bash version detection is locale independent

[janLo]

The BASH_VERSION variable holds the bash version number in a much
better consumable and especially locale independent format than the
--version flag.

This fixes #1940

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[davidism]

Please rebase to 8.0.x and add a changelog entry.

[janLo]

Please rebase to 8.0.x and add a changelog entry.

Done.

[igrr]

Please note that this does not always return the version of bash where autocompletion has been invoked from.

Here's an example, taken from one macOS system:

$ which -a bash
/usr/local/bin/bash
/bin/bash
$ help | head -1
GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin18)
$ bash -c 'echo ${BASH_VERSION}'
4.4.23(1)-release
$ echo $BASH_VERSION
3.2.57(1)-release

Here, /bin/bash is the OS-provided bash 3.2.57, while /usr/local/bin/bash is bash 4.4.23 installed using Homebrew. The user profile is configured with /bin/bash as the login shell.

I don't see a good way to resolve this issue other than to tell the user that such setup is "broken". I'm mentioning it anyway in case someone else has an idea.

(This PR is still an important improvement over the previous bash --version approach, which also suffers from the issue I mentioned. So this comment shouldn't be considered as a blocker for merging the PR.)

[davidism]

If BASH_VERSION is available in the env, then couldn't we look at that directly?

[igrr]

Unfortunately BASH_VERSION is not exported by default, so while

$ echo ${BASH_VERSION}
4.4.23(1)-release

works,

$ python -c 'import os; print(os.environ.get("BASH_VERSION"))'
None

doesn't. This can be worked around by doing export BASH_VERSION before invoking click for autocompletion, but I think this would be a breaking change.

[janLo]

But the current code has the very same problem, as you initially mentioned @igrr, as it also just calls the first bash in $PATH.

[igrr]

I agree with that and with your proposed solution @janLo, I only mentioned this (extra) issue in case anyone looking at this PR comes up with a way to deal it.

[sscherfke]

I would change it to that for now:

        output = subprocess.run(["bash", "--version"], stdout=subprocess.PIPE)
        match = re.search(r"(\d+)\.(\d+)\.\d+", output.stdout.decode())

This solves the locale dependent spelling of version ans also adds support for multi-digit versions (e.g. 5.12.23). Since this still uses bash --version, it won’t introduce new issues.

So while this may not be the perfect solution, it will certainly improve the current state. :)

[janLo]

Why not use the version variable. This would completely avoid future surprises that always happen when one tries to parse data out of human readable descriptions one does not control.

The variable is at least clearly defined:

BASH_VERSION
The version number of the current instance of Bash.

(From https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html)