applies to prepending text only if there are positional

  parameters with something like `"${1+foobar $@}"`.

* `BUG_PP_MDIGIT`: Multiple-digit positional parameters don't require expansion

  braces, so e.g. `$10` = `${10}` (dash; Busybox ash). This is classed as a bug

  because it causes a straight-up incompatibility with POSIX scripts. POSIX

  [says](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02):

  "The parameter name or symbol can be enclosed in braces, which are

  optional except for positional parameters with more than one digit [...]".

* `BUG_PP_MDLEN`: For `${#x}` expansions where x >= 10, only the first digit of

  the positional parameter number is considered, e.g. `${#10}`, `${#12}`,

  `${#123}` are all parsed as if they are `${#1}`. Then, string parsing is

  aborted so that further characters or expansions, if any, are lost.

  Bug found in: dash 0.5.11 - 0.5.11.4 (fixed in dash 0.5.11.5)

* `BUG_PSUBASNCC`: in an assignment parameter substitution of the form

  `${foo=value}`, if the characters `$CC01` (^A) or `$CC7F` (DEL) are in the

  value, all their occurrences are stripped from the expansion (although the

  assignment itself is done correctly). If the expansion is quoted, only

  `$CC01` is stripped. This bug is independent of the state of `IFS`, except if

  `IFS` is null, the assignment in `${foo=$*}` (unquoted) is buggy too: it

  strips `$CC01` from the assigned value. (Found on bash 4.2, 4.3, 4.4)

* `BUG_PSUBBKSL1`: A backslash-escaped `}` character within a quoted parameter

  substitution is not unescaped. (bash 3.2, dash \<= 0.5.9.1, Busybox 1.27 ash)

* `BUG_PSUBEMIFS`: if `IFS` is empty (no split, as in safe mode), then if a

  parameter substitution of the forms `${foo-$*}`, `${foo+$*}`, `${foo:-$*}` or

  `${foo:+$*}` occurs in a command argument, the characters `$CC01` (^A) or

  `$CC7F` (DEL) are stripped from the expanded argument. (Found on: bash 4.4)

* `BUG_PSUBEMPT`: Expansions of the form `${V-}` and `${V:-}` are not

  subject to normal shell empty removal if that parameter is unset, causing

  unexpected empty arguments to commands. Workaround: `${V+$V}` and

  `${V:+$V}` work as expected. (Found on FreeBSD 10.3 sh)

* `BUG_PSUBIFSNW`: When field-splitting unquoted parameter substitutions like

  `${var#foo}`, `${var##foo}`, `${var%foo}` or `${var%%foo}` on non-whitespace

  `IFS`, if there is an initial empty field, a spurious extra initial empty

  field is generated. (mksh)

* `BUG_PSUBNEWLN`: Due to a bug in the parser, parameter substitutions

  spread over more than one line cause a syntax error.

  Workaround: instead of a literal newline, use [`$CCn`](#user-content-control-character-whitespace-and-shell-safe-character-constants).

  (found in dash \<= 0.5.9.1 and Busybox ash \<= 1.28.1)

* `BUG_PSUBSQUOT`: in pattern matching parameter substitutions

  (`${param#pattern}`, `${param%pattern}`, `${param##pattern}` and

  `${param%%pattern}`), if the whole parameter substitution is quoted with

  double quotes, then single quotes in the *pattern* are not parsed. POSIX

  [says](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02)

  they are to keep their special meaning, so that glob characters may

  be quoted. For example: `x=foobar; echo "${x#'foo'}"` should yield `bar`

  but with this bug yields `foobar`. (dash \<= 0.5.9.1; Busybox 1.27 ash)

* `BUG_PSUBSQHD`: Like BUG_PSUBSQUOT, but included a here-document instead of

  quoted with double quotes. (dash \<= 0.5.9.1; mksh)

* `BUG_PUTIOERR`: Shell builtins that output strings (`echo`, `printf`, ksh/zsh

  `print`), and thus also modernish `put` and `putln`, do not check for I/O

  errors on output. This means a script cannot check for them, and a script

  process in a pipe can get stuck in an infinite loop if `SIGPIPE` is ignored.

