Friday, April 26, 2019

Using Git hooks to alter commit messages

As developers we try to get the repetitive boring stuff out of our ways. Hence we try to use tools that automate some of our workflows, or if no tools is available for our specific needs, no problem, we automate them ourselves, we're developers after all!

In one of the projects I worked on, there was a convention to add the task id as part of each commit message because some tools are used to generate reports based on it. I'm not sure why this was required in that situation, but I had to follow the convention anyway. Since I tend to make many small commits every day, I was sure I'll forget to add the task id most of the time. So I started investigating Git hooks.

Git provides many hooks that could be used to automate some of the repetitive behaviors that are required to happen with the different life cycle steps of Git usage. For example:
    • Pre-commit
    • Pre-push
    • Prepate-commit-message
    • Commit-message

The folder ".git/hooks" within the git repository folder contains many sample commit hook files which are good starting points. The one of interest in this case was the commit-msg hook.

In my scenario, we had a convention to name our branches using the patterns "feature/" or "bug/".

So I decided to deduce the task id from the branch name and prepend it to the commit message.
I created a file with the name commit-msg in the .git/hooks folder, the code inside this file is similar to:

message=$(cat $1)
branch=$(git branch | grep \* | cut -d ' ' -f2-)
task=$(echo $branch | cut -d / -f2-)
echo "$task - $message" > $1
  • Line 2: reads the original commit message from the temp file, whose name is passed as the first parameter to the script.
  • Line 3: reads the current branch name. Thanks to StackOverflow.
  • Line 4: extracts the task id from the branch name by splitting the string by the "/" character and taking the second part.
  • Line 5: overwrites the commit message with the required format.

Now when I commit code using:
git commit -m"test message"
And then inspect the logs using git log command, the commit message is modified as needed:
commit f1fe8918c754ca89649a2a86ef4ab0a9a53c0496 (HEAD -> feature/1234)
Author: Hesham A. Amin
Date:   Fri Apr 26 08:24:40 2019 +0200

    1234 - test message

commit 4e3e180d3a27772a32230bf6dbbd039b949dc30e

Investing few minutes to automate daunting repetitive tasks pays off on the long term.

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