Pre-commit Hooks in Git

Remembering to run your tests before you commit is hard:

* 84f7e2e 2016-02-06 | Another typo. And test[David Wickes]
* ef215f8 2016-02-06 | OMFG semicolons WAAAAAT [David Wickes]
* c20b65d 2016-02-06 | Typo [David Wickes]
* fca4aa8 2016-02-06 | Another fix for the same test[David Wickes]
* f0206a9 2016-02-06 | Fixes failing test [David Wickes]
* 657cc48 2016-02-06 | Amazing new feature [David Wickes]

Yes, I suck. It’s even worse when you’ve just pushed that teensy-tiny, insignificant change to the CI pipeline and it throws a strop because the JS linter is super fussy about semi-colons.

Don’t get angry. Get even.

Wait, wut? Don’t get even. Automate all the things!

Inside the unexamined recesses of the .git directory of every repo is a folder called hooks. You should look inside it.

applypatch-msg.sample
commit-msg.sample
post-update.sample
pre-applypatch.sample
pre-commit.sample # <--- This one here!
pre-push.sample
pre-rebase.sample
prepare-commit-msg.sample
update.sample

You’ll see some pretty self-explanatory instructions on what it does, but the tl;dr is:

• It is a shell script that runs before you commit
• You activate it by removing the .sample bit.

So say we have a Node project we’re working on. A cool pre-commit hook would look like:

#!/bin/sh

npm test

Pop that in a file called pre-commit inside that hooks directory - make sure it’s executable like the sample ones - and see what you get.

So as long as you’ve set up you package.json file sensibly to run tests when you run npm test you’re golden. Same idea holds for rake or gradle or whatever you’re using as a task runner.

But you’d hate to do that for every project, right? Automate that too.

Try this:

$ git config --global init.templatedir '~/.git-templates'
$ mkdir -p ~/.git-templates/hooks

Inside the equally unexamined .gitsettings in your home directory you should now see:

[init]
    templatedir = ~/.git-templates

(you could’ve just written that in there by hand… but here we are)

What this’ll do is copy the contents of .git-templates to the .git directories of new projects you clone or initialize.

We now need to make our hook more generic. Let’s save the below off in ~/.git-templates/hooks/pre-commit:

#!/bin/sh

if [ -f package.json ]; then
    echo "detected package.json... running npm test"
    npm test
else
    echo "no testable project detected... so no tests before commit"
fi

[ -f package.json ] asks if there’s a file called package.json, and if there is we run npm test. The rest of the script is just a little logging so we can see what’s happening. Just remember to make it executable¹ before you start to use it.

There we have it - the bare bones of a “generic” pre-commit hook. You can easily embelish this with more tests and other exciting/useful/amusing things to execute (is there a Rakefile present? Do any of the files I’m commiting still contain a console.log or a puts?).

As I said, this template will get added to everything that gets cloneed or initialized by Git from now on. For those projects that have already been started, just run git init again to pull in the template.

And there we are - have fun exploring the other sample templates, read the docs, and experiment with other useful scripts. Then tell me about them on Twitter so I can use them.