Updated on August 2nd, 2022 in #linux

git diff Has a Quiet Flag to Halt a Script If a File Was Updated

This can be really handy if you want to do something specific in a shell script only if a file has been changed in git.

Quick Jump: Demo Video

Recently I wanted to set up this type of workflow in CI:

• Script A maybe updates a Dockerfile
• Script B opens a pull request if Script A updated the Dockerfile
• The CI pipeline will continue to run with or without the Dockerfile being updated

It was important that these scripts were completely decoupled.

I created a little wrapper script to do the above and all I had to do was drop git diff --quiet Dockerfile && exit 0 in between running Script A and Script B. This way the wrapper script will halt and not open the PR if the file didn’t change.

There’s a couple of related flags with git diff:

• --quiet prevents the diff from showing and exits with 1 if there was a diff, 0 if not
• --exit-code will exit with 1 if there’s a diff and 0 if not but it’ll show the diff
• --no-patch will hide the output of the diff

The --quiet flag combines both --exit-code and --no-patch.

By the way I added && exit 0 so that the CI pipeline didn’t fail. I wanted it to continue if the Dockerfile didn’t change. If you didn’t have that requirement you can drop the && exit 0 and let the git diff --quiet Dockerfile command exit with 1 or 0 depending on the result.

Demo Video

Code

If you want to follow along, create a demo.sh file in a directory of your choosing with the contents below:

#!/usr/bin/env bash

set -o errexit
set -o pipefail
set -o nounset

git diff --quiet demo.sh && exit 0

echo "This file has changed, feel free to do whatever you need to do!"

Then run: chmod +x demo.sh && git init && git add -A && git commit -m "Initial commit"

And now you can run ./demo.sh to run the script.

Timestamps

• 0:20 – Going over the use case of maybe updating a Dockerfile and opening a PR
• 1:35 – Running the script to see how it works
• 2:44 – Using && exit 0 to prevent CI from halting
• 3:35 – Going over a few different related git diff flags

Reference Links

• https://nickjanetakis.com/blog/diff-selections-files-directories-and-git-history-with-vim

What are you going to diff using this tactic? Let me know below!