* `BUG_READWHSP`: If there is more than one field to read, `read` does not

   trim trailing `IFS` whitespace. (dash 0.5.7, 0.5.8)

* `BUG_REDIRIO`: the I/O redirection operator `<>` (open a file descriptor

  for both read and write) defaults to opening standard output (i.e. is

  short for `1<>`) instead of defaulting to opening standard input (`0<>`) as

  [POSIX specifies](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_07_07).

  (AT&T ksh93)

* `BUG_REDIRPOS`: Buggy behaviour occurs if a *redir*ection is *pos*itioned

  in between to variable assignments in the same command. On zsh 5.0.x, a

  parse error is thrown. On zsh 5.1 to 5.4.2, anything following the

  redirection (other assignments or command arguments) is silently ignored.

* `BUG_SCLOSEDFD`: bash \< 5.0 and dash fail to establish a block-local scope

  for a file descriptor that is added to the end of the block as a redirection

  that closes that file descriptor (e.g. `} 8<&-` or `done 7>&-`). If that FD

  is already closed outside the block, the FD remains global, so you can't

  locally `exec` it. So with this bug, it is not straightforward to make a

  block-local FD appear initially closed within a block. Workaround: first open

  the FD, then close it – for example: `done 7>/dev/null 7>&-` will establish

  a local scope for FD 7 for the preceding `do`...`done` block while still

  making FD 7 appear initially closed within the block.

* `BUG_SETOUTVAR`: The `set` builtin (with no arguments) only prints native

  function-local variables when called from a shell function. (yash \<= 2.46)

* `BUG_SHIFTERR0`: The `shift` builtin silently returns a successful exit

  status (0) when attempting to shift a number greater than the current

  amount of positional parameters. (Busybox ash \<= 1.28.4)

* `BUG_SPCBILOC`: Variable assignments preceding

  [special builtins](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_14)

  create a partially function-local variable if a variable by the same name

  already exists in the global scope. (bash \< 5.0 in POSIX mode)

* `BUG_TESTERR1A`: `test`/`[` exits with a non-error `false` status

  (1) if an invalid argument is given to an operator. (AT&T ksh93)

* `BUG_TESTILNUM`: On dash (up to 0.5.8), giving an illegal number to `test -t`

  or `[ -t` causes some kind of corruption so the next `test`/`[` invocation

  fails with an "unexpected operator" error even if it's legit.

* `BUG_TESTONEG`: The `test`/`[` builtin supports a `-o` unary operator to

  check if a shell option is set, but it ignores the `no` prefix on shell

  option names, so something like `[ -o noclobber ]` gives a false positive.

  Bug found on yash up to 2.43. (The `TESTO` feature test implicitly checks

  against this bug and won't detect the feature if the bug is found.)

* `BUG_TRAPEMPT`: The `trap` builtin does not quote empty traps in its

  output, rendering the output unsuitable for shell re-input. For instance,

  `trap '' INT; trap` outputs "`trap --  INT`" instead of "`trap -- '' INT`".

  (found in mksh \<= R56c)

* `BUG_TRAPEXIT`: the shell's `trap` builtin does not know the EXIT trap by

  name, but only by number (0). Using the name throws a "bad trap" error. Found in

  [klibc 2.0.4 dash](https://git.kernel.org/pub/scm/libs/klibc/klibc.git/tree/usr/dash).

* `BUG_TRAPFNEXI`: When a function issues a signal whose trap exits the

  shell, the shell is not exited immediately, but only on return from the

  function. (zsh)

* `BUG_TRAPRETIR`: Using `return` within `eval` triggers infinite recursion if

  both a RETURN trap and the `functrace` shell option are active. This bug in

  bash-only functionality triggers a crash when using modernish, so to avoid

  this, modernish automatically disables the `functrace` shell option if a

  `RETURN` trap is set or pushed and this bug is detected. (bash 4.3, 4.4)

* `BUG_TRAPSUB0`: Subshells in traps fail to pass down a nonzero exit status of

  the last command they execute, under certain conditions or consistently,

  depending on the shell. (bash \<= 4.0; dash 0.5.9 - 0.5.10.2; yash \<= 2.47)

