A git pre-commit hook for automatically formatting Go code

One of the (many) wonderful things about the Go programming language is the gofmt tool, which formats your source in a canonical way. I thought it would be nice to integrate this in my git workflow by adding it in a pre-commit hook to automatically format my source code when I committed it.

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.

I found a few solutions online, but they were all missing something—support for partial commits. I frequently use git add -p/git gui 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.

I ended up writing a solution that only formats the version of the that is staged for commit; here's my .git/hooks/pre-commit:

#!/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

It's still not perfect. It will try to operate on every *.go 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.

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.

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