cmd/shfmt: add ability to skip unknown (non-shell) files

[tbcs]

If scripts in a git repository don't have a .sh extension and are spread across various directories it's cumbersome to use shfmt in a pre-commit hook. It would be useful if shfmt could skip non-shell files when invoked from a git hook with the list of files being changed (as it is the case with pre-commit).

This is similar to #288, but different, because the desired behavior here would be that not explicit configuration is required. Rather, shfmt should "magically" skip non-shell files.

[mvdan]

Hm. I really don't want to add more options, particularly when the use case is a bit niche, and when we already have very similar flags like -f.

Here's another idea. What about allowing -f to work on files too? For example:

$ shfmt -f no-shebang shell-shebang extension.sh
shell-shebang
extension.sh

That way, you could do something like shfmt -w $(shfmt -f $FILES). It does invoke the tool twice, but I doubt the overhead in this case matters much, and it allows the tool to remain simpler and more composable.

[tbcs]

Here's another idea. What about allowing -f to work on files too?

What do you mean by "allowing"? Because the -f flag "works" on files as of the current (3.0.1) version as well, it's just that the output is nonsensical: it would list any file specified as part of the result set, regardless of its detected type (it happily echoes the name of a .jar file).

If you meant that the behavior of -f could be changed to the one you described then yes, I think that would be a good thing to do! It would still require a wrapper script for a generic "reformat all shell scripts" pre-commit hook, but at least it would not need to scan the whole repository for scripts just to discard (most likely) the majority of them.

[tbcs]

@mvdan: please see #485

[mvdan]

I could have sworn I replied to your comment a week ago, but I assume it got lost.

Yes, I do mean changing the behavior of -f. Requiring a bit of extra glue is fine, I think. The tool is general purpose, and not just for pre-commit scripts.