Stop a broken change before it is shared
A shop lists its prices in a file. Someone deletes the tea price by mistake and pushes. Now the whole team has a broken list.
A check that runs when you push would have said no. Real teams use checks like this all day.
By the end of this lesson
Note
I can make Git run a check before a push goes through.
Key words
| Word | What it means |
|---|---|
| Hook | a script Git runs at a set moment |
| Check | a test that passes or fails |
| Exit status | the number a program ends with, 0 for success |
| CI | a server that runs checks on every push |
| Runner | the machine that carries out a CI job |
The cells students fill in are left blank in this preview.
A hook is a script Git runs
Git looks in the folder .git/hooks for scripts with set names. A script called pre-push runs just before a push. If it ends with a non-zero exit status, Git stops the push.
The script must be executable. Here the check looks for the tea price in prices.txt.
Make a shared folder to push to
git init --bare -b main shared.gitShown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
Clone it as shop
git clone shared.git shopShown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
Make a file with two prices
printf 'Latte 3.20\nTea 2.10\n' > shop/prices.txtShown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
Write the check
printf '#!/bin/sh\ngrep -q "Tea" prices.txt || { echo "check failed: no Tea price"; exit 1; }\necho "check passed"\n' > shop/.git/hooks/pre-pushShown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
Make the check executable
chmod +x shop/.git/hooks/pre-pushShown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
Stage the file
git -C shop add prices.txtShown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
Commit it
git -C shop commit -m "Add prices"Shown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
Push and watch the check run
git -C shop push -u origin mainShown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
A failing check refuses the push
Now break the file on purpose. Delete the tea line and commit. The check fails, and Git refuses the push, so the broken commit stays on your computer.
You then cancel the bad commit with git revert, which you met in the lesson on undoing, and push again.
Delete the tea line
sed -i '/Tea/d' shop/prices.txtShown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
Commit the mistake
git -C shop commit -am "Remove tea by mistake"Shown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
Try to push it
git -C shop pushShown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
Cancel the mistake
git -C shop revert --no-edit HEADShown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
Push again
git -C shop pushShown here, not run. Create a free account to press Run and type it into your own lab’s terminal.
A common mistake
Warning
Many people think a hook is part of the project, so the whole team gets it. It is not. A hook lives in .git, and a clone does not copy it. Anyone can skip it with --no-verify. A team needs the same check on the server.
CI runs the same check on the server
CI means continuous integration: a server that runs checks every time somebody pushes. You write the steps in a file kept in the repository, so every clone has the same rules.
In Gitea the file lives in .gitea/workflows. The server hands each job to a runner. The result shows on the pull request, so a reviewer sees a failure at once.
Worked example: a workflow file
| Line of the file | What it says |
|---|---|
| `name: check` | the name shown on the page |
| `on: [push]` | run on every push |
| `runs-on: ubuntu-latest` | the kind of machine to use |
| `- uses: actions/checkout@v4` | fetch the project files first |
| `- run: grep -q Tea prices.txt` | the check itself, the same test as the hook |
The cells students fill in are left blank in this preview.
The lab's Gitea has no runner
This file does the job of the hook. Each step runs in order, and a step that exits non-zero fails the job.
The lab's Gitea accepts the file and lists a run for every push, but the run stays queued. No runner is attached, so nothing carries it out. A real team attaches one.