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https://github.com/tokenrove/build-your-own-shell

Guidance for mollusks (WIP)
https://github.com/tokenrove/build-your-own-shell

shell tutorial unix workshop

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Guidance for mollusks (WIP)

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README 

        
> All the broken  

> Too many broken  

> Shells  

> In our shellmounds  

 — [Grayceon, Shellmounds](https://grayceon.bandcamp.com/track/shellmounds)



# Build Your Own Shell



This is the material for a series of workshops I ran at my workplace

on how to write a Unix shell.



The focus is slightly more on building an interactive shell than a

scripting-oriented shell, only because I think this is more

gratifying, even if it's less useful.



Be warned that some of the suggestions and discussion make opinionated

choices without discussing equally-valid alternatives.



This is a work in progress and there may remain many infelicities.

Patches Thoughtfully Considered.  Feel free to report issues via

[Github](https://github.com/tokenrove/build-your-own-shell/issues).



## Why write your own shell?



The shell is at the heart of Unix.  It's the glue that makes all the

little Unix tools work together so well.  Understanding it sheds light

on many of Unix's important ideas, and writing our own is the best

path to that understanding.



This workshop has three goals:



 - to give you a better understanding of how Unix processes work;

   - this will make you better at designing and understanding software

     that runs on Unix;

 - to clarify some common misunderstandings of POSIX shells;

   - this will make you more effective at using and scripting

     ubiquitous shells like bash;

 - to help you build a working implementation of a shell you can be

   excited about working on.

   - there are endless personal customizations you can make to your

     own shell, and can help you think about how you interact with

     your computer and how it might be different.



(some of this rationale is expanded on in my blog post, [Building

shells with a grain of salt])



[Building shells with a grain of salt]: https://www.cipht.net/2017/10/17/build-your-own-shell.html



## How to use this repository



I've tried to break this up into progressive stages that cover mostly

orthogonal topics.  Each stage contains a description of the

facilities that will be discussed, a list of manpages to consult, and

a set of tests.  I've tried to also hint at some functionality that is

fun but not necessary for the tests to pass.



In the root of this repository, there is a script called `validate`;

you can run all the tests against your shell-in-progress by specifying

the path to your shell's executable, like this:



``` shell

$ ./validate ../mysh/mysh

```



It should tell you what stage you need to implement next.  You can

also run a stage by itself, or an individual test:



```shell

$ ./validate ../mysh/mysh stage_2

$ ./validate ../mysh/mysh stage_3 03

```



To run the tests, you will need [`expect`], which is usually in a

package called `expect`, and a C compiler.  The way the tests are

implemented is less robust than one might hope, but should suffice for

our pedagogical goals.  They are unfortunately somewhat timing

sensitive, such that some tests will be flaky.  If you encounter a

specifically flaky test, please let me know.



The tests assume you will be implementing a vanilla Bourne-flavored

shell with some ksh influences.  Feel free to experiment with

alternate syntax, but if so, you may need to adjust the tests.  Except

where specifically noted, `bash` (and `ksh`) should pass all the

tests, so you can "test the tests" that way.  (Try `./validate

/bin/bash`.) Likewise, `cat` should fail all the tests.



Originally, I targeted plain `/bin/sh`, but I decided the material in

stage 5 was too important.  Still, `dash` will pass everything but

stage 5.  There are also some other minor compatibility differences

with some existing shells; you may run into them if you try them out.

Any failure (of a supposedly POSIX shell) that isn't documented in the

comments of a test should be reported as a bug.



[`prove`]: http://perldoc.perl.org/prove.html

[`expect`]: http://wiki.tcl.tk/201



# Stages



## 1: [fork/exec/wait](stage_1.md)



In which we discuss the basics of Unix processes, write the simplest

possible shell, and then lay the foundations for the rest of the

steps.



## 2: [files and pipes](stage_2.md)



In which we add pipes and fd redirection to our shell.



## 3: [job control and signals](stage_3.md)



In which we discuss signals and add support for ever-helpful chords

like `^C`, `^\`, and `^Z`.



## 4: [quoting and expansion](stage_4.md)



In which we discuss environments, variables, globbing, and other

oft-misunderstood concepts of the shell.



## 5: [interactivity](stage_5.md)



In which we apply some polish to our shell to make it usable for

interactive work.



## &: [where to go next](next-steps.md)



In which I prompt you to go further.



