Git comes with support for action sequences based on repository activity. Pushes, pulls, merges, and commits can all be configured to trigger specific custom actions responsively. Often, the custom actions are geared towards promoting intra-team communication, process-gating like enforcing commit log standards and best practices for code content/syntax. Of course, CI workflows already are a popular frontline of defense for bad code pushes by using linters and syntax checks as part of an initial test stage in a pipeline. However, those linting processes have a cost in terms of compute resources they utilize. In a large development environment, this can translate into real money costs pretty quickly. Why spend compute cycles on pipeline jobs that can be just as easily run on a developer’s workstation or laptop? So, let’s take a closer look at git hooks…

Git hooks are configured in a given repo via files located in /.git/hooks. A new repository will be automagically populated with these handy tools, which are inactive by default thanks to the file suffix “.sample”:

[rcrelia@fuji hooks (GIT_DIR!)]$ ls -latr
total 40
-rwxr-xr-x 1 rcrelia staff 3611 Dec 22 2014 update.sample
-rwxr-xr-x 1 rcrelia staff 1239 Dec 22 2014 prepare-commit-msg.sample
-rwxr-xr-x 1 rcrelia staff 4951 Dec 22 2014 pre-rebase.sample
-rwxr-xr-x 1 rcrelia staff 1356 Dec 22 2014 pre-push.sample
-rwxr-xr-x 1 rcrelia staff 1642 Dec 22 2014 pre-commit.sample
-rwxr-xr-x 1 rcrelia staff  398 Dec 22 2014 pre-applypatch.sample
-rwxr-xr-x 1 rcrelia staff  189 Dec 22 2014 post-update.sample
-rwxr-xr-x 1 rcrelia staff  896 Dec 22 2014 commit-msg.sample
-rwxr-xr-x 1 rcrelia staff  452 Dec 22 2014 applypatch-msg.sample
drwxr-xr-x 11 rcrelia staff 374 Dec 22 2014 .
drwxr-xr-x 15 rcrelia staff 510 Jan 4 2015 ..

Having individual hooks like these provides a powerful framework for customizing your repository usage to your specific needs and workflows. Each one is triggered at the stage of git workflow described by the filename (pre-commit, post-update, etc.)

Tools like linters fit nicely into the pre-commit action sequence. By configuring the pre-commit hook with a linter, you are delivering higher quality code to your pipelines which makes for a more efficient use of your compute resource budget.

Hook Management: Yelp’s pre-commit

I recently started using an open source utility released by Yelp’s engineers called simply, “pre-commit”. Essentially, it is a framework for managing the pre-commit hook in a git repository, using a single configuration file with multiple action sequences. This allows for a single pre-commit hook to do many different sorts of actions. It includes some basic linter capabilities as well as other code quality control features, but also is integrated with other projects (e.g., ansible-lint has a pre-commit hook available).

Setup is straightforward, as is the usage. Here’s how I did it:

pip install pre-commit
cd repodir
pre-commit install
# edit a new file called .pre-commit-config.yaml in the root of your repo
git add .pre-commit-config.yaml
git commit -m "turn on pre-commit hook"

Immediately you should see the pre-commit utility do its thing when you commit this or any other change to your repository.

Here is the current working version of my pre-commit config file (one per repo):

- repo: git://github.com/pre-commit/pre-commit-hooks
  sha: v0.7.1
  hooks:
   - id: trailing-whitespace
     files: \.(js|rb|md|py|sh|txt|yaml|yml)$
   - id: check-json
     files: \.(json|template)$
   - id: check-yaml
     files: \.(yml|yaml)$
   - id: detect-private-key
   - id: detect-aws-credentials
- repo: git://github.com/detailyang/pre-commit-shell
  sha: 6f03a87e054d25f8a229cef9005f39dd053a9fcb
  hooks:
   - id: shell-lint

So, I’m using some of pre-commit’s built-in handlers for whitespace cleanup, JSON linting, YAML linting, and checking to make sure I don’t include any private keys or AWS credentials in my commits. Also, I’ve integrated a third-party tool, pre-commit-shell, that is a wrapper to shellcheck for syntax checking and enforcing best practices in any shell scripts I might add to the repo.

And here is an example of a code commit that triggers pre-commit’s operation:

[rcrelia@fuji aws-mojo (master +=)]$ git commit -m "pre-commit"
[INFO] Installing environment for git://github.com/pre-commit/pre-commit-hooks.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
Trim Trailing Whitespace.................................................Passed
Check JSON...........................................(no files to check)Skipped
Check Yaml...............................................................Passed
Detect Private Key.......................................................Passed
Detect AWS Credentials...................................................Passed
Shell Syntax Check...................................(no files to check)Skipped
[master 7d837e7] pre-commit
 1 file changed, 15 insertions(+)
 create mode 100644 .pre-commit-config.yaml

While pre-commit doesn’t handle management of the other available Git hooks, it does a very good job with what it does control, with a robust plugin interface and the ability to write custom hooks.

If you find yourself in need of some automated linting of your code before you push to your remote repositories, I highly recommend the use of pre-commit for its ease of use and operational flexibility.

Happy coding!