* `BUG_TRAPUNSRE`: When a trap *uns*ets itself and then *re*sends its own signal,

  the execution of the trap action (including functions called by it) is

  not interrupted by the now-untrapped signal; instead, the process

  terminates after completing the entire trap routine. (bash \<= 4.2; zsh)

* `BUG_UNSETUNXP`: If an unset variable is given the export flag using the

  `export` command, a subsequent `unset` command does not remove that export

  flag again. Workaround: assign to the variable first, then unset it to

  unexport it. (Found on AT&T ksh JM-93u-2011-02-08; Busybox 1.27.0 ash)

* `BUG_VARPREFIX`: On a shell with the `VARPREFIX` feature, expansions of type

  `${!`*prefix*`@}` and `${!`*prefix*`*}` do not find the variable name

  *prefix* itself. (AT&T ksh93)

* `BUG_ZSHNAMES`: A series of lowerase names, normally okay for script use

  as per POSIX convention, is reserved for special use. Unsetting these

  names is impossible in most cases, and changing them may corrupt important

  shell or system settings. This may conflict with

  [simple-form](#user-content-simple-form) modernish scripts.

  This bug is detected on zsh when it was not initially invoked in emulation

  mode, and emulation mode was enabled using `emulate sh` post invocation

  instead (which does not disable these conflicting parameters).

  As of zsh 5.6, the list of variable names affected is: `aliases` `argv`

  `builtins` `cdpath` `commands` `dirstack` `dis_aliases` `dis_builtins`

  `dis_functions` `dis_functions_source` `dis_galiases` `dis_patchars`

  `dis_reswords` `dis_saliases` `fignore` `fpath` `funcfiletrace`

  `funcsourcetrace` `funcstack` `functions` `functions_source` `functrace`

  `galiases` `histchars` `history` `historywords` `jobdirs` `jobstates`

  `jobtexts` `keymaps` `mailpath` `manpath` `module_path` `modules` `nameddirs`

  `options` `parameters` `patchars` `path` `pipestatus` `prompt` `psvar`

  `reswords` `saliases` `signals` `status` `termcap` `terminfo` `userdirs`

  `usergroups` `watch` `widgets` `zsh_eval_context` `zsh_scheduled_events`

* `BUG_ZSHNAMES2`: Two lowercase variable names `histchars` and `signals`,

  normally okay for script use as per POSIX convention, are reserved for

  special use on zsh, *even* if zsh is initialised in sh mode (via a `sh`

  symlink or using the `--emulate sh` option at startup).

  Bug found on: zsh <= 5.7.1. The bug is only detected if `BUG_ZSHNAMES` is

  not detected, because this bug's effects are included in that one's.



### Warning IDs ###



Warning IDs do not identify any characteristic of the shell, but instead

warn about a potentially problematic system condition that was detected at

initialisation time.



* `WRN_EREMBYTE`: The current system locale setting supports Unicode UTF-8

  multi-byte/variable-length characters, but the utility used by

  [`str ematch`](#user-content-string-tests)

  to match extended regular expressions (EREs) does not support them

  and treats all characters as single bytes. This means multi-byte characters

  will be matched as multiple characters, and character `[:`classes`:]`

  within bracket expressions will only match ASCII characters.

* `WRN_MULTIBYTE`: The current system locale setting supports Unicode UTF-8

  multi-byte/variable-length characters, but the current shell does not

  support them and treats all characters as single bytes. This means

  counting or processing multi-byte characters with the current shell will

  produce incorrect results. Scripts that need compatibility with this

  system condition should check `if thisshellhas WRN_MULTIBYTE` and resort

  to a workaround that uses external utilities where necessary.

* `WRN_NOSIGPIPE`: Modernish has detected that the process that launched

  the current program has set `SIGPIPE` to ignore, an irreversible condition

  that is in turn inherited by any process started by the current shell, and

  their subprocesses, and so on. The system constant

  [`$SIGPIPESTATUS`](#user-content-modernish-system-constants)

  is set to the special value 99999 and neither the current shell nor any

  process it spawns is now capable of receiving `SIGPIPE`. The

  [`-P` option to `harden`](#hardening-while-allowing-for-broken-pipes)

  is also rendered ineffective.

