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<title>A git pre-commit hook for automatically formatting Go code — Luke Shumaker</title>
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<header><a href="/">Luke Shumaker</a> » <a href=/blog>blog</a> » git-go-pre-commit</header>
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<h1 id="a-git-pre-commit-hook-for-automatically-formatting-go-code">A git pre-commit hook for automatically formatting Go code</h1>
<p>One of the (many) wonderful things about the Go programming language is the <code>gofmt</code> tool, which formats your source in a canonical way. I thought it would be nice to integrate this in my <code>git</code> workflow by adding it in a pre-commit hook to automatically format my source code when I committed it.</p>
<p>The Go distribution contains a git pre-commit hook that checks whether the source code is formatted, and aborts the commit if it isn't. I don't remember if I was aware of this at the time (or if it even existed at the time, or if it is new), but I wanted it to go ahead and format the code for me.</p>
<p>I found a few solutions online, but they were all missing something—support for partial commits. I frequently use <code>git add -p</code>/<code>git gui</code> to commit a subset of the changes I've made to a file, the existing solutions would end up adding the entire set of changes to my commit.</p>
<p>I ended up writing a solution that only formats the version of the that is staged for commit; here's my <code>.git/hooks/pre-commit</code>:</p>
<pre><code>#!/bin/bash
# This would only loop over files that are already staged for commit.
# git diff --cached --numstat |
# while read add del file; do
# …
# done
shopt -s globstar
for file in **/*.go; do
tmp="$(mktemp "$file.bak.XXXXXXXXXX")"
mv "$file" "$tmp"
git checkout "$file"
gofmt -w "$file"
git add "$file"
mv "$tmp" "$file"
done</code></pre>
<p>It's still not perfect. It will try to operate on every <code>*.go</code> file—which might do weird things if you have a file that hasn't been checked in at all. This also has the effect of formatting files that were checked in without being formatted, but weren't modified in this commit.</p>
<p>I don't remember why I did that—as you can see from the comment, I knew how to only select files that were staged for commit. I haven't worked on any projects in Go in a while—if I return to one of them, and remember why I did that, I will update this page.</p>
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