# Shells written from this workshop



I'll link to some of the shells that were written as a result of this

workshop here shortly, including a couple I wrote to serve as examples

of different approaches.



# Supplementary Material



## Tools



 - The [shtepper] is a great resource for understanding shell

   execution.  You can input an expression and see in excruciating

   detail how it should be evaluated.



[shtepper](http://shell.cs.pomona.edu/shtepper)



## Documents



 - [Advanced Programming in the Unix Environment] by Stevens covers

   all this stuff and is a must-read.  I call this *APUE* throughout

   this tutorial.

 - Chet Ramey describes [the Bourne-Again Shell] in [the Architecture

   of Open Source Applications]; this is probably the best thing to

   read to understand the structure of a real shell.

 - Michael Kerrisk's [the Linux Programming Interface], though fairly

   Linux-specific, has some great coverage of many of the topics we'll

   touch on.  I call this *LPI* throughout this tutorial.

 - [Unix system programming in OCaml] shows the development of a simple shell.

 - [Advanced Unix Programming] by Rochkind; chapter 5 has a simple shell.

 - the [tour of the Almquist shell] is outdated but may help you find

   where some things are implemented in `dash` and other `ash`

   descendants.



### Other Tutorials



I wrote this workshop partially because I felt other tutorials don't

go far enough, but all of these are worth reading, especially if

you're having trouble with a stage they cover:



 - Stephen Brennan's [Write a Shell in C] is a more detailed look at

   what is [stage 1](stage_1.md) here.

 - Jesse Storimer's [A Unix Shell in Ruby] gets as far as pipes;

 - Kamal Marhubi's [Let's Build a Shell] also goes about that far;

 - glibc's [Implementing a Job Control Shell] shows specifically how

   to implement job control;

 - Nelson Elhage's [Signalling and Job Control] covers some

   of [stage 3](stage_3.md)'s material.



### References



 - the [POSIX standard] explains the expectations for the shell and

   its utilities in reasonable detail.

 - there are [POSIX conformance test suites] but they don't seem to be

   available in convenient, non-restricted forms.

 - [yash's posix-shell-tests] are only runnable with yash, but the

   tests themselves are full of useful ideas.



[A Unix Shell in Ruby]: http://www.jstorimer.com/blogs/workingwithcode/7766107-a-unix-shell-in-ruby

[Advanced Programming in the Unix Environment]: http://www.apuebook.com/

[Advanced Unix Programming]: http://basepath.com/aup/

[Implementing a Job Control Shell]: https://www.gnu.org/software/libc/manual/html_node/Implementing-a-Shell.html

[Let's Build a Shell]: https://github.com/kamalmarhubi/shell-workshop

[POSIX standard]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/contents.html

[Signalling and Job Control]: https://blog.nelhage.com/2010/01/a-brief-introduction-to-termios-signaling-and-job-control/

[the Architecture of Open Source Applications]: http://www.aosabook.org/en/index.html

[the Bourne-Again Shell]: http://www.aosabook.org/en/bash.html

[the Linux Programming Interface]: http://man7.org/tlpi/index.html

[tour of the Almquist shell]: http://git.kernel.org/cgit/utils/dash/dash.git/tree/src/TOUR

[Unix system programming in OCaml]: https://ocaml.github.io/ocamlunix/

[Write a Shell in C]: https://brennan.io/2015/01/16/write-a-shell-in-c/

[POSIX conformance test suites]: https://www.opengroup.org/testing/testsuites/vscpcts2003.htm

[yash's posix-shell-tests]: https://github.com/posix-shell-tests/posix-shell-tests



## Shells to Examine



 - [busybox]: C; contains both ash and hush, and test suites.

 - [mksh]: C; non-interactive tests.

 - [rc]: C; fairly minimal.

 - [zsh]: C; extremely maximal.

 - [bash](https://savannah.gnu.org/git/?group=bash): C.

 - [fish]: C++11; has expect-based interactive tests.

 - [Thompson shell]: C; the original Unix shell; very minimal.

 - [scsh]: Scheme and C; intended for scripting.

 - [cash]: OCaml; based on scsh.

 - [eshell]: Emacs Lisp.

 - [oil]: Python and C++; has an extensive test suite.

 - [xonsh](http://xon.sh/): Python.

 - [oh](https://github.com/michaelmacinnis/oh): Go.

