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Writing a Git Hook in Bash

Git Hooks can be a very useful addition to a developer's workflow.

For instance, as I'm working on rewriting my new website using Next.js, I've set up eslint and have pretty strict linting rules when running Next builds.

Unfortunately, Git doesn't follow these rules by default. So it's easy to forget to run the linter, run a git push, and end up with a failing production build on Vercel.

To avoid this, I've set up a simple Git Hook which runs eslint before every git push, and fails the push if the linter returns any errors:

# .git/hooks/pre-push

# pipe output (with colors) to .temp file
RESULT=$(script -q .temp npm run lint)

# if .temp file contains "error", fail
if grep --quiet error .temp; then
  printf "\n  !! Will not push -- found linting errors:\n"
  sed '1,5d' .temp | cat
  rm .temp
  exit 1
else
  rm .temp
  exit 0
fi

The above script runs npm run lint, captures the output (with text colors) to a .temp file, and then greps for error in that file. If it finds any instances of the word "error", it won't push, but will instead print the output of the npm run lint command:

$ git push

  !! Will not push -- found linting errors:

/Users/andrew/Git/awwsmm.com/components/CommitGroupComponent.tsx
   2:1  error  Imports should be sorted alphabetically  sort-imports
  24:4  error  Missing semicolon                        semi

✖ 2 problems (2 errors, 0 warnings)
  1 error and 0 warnings potentially fixable with the `--fix` option.

error: failed to push some refs to 'github.com:awwsmm/awwsmm.com.git'

This prevents useless builds on Vercel which will fail because of linting errors.

Note: make sure your .git/hooks/<file> is executable (chmod +x .git/hooks/<file>), or it will not prevent the particular action from occurring. It's probably best to play around with a pre-commit hook rather than a pre-push one, when testing.

Writing a Git Hook in Java

We can do a similar thing using Java instead of bash scripting. For this example, we'll look at writing a pre-commit hook in Java. Instead of writing Java code to the pre-commit file directly, we'll add a bit of indirection.

First, create a file called PreCommit.java in the .git/hooks/ directory and add some pre-commit logic to it. In this case, we're just going to fail every commit:

public class PreCommit {
  public static void main (String[] args) {
    System.out.println("Java says: no committing please.");
    System.exit(1);
  }
}

The System.exit(1) gives the same result as exit 1 in the Bash script -- the commit will fail to complete.

Next, we'll make the .git/hooks/pre-commit file executable with

$ chmod +x .git/hooks/pre-commit

...and modify its contents to read:

#!/bin/sh

# get the path to this script file
DIR=$(dirname "$0")

# Java >= 11
exec java $DIR/PreCommit.java "$@"

If you're using Java 11+, you can also write this all as a single file, though I personally don't recommend this method as it combines Bash and Java syntax in a single file, which can be difficult for IDEs to handle in terms of syntax highlighting, etc.

With this in place, we'll now see a message like

$ git commit -m "update gitignore, more hook testing"
Java says: no committing please.

Note that in Java versions < 11, we need to compile this script first with javac, so pre-commit should instead look something like

#!/bin/sh
DIR=$(dirname "$0")

# Java < 11
javac $DIR/PreCommit.java "$@"
exec java -classpath $DIR PreCommit
!#

Writing a Git Hook in Scala

The process for writing a Git Hook in Scala is very similar to writing one in Java. First, define a pre-commit.scala file with the following contents

object Hook {
  def main (args: Array[String]): Unit = {
    System.out.println("Scala says: no committing please.");
    System.exit(1);
  }
}

(Note that, in Scala, the name of the class doesn't have to match the name of the file.)

Then, modify the contents of pre-commit to read

#!/bin/sh
DIR=$(dirname "$0")
exec scala $DIR/pre-commit.scala "$@"
!#

Finally, try to commit:

$ git commit -m "update gitignore, more hook testing"
Scala says: no committing please.

That's all there is to it.

As it takes a while for the JVM to boot up when running Git Hooks in Java or Scala, it's probably not extremely practical to do so... unless you have a lot of logic in your hook which is easier to read in Java / Scala than in a Bash script.

But anyway, it's good to know that it can be done.