  Depending on how a given command `foo` is implemented, it is now possible

  that a pipeline such as `foo | head -n 10` never ends; if `foo` doesn't

  check for I/O errors, the only way it would ever stop trying to write

  lines is by receiving `SIGPIPE` as `head` terminates.

  Programs that use commands in this fashion should check `if thisshellhas

  WRN_NOSIGPIPE` and either employ workarounds or refuse to run if so.



## Appendix B: Regression test suite ##



Modernish comes with a suite of regression tests to detect bugs in modernish

itself, which can be run using `modernish --test` after installation. By

default, it will run all the tests verbosely but without tracing the command

execution. The `install.sh` installer will run `modernish --test -eqq` on the

selected shell before installation.



A few options are available to specify after `--test`:



* `-h`: show help.

* `-e`: disable or reduce expensive (i.e. slow or memory-hogging) tests.

* `-q`: quieter operation; report expected fails [known shell bugs]

  and unexpected fails [bugs in modernish]). Add `-q` again for

  quietest operation (report unexpected fails only).

* `-s`: entirely silent operation.

* `-t`: run only specific test sets or tests. Test sets are those listed

  in the full default output of `modernish --test`. This option requires

  an option-argument in the following format:    

  *testset1*`:`*num1*`,`*num2*`,`…`/`*testset2*`:`*num1*`,`*num2*`,`…`/`…    

  The colon followed by numbers is optional; if omitted, the entire set

  will be run, otherwise the given numbered tests will be run in the given

  order. Example: `modernish --test -t match:2,4,7/arith/shellquote:1` runs

  test 2, 4 and 7 from the `match` set, the entire `arith` set, and only

  test 1 from the `shellquote` set.

  A *testset* can also be given as the incomplete beginning of a name or as

  a shell glob pattern. In that case, all matching sets will be run.

* `-x`: trace each test using the shell's `xtrace` facility. Each trace is

  stored in a separate file in a specially created temporary directory. By

  default, the trace is deleted if a test does not produce an unexpected

  fail. Add `-x` again to keep expected fails as well, and again to

  keep all traces regardless of result. If any traces were saved,

  modernish will tell you the location of the temporary directory at the

  end, otherwise it will silently remove the directory again.

* `-E`: don't run any tests, but output a command to open the tests that would

  have been run in your editor. The editor from the `VISUAL` or `EDITOR`

  environment variable is used, with `vi` as a default. This option should be

  used together with `-t` to specify tests. All other options are ignored.

* `-F`: takes an argument with the name or path to a `find` utility to

  prefer when testing [`LOOP find`](#user-content-the-find-loop).

  [More info here](#user-content-picking-a-find-utility).



These short options can be combined so, for example,

`--test -qxx` is the same as `--test -q -x -x`.



### Difference between capability detection and regression tests ###



Note the difference between these regression tests and the cap tests listed in

[Appendix A](#user-content-appendix-a-list-of-shell-cap-ids). The latter are

tests for whatever shell is executing modernish: they detect capabilities

(features, quirks, bugs) of the current shell. They are meant to be run via

[`thisshellhas`](#user-content-shell-capability-detection) and are designed to

be taken advantage of in scripts. On the other hand, these tests run by

`modernish --test` are regression tests for modernish itself. It does not

make sense to use these in a script.



New/unknown shell bugs can still cause modernish regression tests to fail,

of course. That's why some of the regression tests also check for

consistency with the results of the capability detection tests: if there is a

shell bug in a widespread release version that modernish doesn't know about

yet, this in turn is considered to be a bug in modernish, because one of its

goals is to know about all the shell bugs in all released shell versions

currently seeing significant use.



### Testing modernish on all your shells ###



The `testshells.sh` program in `share/doc/modernish/examples` can be used to

run the regression test suite on all the shells installed on your system.

You could put it as `testshells` in some convenient location in your

`$PATH`, and then simply run:



    testshells modernish --test



(adding any further options you like – for instance, you might like to add

`-q` to avoid very long terminal output). On first run, `testshells` will

generate a list of shells it can find on your system and it will give you a

chance to edit it before proceeding.



