git diff can emit different exit codes

20 July 2020
github-actions,
git,
tools,
learning,
til

Combining git diff exit codes and the POSIX negation operator to control GitHub Actions job step execution based on git changes.

When defining a job step in GitHub Actions, you can specify a condition that must be met for a job step to run (in a broadly similar way to how things were in Job Control Language). In my profile repo's builder workflow, I wanted only to proceed with a git commit step if there were actual changes that had been made in a previous step.

Supplying the --exit-code option to git diff makes it emit an exit code of 1 if there are differences, and 0 if not. This option is implicit in --quiet, too.

As exit code 1 represents a fail in job step conditionals, the output of git diff can be reversed with the POSIX ! operator. So in this pair of steps, the second one will only run if there are no differences detected in the first one:

- name: Check for changes (fail if none)
  run: |
    ! git diff --quiet

- name: Commit changes if required
  if: ${{ success() }}
  run: |
    git config --global user.email "qmacro-bot@example.com"
    git config --global user.name "qmacro bot"
    git add README.md
    git commit -m 'update README' || exit 0
    git push

I had to write the ! git diff --quiet within a YAML multiline expression (introduced with |) as the GitHub Actions runner didn't like it on the same line, i.e. run: ! git diff --quiet.