GIT: Using Git Hooks

Git hooks are similar to SVN hooks; you can execute a script at a point in the Git routine. Git hooks are both local and server-side. The local hooks pertain to local activities, like committing, merging, checking out, etc. The server-side hooks deal with receiving pushes from a local client.

The scripts that Git hooks execute are stored in the .git/hooks directory of your project. In every repository there are a collection of sample hooks that you can rename and use as inspiration for your own.

The scripts can be in any language that support executable scripts. The samples are mostly shell scripts, but you can use Perl, Ruby, Python, or something else that is familiar to you.

Here are all the hooks available in Git:

• pre-commit
• prepare-commit-msg
• commit-msg
• post-commit
• applypatch-msg
• pre-applypatch
• post-applypath
• pre-rebase
• post-rewrite
• post-checkout
• post-merge
• pre-push
• pre-receive
• update
• post-receive

Local Git Hooks

Local Git hooks reside in the .git/hooks directory and are not treated as project files. They are not tracked in the Index and because of that they are not included when someone clones a repository.

Because of this there are some limitations to keep in mind while working with Git hooks. First and foremost: local hooks are not a reliable way to enforce policies, like a commit message structure. And, because you are unable to include the contents of the .git/hooks directory in version control, you will need to consider a way to share hooks that you’d like your team to use.

Here are the local hooks:

• pre-commit
• prepare-commit-msg
• commit-msg
• post-commit
• applypatch-msg
• pre-applypatch
• post-applypath
• pre-rebase
• post-rewrite
• post-checkout
• post-merge
• pre-push

Server-side Hooks

Server-side hooks are set up to kick off scripts when a typical remote repository actions takes place.

There are only three server-side hooks:

• pre-receive
• update
• post-receive

They all operate around when the server receives a git-push from a client. These are reliable methods for enforcing some sort of repository or standard. You can run checks, kick of an integration script, testing, etc.

Implement a Git Hook

For our example we’re going to implement a local hook. We’d like to display a message after each commit, reminding us to push our commits to the remote repository.

The setup for this message will be to use the post-commit hook. This hook is triggered after each successful commit. For our implementation, we want the hook to trigger and display a message for us. 
There isn’t a sample file for post-commit already in place. That’s okay, we can create it.

Create a new file called post-commit in .git/hooks.

I’m going to use Ruby as the scripting language. Remember, Git Hooks can use any executable script. But I like Ruby so we’ll use that. This Ruby code is as simple as it comes, with just a single puts statement.

Add this code to the post-commit file:

#!/usr/local/bin
puts "======================="
puts "Please remember to push your commits!"
puts "======================="

With the code in place, we can then save the hook file and set it as executable.

$ chmod +x post-commit

Test it by making a change to our project and creating a commit. If you see the message, it worked!

If you didn’t, check your code, that the hook file is executable, and that you have it properly named.

In another article I’ll talk about triggering hooks on the server.

GIT: Git commit and other hooks

50.1. Usage of Git hooks

Git provides commit hooks, e.g., programs which can be executed at a pre-defined point during the work with the repository. For example, you can ensure that the commit message has a certain format or trigger an action after a push to the server.

These programs are usually scripts and can be written in any language, e.g., as shell scripts or in Perl, Python etc. You can also implement a hook, for example, in C and use the resulting executables. Git calls the scripts based on a naming convention.

50.2. Client and server side commit hooks

Git provides hooks for the client and for the server side. On the server side you can use the pre-receive and post-receive script to check the input or to trigger actions after the commit. The usage of a server commit hook requires that you have access to the server. Hosting providers like GitHub or Bitbucket do not offer this access.

If you create a new Git repository, Git creates example scripts in the .git/hooks directory. The example scripts end with .sample. To activate them make them executable and remove the .sample from the filename.

50.3. Restrictions

Not all Git server implementations support server side commit hooks. For example Gerrit (a Git server which also provides the ability to do code review) does not support hooks in this form. Also Github and Bitbucket do not support server hooks at the time of this writing.

Local hooks in the local repository can be removed by the developer.