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shellcheck-repl

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Validation of Shell Commands at the Prompt Before Evaluation

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# ShellCheck REPL: Validation of Shell Commands Before Evaluation

[ShellCheck] is a great tool for validating your Unix shell scripts.
It will parse the scripts and warn about mistakes, errors, and
potential problems.  This tool - **shellcheck-repl** - brings
ShellCheck validation to the [Bash] read-eval-print loop (REPL),
i.e. the [Bash] prompt.  Getting this type of validation and feedback
at the prompt lowers the risk of damaging mistakes and will help you
become a better Bash user and developer.

The **shellcheck-repl** tool injects itself into the Bash REPL where
it intercepts the read command line, validates the content via
ShellCheck, and if it is all OK, then the command is evaluated and
printed as usual.  However, if there is a mistake, then the command
will _not_ be evaluated and an informative error message is displayed
instead.  For example, assume we do:

```sh
$ words="lorem ipsum dolor"
$ for w in "$words"; do echo $w; done
```

Although this looks like a simple for loop, it might not be clear to
you what the outcome of it will be.  What values will `$w` take?
However, with **shellcheck-repl** enabled, we will get the following
if we try call it:

```sh
$ for w in "$words"; do echo "$w"; done
           ^-- SC2066: Since you double quoted this, it will
           not word split, and the loop will only run once.
```

So, what [SC2066] suggests is that the output will be a single line
`lorem ipsum dolor` and not three words on three separate lines.  We
probably meant to use:

```sh
$ for w in $words; do echo "$w"; done
lorem
ipsum
dolor
```

## Bypassing the ShellCheck validation

You can bypass the ShellCheck validation by appending two spaces to
the command with.  For instance, assume we get:

```sh
$ words="lorem ipsum dolor"
$ echo $words
       ^-- SC2086: Double quote to prevent globbing and word splitting.
```

Ideally we should call:

```sh
$ echo "$words"
lorem ipsum dolor
```

but if we find that too tedious, we can skip the validation by
appending two or more spaces at the end:

```sh
$ echo $words␣␣
lorem ipsum dolor
```

By the way, one example where [SC2086] is crucial is when you work
with filenames.  Using:

```sh
$ rm $file␣␣
```

can be very risky if `$file` contains spaces - in addition to not
removing the file intended, you might end up removing files that you
did not intend to remove.

## Disable and enable checks

To disable the ShellCheck REPL tool, do:

```sh
$ sc_repl_disable
```

To reenable it, do:

```sh
$ sc_repl_enable
```

## Settings

### ShellCheck rules to ignore

Some of the ShellCheck rules may be too tedious to follow when on the
command line.  For example, when trying to change directory to a
non-existing directory, `cd` will produce a non-zero exit code.  If
you do not handle this type of error in a script, ShellCheck will
report on [SC2164];

```sh
cd /path/to
^-- SC2164: Use 'cd ... || exit' or 'cd ... || return' in case cd fails.
```

The suggestion is really valid for scripts, but for the command line
it is just annoying.  Because of this, **shellcheck-repl** disables
the check for SC2164 by default.  In addition, it also disables the
validation of other ShellCheck rules that are too tedious or simply
false-positives when used at the command line. Here is the complete
list:

* [SC1001]: This \= will be a regular '=' in this context.
* [SC1090]: Can't follow non-constant source. Use a directive to
  specify location.
* [SC1091]: Not following: (error message here).
* [SC1113]: Use #!, not just #, for the shebang.
* [SC2034]: 'var' appears unused. Verify it or export it.
* [SC2096]: On most OS, shebangs can only specify a single parameter.
* [SC2155]: Declare and assign separately to avoid masking return
  values.
* [SC2164]: Use 'cd ... || exit' or 'cd ... || return' in case cd
  fails.

This set of rules that are disabled by default can be configured via
environment variable `SHELLCHECK_REPL_EXCLUDE` by specifying rules
(without `SC` prefix) as a comma-separated list.  The default
corresponds to
`SHELLCHECK_REPL_EXCLUDE=1001,1090,1091,1113,2034,2155,2164`.

### Disable and enable hints

When there's a ShellCheck issue, a hint on how to disable that issue
is also outputted, e.g.

```sh
$ msg="Value: $value"
              ^----^ SC2154 (warning): value is referenced but not assigned.

To skip a check, add its SC number to 'SHELLCHECK_REPL_EXCLUDE', e.g.

export SHELLCHECK_REPL_EXCLUDE="${SHELLCHECK_REPL_EXCLUDE},4038"

Currently, SHELLCHECK_REPL_EXCLUDE=1001,1090,1091,1113,2034,2155,2164

To skip ShellCheck validation for this call, append two spaces
```

This message can be disabled by setting:

```sh
SHELLCHECK_REPL_VERBOSE=false
```

## Requirements

* [ShellCheck]
* [Bash] (>= 4.4)

Bash is the only supported shell right now.

## Installation

Download the `shellcheck-repl.bash` script and source it in your
`~/.bashrc` startup script, e.g.

```sh
$ curl -L -O https://github.com/HenrikBengtsson/shellcheck-repl/archive/refs/tags/0.4.3.tar.gz
$ tar xf 0.4.3.tar.gz
$ echo ". /path/to/software/shellcheck-repl-0.4.3/shellcheck-repl.bash" >> ~/.bashrc
```

Or, similarly, via Git:

```sh
$ cd /path/to/software
$ git clone https://github.com/HenrikBengtsson/shellcheck-repl.git
$ echo ". /path/to/software/shellcheck-repl/shellcheck-repl.bash" >> ~/.bashrc
```

## Authors

* GitHub user [xPMo](https://github.com/xPMo) - [original code from 2019-03-28](https://github.com/koalaman/shellcheck/issues/1535#issuecomment-477633465)
* Henrik Bengtsson

## Appendix

### Example: ShellCheck REPL prevents Bash fork bomb

A well-known [fork bomb](https://en.wikipedia.org/wiki/Fork_bomb) in Bash is `:(){ :|:& };:`.  If launched, it will recursively relaunch itself via piping and background jobs.  With ShellCheck REPL enable, we will be prevented from executing the call, e.g.

```sh
$ :(){ :|:& };:
     ^-- SC2264 (error): This function unconditionally re-invokes itself. Missing 'command'?
       ^-- SC2264 (error): This function unconditionally re-invokes itself. Missing 'command'?
```

[ShellCheck]: https://github.com/koalaman/shellcheck
[Bash]: https://www.gnu.org/software/bash/
[SC2066]: https://github.com/koalaman/shellcheck/wiki/SC2066
[SC2086]: https://github.com/koalaman/shellcheck/wiki/SC2086
[SC1001]: https://github.com/koalaman/shellcheck/wiki/SC1001
[SC1090]: https://github.com/koalaman/shellcheck/wiki/SC1090
[SC1091]: https://github.com/koalaman/shellcheck/wiki/SC1091
[SC1113]: https://github.com/koalaman/shellcheck/wiki/SC1113
[SC2034]: https://github.com/koalaman/shellcheck/wiki/SC2034
[SC2096]: https://github.com/koalaman/shellcheck/wiki/SC2096
[SC2155]: https://github.com/koalaman/shellcheck/wiki/SC2155
[SC2164]: https://github.com/koalaman/shellcheck/wiki/SC2164
[#21]: https://github.com/HenrikBengtsson/shellcheck-repl/issues/21

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