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https://github.com/icy/genvsub

Another way to substitute environment variables in shell format strings ${FOO}, designed for k8s stuff
https://github.com/icy/genvsub

helm k8s kustomization shell substitution template-engine terraform variable

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Another way to substitute environment variables in shell format strings ${FOO}, designed for k8s stuff

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README 

        
![icy](https://github.com/icy/genvsub/workflows/icy/badge.svg)



## Alternative to Gettext's envsubst



Substitutes environment variables in shell format strings,

and can *raise error* if any environment is not set.

There are two kinds of format string `$FOO` and `${FOO}`.

This tool only works with the later form and can serve a simple

template engine for your shell scripts ;)

The program can also limit its action to a small set of variables

whose names match a predefined prefix/regexp.



## TOC



* [Usage](#usage)

  * [Supported options](#supported-options)

  * [Installation. Examples](#installation-examples)

  * [Note on variable prefix](#note-on-variable-prefix)

* [Development](#development)

  * [Smoke tests](#smoke-tests)

* [References](#references)

* [License](#license)



## Usage



### Supported options



* `-v`: Scan and output all occurrences of variables in the input.

* `-u`: Raise error when environment variable is not set.

        When being used with `-v`, the program scans through the whole

        input; otherwise, the program stops immediately when there is

        any undefined (environment) variable.

* `-p regexp`: Limit substitution to variables that match this prefix.

        You can use some regular expression as prefix.

        Default to `[^}]+`. Can be used as an alternative

        to `SHELL-FORMAT` option in the original GNU `envsubst`



It's highly recommended to use `-u` option. It's the original idea

why this tool was written.



### Installation. Examples



Starting from `v1.2.2`, you can download binary files generated automatically

by Github-Action action (via goreleaser tool). You find the files from

the release listing page: https://github.com/icy/genvsub/releases



To install on your laptop by local compiling process, please try the popular way



    $ go get -v github.com/icy/genvsub

    $ export PATH=$PATH:$(go env GOPATH)/bin



If you don't want to install, you can use it like this:



    $ echo '${HOME}' |  go run github.com/icy/genvsub@latest -u

    

The program works with `STDIN` and write their output to `STDOUT`



    $ echo 'My home is $HOME'   | ./genvsub # This will output "My home is $HOME"

    $ echo 'My home is ${HOME}' | ./genvsub # This will output "My home is /home/your-home-path"

    $ echo 'Raise error with unset variable ${XXHOME}' | ./genvsub -u



To limit substitution to variables that match some prefix, use `-p` option:



    $  echo 'var=${TEST_VAR}' | ./genvsub -u -p SAFE_

    :: genvsub is reading from STDIN and looking for variables with regexp '\${(SAFE_)}'

    var=${TEST_VAR}



    $ echo '${TEST_VAR}' | ./genvsub -u -p 'TEST_.*'

    :: genvsub is reading from STDIN and looking for variables with regexp '\${(TEST_.*)}'

    

    :: Environment variable 'TEST_VAR' is not set.



The second command raises an error because the variable `TEST_VAR` matches

the expected prefix `TEST_` and its value is not set.



You can also specify exactly a few variables to be substituted

(which is exactly an alternative to the `shell-format` option

in the original GNU tool `envsubst`):



    $ echo '${TEST_VAR}' | ./genvsub -u -p 'VAR_NAME_3|VAR_NAME_3|VAR_NAME_3'



### Note on variable prefix



When using `-p string` to specify the variable prefix, you can also use

some simple regular expression. However, please note that for the given

input argument `-p PREFIX`, the  program will build the final regexp

`\${(PREFIX)}`.



1. Hence you can't use for example `-p '^FOO'`.

2. You can also easily trick the program with some fun `PREFIX` ;)

   However, as seen in

   https://github.com/icy/genvsub/blob/33e68048c6fe4b6ca0befadbc9fa5c19055ede8b/sub.go#L42

   the program enforces input data to follow the form `${VARIABLE_NAME}`.

   I'm still thinking if we can allow more tricks here.



## Problems



Clean up on Dec 18th 2020.



## Development



### Smoke tests



We don't likely have tests with `Golang`. We have some smoke tests instead.



```

$ make build tests

```



Tests are written in `shell` script.

Please have a look at [tests/test.sh](tests/test.sh).



### Ruby version



Removed on Dec 18th 2020.



## References



- [ ] Original tool: https://www.gnu.org/software/gettext/manual/html_node/envsubst-Invocation.html

    (which can't raise error if some variable is not set.)

- [ ] https://github.com/a8m/envsubst : A Go tool; It tries to behave like Bash with some

     advanced feature dealing with empty/non-set/default values

     https://github.com/a8m/envsubst#docs; it can also raise error if some variable

     is empty and/or not-set. This tool has a few features that I don't need,

     and at the same time it doesn't have some features I need (reg-exp variable name

     filter, strict variable naming format, bla bla)

- [ ] https://github.com/gdvalle/envsub : A Rust tool; It introduces new syntax `%VAR%`,

      which can be refined with `ENVSUB_PREFIX=%` and `ENVSUB_SUFFIX=%`.

      When hitting unset variables it will exit rather than expanding as empty strings.

      It also fully buffers input before writing, so in-place replacement is possible.

- [ ] https://github.com/s12v/exec-with-secrets: Fetch configuration/secrets/variables

      at run time, and provides them to your program. Now you have to rebuild your

      docker images with the tool atop your original `executable` command.



## License



This work is writtedn by Ky-Anh Huynh

and it's released under a MIT license.