## Appendix C: Supported locales ##



modernish, like most shells, fully supports two system locales: POSIX

(a.k.a. C, a.k.a. ASCII) and Unicode's UTF-8. It will work in other locales,

but things like converting to upper/lower case, and matching single

characters in patterns, are not guaranteed.



*Caveat:* some shells or operating systems have bugs that prevent (or lack

features required for) full locale support. If portability is a concern,

check for `thisshellhas WRN_MULTIBYTE` or `thisshellhas BUG_NOCHCLASS`

where needed. See [Appendix A](#user-content-appendix-a-list-of-shell-cap-ids).



Scripts/programs should *not* change the locale (`LC_*` or `LANG`) after

initialising modernish. Doing this might break various functions, as

modernish sets specific versions depending on your OS, shell and locale.

(Temporarily changing the locale is fine as long as you don't use

modernish features that depend on it – for example, setting a specific

locale just for an external command. However, if you use `harden`, see

the [important note](#user-content-important-note-on-variable-assignments)

in its documentation!)



## Appendix D: Supported shells ##



Modernish builds on the

[POSIX 2018 Edition](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/contents.html)

standard, so it should run on any sufficiently POSIX-compliant shell and

operating system. It uses both

[bug/feature detection](#user-content-shell-capability-detection)

and

[regression testing](#user-content-appendix-b-regression-test-suite)

to determine whether it can run on any particular shell, so it does not

block or support particular shell versions as such. However, modernish has

been confirmed to run correctly on the following shells:



-   [bash](https://www.gnu.org/software/bash/) 3.2 or higher

-   [Busybox](https://busybox.net/) ash 1.20.0 or higher, excluding 1.28.x

    (also possibly excluding anything older than 1.27.x on UTF-8 locales,

    depending on your operating system)

-   [dash](http://gondor.apana.org.au/~herbert/dash/) (Debian sh)

    0.5.7 or higher, *excluding* 0.5.10, 0.5.10.1, 0.5.11-0.5.11.4

-   [FreeBSD](https://www.freebsd.org/) sh 11.0 or higher

-   [gwsh](https://github.com/hvdijk/gwsh)

-   [ksh](http://github.com/ksh93/ksh) 93u+ 2012-08-01, 93u+m

-   [mksh](http://www.mirbsd.org/mksh.htm) version R55 or higher

-   [NetBSD](https://www.netbsd.org/) sh 9.0 or higher

-   [yash](http://yash.osdn.jp/) 2.40 or higher (2.44+ for POSIX mode)

-   [zsh](http://www.zsh.org/) 5.3 or higher



Currently known *not* to run modernish due to excessive bugs:



-   bosh ([Schily](http://schilytools.sourceforge.net/) Bourne shell)

-   [ksh](http://www.kornshell.com/) A 2020.0.0

-   pdksh, including [NetBSD](https://www.netbsd.org/) ksh and

    [OpenBSD](https://www.openbsd.org/) ksh



## Appendix E: zsh: integration with native scripts ##



This appendix is specific to [zsh](http://zsh.sourceforge.net/).



While modernish duplicates some functionality already available natively

on zsh, it still has plenty to add. However, writing a normal

[simple-form](#user-content-simple-form) modernish script turns

`emulate sh` on for the entire script, so you lose important aspects

of the zsh language.



But there is another way – modernish functionality may be integrated

with native zsh scripts using 'sticky emulation', as follows:



```sh

emulate -R sh -c '. modernish'

```



This causes modernish functions to run in sh mode while your script will still

run in native zsh mode with all its advantages. The following notes apply:



* Using the [safe mode](#user-content-use-safe) is *not* recommended, as zsh

  does not apply split/glob to variable expansions by default, and the

  modernish safe mode would defeat the `${~var}` and `${=var}` flags that apply

  these on a case by case basis. This does mean that:

    * The `--split` and `--glob` operators to constructs such as

      [`LOOP find`](#user-content-the-find-loop)

      are not available. Use zsh expansion flags instead.

    * Quoting literal glob patterns to commands like `find` remains necessary.

* Using [`LOCAL`](#user-content-use-varlocal) is not recommended.

  [Anonymous functions](http://zsh.sourceforge.net/Doc/Release/Functions.html#Anonymous-Functions)

  are the native zsh equivalent.

