Today I Learned: collection of notes, tips and tricks and stuff I learn from day to day working with computers and technology as an open source contributor and product manager

View project on GitHub

Use pre-commit

The use of hooks in Git has fascinated me for a long time, but I have never gotten around do doing something awesome with this readily available tool. My very basic experiments has fallen victim to not fully getting it running or administration of hooks.

The other day I was listening to the Test & Code podcast, which had a guest: Anthony Sottile (@codewithanthony) who presented a framework for working with Git hooks, named: pre-commit.

A tool sounding like it offers exactly what I need…

This is lifted from the website of the pre-commit framework to give you a picture

As we created more libraries and projects we recognized that sharing our pre-commit hooks across projects is painful. We copied and pasted unwieldy bash scripts from project to project and had to manually change the hooks to work for different project structures.

And believe me this is a easy as 1-2-3.

The framework is installed on you local machine together with your hooks, but you can configure their use on a repository-by-repository basis as they are actually installed per repository.

What really got me excited was the sheer number of different hooks and many of the hooks support or use existing parts of my toolchain, like:

And I discovered some, which I had on my TODO list to evaluate:

And finally I discovered some tools I did not know at all:

And believe there are many more

Getting Started

Now I will demonstrate how easy it is go getting started

1. Install

When you have pre-commit installed (please refer to the website for details, I simply installed from Homebrew, both on macOS and in WSL).

$ brew install pre-commit

You might platform might require a different approach and good thing is that there are options to choose from.

2. Configure

You can copy-paste from other repositories or from the website.

Or you can get a skeleton configuration to get you started from pre-commit itself.

$ pre-commit sample-config
# See https://pre-commit.com for more information
# See https://pre-commit.com/hooks.html for more hooks
-   repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v3.2.0
    -   id: trailing-whitespace
    -   id: end-of-file-fixer
    -   id: check-yaml
    -   id: check-added-large-files

Let’s write the above configuration into a pre-commit configuration file, which can commit to our repository.

$ pre-commit sample-config > .pre-commit-config.yaml

The filename is uniform, so you can easily do a search on GitHub for examples and inspiration.

3. Test

The hooks can be for pre-commits or whatever phase you decide you want to handle, but you can actually test without committing, which is really useful for and when getting started.

$ pre-commit run  --all-files
[INFO] Initializing environment for https://github.com/pre-commit/pre-commit-hooks.
[INFO] Installing environment for https://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.................................................Failed
- hook id: trailing-whitespace
- exit code: 1
- files were modified by this hook

Fixing .github/dependabot.yml

Fix End of Files.........................................................Failed
- hook id: end-of-file-fixer
- exit code: 1
- files were modified by this hook

Fixing _config.yml
Fixing .github/dependabot.yml

Check Yaml...............................................................Passed
Check for added large files..............................................Passed

If this was a commit it would not go through. If you however was in a situation where you wanted the commit to go through git has the option --no-verify or -n to do exactly that.

So you can always do:

$ git commit -n iceberg_data.json -m "Latest data on icebergs"

pre-commit is not a part of our toolchain for this repository and we are ready to Go!


Now you have to consider a few things when forming a strategy for use of pre-commit. this is founded in why I have been reluctant to using pre-commit hooks.

  1. The hooks should not slow you down

    • I actually think that pre-commit is pretty fast, but the more evaluation you stack up, I expect it will eventually become slower

    • You can override the blocking behaviour as demonstrated above with --no-verify / -n

  2. The hooks you use, bring you feedback and value

    • Use only hooks that you need, the moment you can see that you often override them for pushing through commits it is time for them to go

    • The pre-commit hooks are installed in a non-intrusive manner, they are for you not necessarily your collaborators or colleagues, but the configuration is in the repository, so it not stopping them from using same configuration - do note the configuration is therefor a bit intrusive if you cannot decide on a common configuration

  3. The hooks are a supplement to your CI

    • pick hooks that give your fast actionable feedback and perhaps you can remove steps from your CI configuration and speed that up

    • Remember that CI helps you with PRs etc. coming from others non-necessarily using pre-commit, so it is a supplement

  4. A subset of hooks alter contents

    • Do note that some hooks alter contents of files. These are the hooks that do formatting and beautification etc.

    • If you are in a collaborative setting, use these hooks with caution and see to that you agree on formatting/use, so you do not end up in, tabs vs. spaces like discussions

    • Make sure that your diffs can be read and are useful, formatting should not be obfuscation, format for readability


Maintenance of hooks is just as easy as installing and using. Jump into your repository directory.

$ cd til
$ pre-commit autoupdate
Updating https://github.com/pre-commit/pre-commit-hooks ... [INFO] Initializing environment for https://github.com/pre-commit/pre-commit-hooks.
updating v2.3.0 -> v4.0.1.
Updating https://github.com/igorshubovych/markdownlint-cli ... already up to date.

Next steps

In addition to using all of the existing and marvelous hooks you can write your own.

I have just started on a hook myself, to remind me not to commit directly to main (master), but I can always overwrite you know.

If your hook offers something awesome, remember to share it a get it added to the listing so others can check it out and put it to use.

I did a lot of experiments with using my experimental hook from another repository, so it required some early releases and quite a few before I got it working. pre-commit does however offer a lot of testing facilities, I discovered afterwards.

Have fun and keep committing, now in style and hopefully with fewer stupid mistakes

Resources and References

  1. pre-commit.com
  2. pre-commit.com: Listing of hooks
  3. Test & Code Podcast
  4. Test & Code Podcast: Episode 157
  5. jonasbn/pre-commit-hooks