How does the git diff –exit-code option work and when would it be useful?

The git diff --exit-code option is an interesting flag you can use with the git diff command to produce a useful exit status. To understand how this option works, you first need to understand the purpose of the git diff command and what exit codes are in the context of git or command line programs generally.

The git diff command is typically used to show changes between commits, commit and working tree, etc. It shows the changes between the source and destination specified, be it commits, branches, or the staging area.

On the other hand, exit codes are a way for command line programs to communicate the success or failure of a command. By convention, an exit code of 0 usually indicates that a program has executed successfully, whereas any non-zero exit code suggests some kind of error or issue.

So, when you use the git diff --exit-code option, it changes the behavior of the git diff command slightly: it will return an exit status of 0 if there are no changes and a non-zero status if there are changes. By default, git diff will always return a 0 exit status, regardless of whether there are differences or not, because its primary purpose is to display those differences, not to determine whether they exist.

The git diff --exit-code command becomes incredibly useful in scripting and automation scenarios. For instance, in a Continuous Integration (CI) setup, you might want to check if there are uncommitted changes in the repository before proceeding with a build or a deploy process. In this case, the script could look something like this:

if git diff --exit-code; then
    echo "No changes detected, proceeding with the build..."
    echo "Changes detected! Please commit them before building."
    exit 1

In this script, if git diff --exit-code returns 0 (i.e., no changes detected), the build process continues. But if there are changes (exit status is non-zero), it stops the process and prompts the user to commit changes before proceeding.

In conclusion, git diff --exit-code is a powerful tool that you can use to automate workflows and ensure that operations like build and deployment occur only when the working tree is clean and all changes are committed.