* Native zsh loops should be preferred over modernish loops, except where

  modernish adds functionality not available in zsh (such as `LOOP find` or

  [user-programmed loops](#user-content-creating-your-own-loop)).



See `man zshbuiltins` under `emulate`, option `-c`, for more information.



## Appendix F: Bundling modernish with your script ##



The modernish installer `install.sh` can bundle one or more scripts with a

stripped-down version of the modernish library. This allows the bundled scripts

to run with a known version of modernish, whether or not modernish is installed

on the user's system. Like modernish itself, bundling is cross-platform and

portable (or as portable as your script is).



Bundled scripts are not modified. Instead, for each script, a wrapper script is

installed under the same name in the installation root directory. This wrapper

automatically looks for a [suitable](#user-content-appendix-d-supported-shells)

POSIX-compliant shell that passes the modernish battery of fatal bug tests,

then sets up the environment to run the real script with modernish on that

shell. Your modernish script can be run through the supplied wrapper script

from any directory location on any POSIX-compliant operating system, as long as

all files remain in the same location relative to each other.



Bundling is always a non-interactive installer operation, with options

specified on the command line. The installer usage for bundling is as follows:



`install.sh` `-B` `-D` *rootdir* [ `-d` *subdir* ]

[ `-s` *shell* ] *scriptfile* [ *scriptfile* ... ]



The `-B` option enables bundling mode. The option does not itself take an

option-argument. Instead, any number of *scriptfile*s to bundle can be given

as arguments following all other options. All scripts are bundled with a

single copy of modernish. The bundling operation does not deal with any

auxiliary files the scripts may require (other than modernish modules); any

such need to be added manually after bundling is complete.



The `-D` option specifies the path to the bundled installation's root

directory, where wrapper scripts are installed. This option is mandatory.

If the directory doesn't exist, it is created.



The `-d` option specifies the subdirectory of the `-D` root directory where the

bundled scripts and modernish are installed. It can contain slashes to install

the bundle at a deeper directory level. The default subdirectory is `bndl`.

The option-argument can be empty or `/`, in which case the bundle is installed

directly into the installation root directory.



The `-s` option specifies a preferred shell for the bundled scripts. A shell

name or a full path to a shell can be given. Wrapper scripts try the full path

first (if any), then try to find a shell with its basename, and then try to

find a shell with that basename minus any version number (e.g. `bash` instead

of `bash-5.0` or `ksh` instead of `ksh93`). If all that doesn't produce a shell

that passes fatal bugs tests, it continues with the normal shell search.



This means the script won't fail to launch if the preferred shell can't be

found. Instead, it is up to the script itself to refuse to run if required

shell-specific conditions are not met. Script should use the

[`thisshellhas`](#user-content-shell-capability-detection)

function to check for any nonstandard

[capabilities](#user-content-capabilities)

required, or any

[bugs](#user-content-bugs)

or

[quirks](#user-content-quirks)

that the script is incompatible with (or indeed requires!).



Bundling is supported for both

[portable-form](#user-content-portable-form)

and

[simple-form](#user-content-simple-form)

modernish scripts. The installer automatically adapts the wrapper scripts to

the form used. For simple-form scripts, the directory containing the bundled

modernish core library (by default, `.../bndl/bin/modernish`) is prefixed to

`$PATH` so that `. modernish` works. Since simple-form scripts are often more

shell-specific, you may want to specify a preferred shell with the `-s` option.



To save space, the bundled copy of the modernish library is reduced such that

all comments are stripped from the code,

[interactive use](#user-content-interactive-use)

is not supported,

the [regression test suite](#user-content-appendix-b-regression-test-suite)

is not included,

[`thisshellhas`](#user-content-shell-capability-detection)

does not have the `--cache` and `--show` operators,

and the

[`cap/*.t` capability detection scripts](#user-content-appendix-a-list-of-shell-cap-ids)

are "statically linked" (directly included) into bin/modernish instead of

shipped as separate files.

A `README.modernish` file is added with a short explanation, the licence,

and a link for people to get the complete version of modernish. Please do

not remove this when distributing bundled scripts.



---



`EOF`