til/git/check-if-a-file-has-changed-in-a-script.md

Check If A File Has Changed In A Script

If I'm at the command line and I want to check if a file has changed, I can run git diff and see what has changed. If I want to be more specific, I can run git diff README.md to see if there are changes to that specific file.

If I'm trying to do this check in a script though, I want the command to clearly tell the script Yes or No. Usually a script looks for an exit code to determine what path to take. But as long as git diff runs successfully, regardless of whether or not their are changes, it is going to have an affirmative exit code of 0.

This is why git diff offers the --exit-code flag.

Make the program exit with codes similar to diff(1). That is, it exits with 1 if there were differences and 0 means no differences.

With that in mind, we can wire up a script with git diff that takes different paths depending on whether or not there are changes.

if ! git diff --exit-code README.md; then
  echo "README.md has changes"
else
  echo "README.md is clean"
fi

We can take this a step further and instead use the --quiet flag.

Disable all output of the program. Implies --exit-code. Disables execution of external diff helpers whose exit code is not trusted

This exhibits the same behavior as --exit-code and goes the additional step of silencing diff output and disabling execution of external diff helpers like delta.

See man git-diff for more details.