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3rd chunk of `cookbook/foreign_shell_scripts.md`
e8d79de31bfe0ab7603253b0205874cdf985f1fdf8c9ff9d0000000100000879
    # The shell to run the script in
    # (has to support '-c' argument and POSIX 'env', 'echo', 'eval' commands)
    --arguments (-a): list<string> = []
    # Additional command line arguments to pass to the foreign shell
] {
    let script_contents = $in;
    # ...
}
```

We're declaring a custom command that takes two optional flags:

- `--shell` to specify a shell to run the script in, (e.g. `bash`)
- `--arguments` to parse further command line arguments to that shell.

The actual script is not mentioned here, because it is read using the special
`$in` variable that represents anything passed to Standard Input (`stdin`), e.g.
via a pipe.

The shell is set to `/bin/sh` by default, because this is often considered the
"default" POSIX-compatible shell of UNIX-like systems, e.g. macOS or Linux. It
is often not running the original Bourne shell (`sh`), but linking to a
different shell, like `bash`, with some compatibility flags turned on.

As such, many "generic" shell scripts to source are compatible with the system'
s `/bin/sh`.

Now, let's have a look at where the shell is actually run:

```nu
let env_out = with-env { SCRIPT_TO_SOURCE: $script_contents } {
    ^$shell ...$arguments -c ` ... `
}
```

Essentially, this calls the specified shell (using `^` to run the value as a
command) with any arguments specified. It also passes `-c` with an inlined
script for the shell, which is the syntax to immediately execute a passed script
and exit in most shells.

The `with-env { SCRIPT_TO_SOURCE: $script_contents }` block defines an
additional environment variable with the actual script we want to run. This is
used to pass the script in an unescaped string form, where the executing shell is entirely responsible for parsing it. The alternatives would have been:

- Passing the script via `-c $script`, but then we couldn't (safely) add our own
  commands to log out the environment variables after the script ran.
- Using string interpolation, but then we would be responsible for fully
  escaping the script, so that the `eval "($script)"` line doesn't break due to
  quotation marks. With the variable expansion in the foreign shell, that shell
Title: Detailed Explanation of Shell Execution in `capture-foreign-env`
Summary
The `capture-foreign-env` command utilizes the `--shell` and `--arguments` flags to define how the foreign shell script is executed. The script content is passed via stdin using the `$in` variable. The shell defaults to `/bin/sh`, which is generally POSIX-compatible. The actual execution happens within a `with-env` block, where the script is assigned to the `SCRIPT_TO_SOURCE` environment variable. The shell is invoked with `-c` and an inlined script, enabling the execution of the script and additional commands to log environment variables. This approach avoids the complexity of escaping the script content, as the shell is responsible for parsing it.