git fmt-merge-msg

Used by git merge to compose its commit message.

🎯 What & why

git fmt-merge-msg builds the commit message git merge uses, reading the refs in FETCH_HEAD. You only invoke it directly when scripting custom merge flows.

🧠 Mental model

Plumbing: takes FETCH_HEAD lines on stdin, emits a merge commit subject and optional body. git merge calls it for you; scripts call it when they want to override the wording.

🛠️ Synopsis

git fmt-merge-msg [--log[=<n>] | --no-log] [--message <msg>] [-F <file>] < $GIT_DIR/FETCH_HEAD

# Driven by config:
#   merge.log = false | <n>
#   merge.summary = (deprecated alias of merge.log)

🎚️ Switches & options

Flag	What it does
`--log[=<n>]`	Append shortlog of up to <n> commits (default 20) being merged
`--no-log`	Suppress the shortlog body — subject only
`--message <msg>`	Use <msg> as the subject instead of the default
`-F <file>`	Read FETCH_HEAD-style input from <file> instead of stdin
`merge.log`	Config: same effect as `--log` for every merge
`merge.summary`	Deprecated alias of `merge.log`; still honored

💡 Use cases

Custom fetch-then-merge scripts that want consistent commit wording.
Generating release-merge messages with a shortlog of included commits.
Replaying merges in automation where you control FETCH_HEAD directly.
Producing merge messages for tools that bypass git merge (e.g., server-side merges).

🧪 Examples

Default message from FETCH_HEAD

git fmt-merge-msg < .git/FETCH_HEAD

Include shortlog of merged commits

git fmt-merge-msg --log=50 < .git/FETCH_HEAD

Always log shortlogs by default

git config --global merge.log true

Use as input to a low-level merge

git fmt-merge-msg < .git/FETCH_HEAD | git commit-tree $TREE -p HEAD -p $OTHER -F -

🎓 Recommendations

Set merge.log = true (or a number) so merge commits document what came in.
When scripting, pipe a curated FETCH_HEAD rather than editing the message after the fact.
Prefer --message over post-hoc git commit --amend for reproducible automation.

🪤 Common pitfalls

Reads FETCH_HEAD only — if you didn't fetch, there's nothing to summarize.
merge.summary still works but is deprecated; use merge.log in new config.
The shortlog respects mailmap; missing .mailmap entries produce duplicated author names.

🔗 Related modules

📝 Quiz

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