git patch-id

Same change, different commit IDs — patch-id matches them up.

🎯 What & why

Computes a stable, content-only hash of a patch so that the same logical change made on different branches (with different commit IDs and timestamps) hashes to the same value.

🧠 Mental model

Strip out everything that varies between equivalent patches - context, line numbers, file headers' noise, whitespace order - then SHA1 what's left. Two cherry-picks of the same change get the same patch-id.

🛠️ Synopsis

git patch-id [--stable | --unstable | --verbatim] < <mbox-or-diff>
# Output: <patch-sha1> <commit-sha1>

🎚️ Switches & options

Flag	What it does
`--stable`	Default. Sums per-hunk hashes so reordered hunks still match
`--unstable`	Hash depends on hunk order; faster, matches pre-1.9 behavior
`--verbatim`	Hash the diff bytes literally - whitespace and order matter
`(stdin)`	Reads `git format-patch` mbox or `git diff` output; one or many patches

💡 Use cases

Detecting that a commit on a feature branch is already upstream (what git cherry does)
De-duplicating a backport queue across release branches
Building tools that rebase/replay patches and need to skip equivalents
Auditing whether two repos contain the same logical history despite different SHAs

🧪 Examples

Patch-id of a single commit

git show HEAD | git patch-id

All commits in a range

git format-patch --stdout origin/main..HEAD | git patch-id --stable

Find a matching upstream commit

git log --format=%H origin/main | while read c; do git show $c | git patch-id; done | grep <id>

Order-sensitive comparison

git diff A B | git patch-id --verbatim

🎓 Recommendations

Stick with the default --stable unless you have a specific reason - it's what git cherry and git rebase use.
Pipe format-patch --stdout (not raw diff) when you want the commit-id field populated correctly.
Cache patch-ids in a small index when comparing many branches; the hashing isn't free.

🪤 Common pitfalls

Identical-looking changes with different file renames produce different patch-ids - rename detection is not applied.
--verbatim hashes whitespace and line endings; CRLF/LF mismatches will diverge silently.
Trailing newline differences between diff and format-patch output can flip results under --verbatim.

🔗 Related modules

📝 Quiz

Hit each option, then Check answers. Score is recorded; Next is always open.