 - [yash-rs](https://github.com/magicant/yash-rs): Rust.



[busybox]: https://git.busybox.net/busybox/tree/shell

[cash]: https://github.com/ShamoX/cash

[eshell]: https://github.com/emacs-mirror/emacs/tree/master/lisp/eshell

[fish]: https://github.com/fish-shell/fish-shell

[mksh]: https://github.com/MirBSD/mksh

[oil]: https://github.com/oilshell/oil

[rc]: https://github.com/rakitzis/rc

[scsh]: https://github.com/scheme/scsh

[Thompson shell]: https://github.com/dspinellis/unix-history-repo/blob/Research-V5-Snapshot-Development/usr/source/s2/sh.c

[zsh]: https://github.com/zsh-users/zsh



## Links to Resources by Language



Although there is an elegant relationship between C and Unix which

makes it attractive to write a shell in the former, to minimize

frustration I suggest trying a higher-level language first.  Ideally

the language will have good support for:



- making POSIX syscalls

- string manipulation

- hash tables



Languages that provide a lot of their own infrastructure with regards

signals or threads may be much more difficult to use.



### C++



http://basepath.com/aup/ex/group__Ux.html



### Common Lisp



The most convenient library would be [iolib], which you can get

through [Quicklisp].  You'll need to install `libfixposix` first.

There's also [sb-posix] in `sbcl` for the daring.



[iolib]: https://github.com/sionescu/iolib

[Quicklisp]: https://www.quicklisp.org/

[sb-posix]: http://www.sbcl.org/manual/#sb_002dposix



### Haskell



 - use [the unix package](https://hackage.haskell.org/package/unix)

 - [Hell] might be a starting point



[Hell]: https://github.com/chrisdone/hell/



### Java / JVM-based languages



You will probably run into issues related to the JVM, particularly

with signals and forking, but as a starting point, you could do worse

than loading libc with JNA.



There's also [jtux](http://basepath.com/aup/jtux/index.htm).



### Lua



There are a variety of approaches, but [ljsyscall] looks promising.

[luaposix] might be sufficient.



[ljsyscall]: https://github.com/justincormack/ljsyscall

[luaposix]: https://github.com/luaposix/luaposix



### OCaml



 - [Unix module](https://caml.inria.fr/pub/docs/manual-ocaml/libref/Unix.html)

 - [lambda-term](https://github.com/diml/lambda-term)

 - [parsing with OCamllex and menhir](https://realworldocaml.org/v1/en/html/parsing-with-ocamllex-and-menhir.html)



See also [Unix system programming in OCaml], [cash].



### perl



See `perlfunc(3perl)`; all the functions we want are at hand, usually

with the same name.



### Python



Although Python provides higher-level abstractions like

[`subprocess`], for the purposes of this workshop you probably want to

use the functions in [`os`].



Please note an important gotcha for stage 2!  Since [Python 3.4], fds

have defaulted to non-inheritable, which means you'll need to

explicitly `os.set_inheritable(fd, True)` any file descriptor you

intend to pass down to a child.



[`os`]: https://docs.python.org/3/library/os.html

[`subprocess`]: https://docs.python.org/3/library/subprocess.html

[Python 3.4]: https://peps.python.org/pep-0446/



### Racket



The implementation seems a little too heavy to do this conveniently,

but see the Scheme section below for alternatives.



### Ruby



`Process` has most of what you need.  You can use `Shellwords` but you

decide if it's cheating or not.



### Rust



Although we use few enough calls that you could just create bindings

directly, either

[to libc with the FFI](https://doc.rust-lang.org/book/ffi.html) or by

[directly making syscalls](https://crates.io/crates/syscall), for just

getting something working, the [nix-rust] library should provide all

the necessary facilities.



[nix-rust]: https://github.com/nix-rust/nix



### Scheme



Guile already has all the calls you need; see

[the POSIX section of the Guile manual].  Another approach would be to

use something like [Chibi Scheme] with bindings to libc calls.



[Chibi Scheme]: https://github.com/ashinn/chibi-scheme

[the POSIX section of the Guile manual]: https://www.gnu.org/software/guile/manual/html_node/POSIX.html#POSIX



### Tcl



Although core Tcl doesn't provide what's necessary, `expect` probably

does.  For example, Tcl doesn't have a way to `exec`, but expect

provides `overlay` to do this.