Though I'm uncertain whether it's the correct syntax, here's a more comprehensive example.

get_value () {
   if [ -z {$1+x} ];
   then
      return $2
   else
      return $1
}

# ...

git add get_value $INPUT_FILE_PATTERN '.'

However, I assume this won't work if $INPUT_FILE_PATTERN is undefined(or whatever it's called in bash), see our discussion here. I'd therefore propose to find the root cause of the issue instead.

Thanks!
A function would be really helpful. Have to be honest here: Haven't used them much yet.

Will do some research and also trying to figure out, how we can tell GitHub Actions to resepect the default values for input options.

I've stumbled upon Publish-Docker-Github-Action which defines a uses() function to check whether the input param is set, so it'd make sense to stick to the suggested temporary workaround, though it's still an unexpected behavior.

Refer to b6dcf94. What are your thoughts on this approach? Would it make sense to make get_value less error-prone? E.g. when neither of the values are defined print a message to the console and exit 1.

Another approach would be to use solely the is_defined function similarly to your solution.

is_defined() {
    [ ! -z "${1}" ]
}

if is_defined "${INPUT_FILE_PATTERN}"; then
   git add  "${INPUT_FILE_PATTERN}"
else
   git add .
fi

if is_defined "${INPUT_COMMIT_OPTIONS}"; then
   git commit -m "$INPUT_COMMIT_MESSAGE" --author="$GITHUB_ACTOR <$GITHUB_ACTOR@users.noreply.github.com>" "${INPUT_COMMIT_OPTIONS}"
else
   git commit -m "$INPUT_COMMIT_MESSAGE" --author="$GITHUB_ACTOR <$GITHUB_ACTOR@users.noreply.github.com>"
fi

It might be a bit more readable and less error-prone.

Would it make sense to make get_value less error-prone? E.g. when neither of the values are defined print a message to the console and exit 1.

Good (and hard) question. I don't know if there's a common convention in bash scripts of what should happen if arguments are missing.

Also see next question/answer ↓.

Another approach would be to use solely the is_defined function similarly to your solution.

I personally like that a bit more. The code looks a bit cleaner and it's immediately clear what the code is doing.

So could you update the script with your if-else-code?

Have you tested your PR within GitHub Actions already? As far as I know, there's currently no easy way to test Actions locally.
(I have some ideas for automated testing, but didn't have the time yet to build it)

So could you update the script with your if-else-code?

Sure, check out af33cfb.

Have you tested your PR within GitHub Actions already?

Not quite, but I've already set up a test repo while I was trying to figure out why the Action doesn't respect default params.

Have you ever had such errors?

https://github.com/gomorizsolt/git-auto-commit-action-test-repo/commit/a27ce8f8ae3909482619b107916d566cace4dad5/checks?check_suite_id=290066630

Not a git repository
To compare two paths outside a working tree:
usage: git diff [--no-index] <path> <path>
fatal: not a git repository (or any parent up to mount point /github)

What do I miss?

Sorry, my bad, forgot to use actions/checkout@v1.

v2.3.0 has been released.

Thanks again!