In Git, what does git format-patch produce, and how does a maintainer apply it?

## Why this exists Many long-lived projects — the Linux kernel and Git itself among them — accept contributions as email rather than as pushed branches. The contributor never needs write access anywhere, and review happens in plain text. `git format-patch` and `git am` are the two halves of that pipeline: one turns commits into transportable text, the other turns that text back into commits. ## What format-patch writes `git format-patch <since>..<until>` (commonly `git format-patch origin/main..HEAD`, or `git format-patch -1` for just the tip commit) creates one file per commit in the current directory, or in the directory given by `-o <dir>`. The names are numbered and derived from the subject line: `0001-...patch`, `0002-...patch`. Each file is formatted as a single-message mbox: - `From <hash> Mon Sep 17 00:00:00 2001` — a fixed magic first line that marks the mbox. - `From:` the **author** of the commit, not whoever ran the command. - `Date:` the author date. - `Subject: [PATCH 2/5] <first line of the message>` — the numbering comes from the series size. - The rest of the commit message as the body, then `---`, a diffstat, and the diff itself, ending with the Git version as a signature line. The diff hunks carry `index <old>..<new> <mode>` lines that name the abbreviated blob hashes on both sides. Those hashes are what makes a later three-way apply possible. ## Useful options `--cover-letter` produces an extra `0000-cover-letter.patch` with a placeholder subject and the series diffstat — the place to explain the *why* of the whole series. `-v2` (a reroll count) changes the subject prefix to `[PATCH v2 n/m]` so reviewers can tell revisions apart. `--base=<commit>` records a `base-commit:` trailer so the recipient knows exactly what the series applies to. `--thread` arranges message headers so the series threads in a mail client. `git send-email 0*.patch [email protected]` sends them using the SMTP settings under `sendemail.*`. ## What git am does `git am <files-or-mbox>` walks the messages in order and, for each one: parses the headers to recover author name, email and author date; parses the subject, stripping the `[PATCH n/m]` prefix, to recover the summary line; applies the diff to the index and working tree; and commits. The resulting commit has the **original author and author date**, while the *committer* is whoever ran `am` and the commit date is now. That asymmetry is exactly why author and committer are separate fields in Git's commit object. Because `am` recreates commits, a maintainer can apply a ten-patch series and get ten commits with the contributor's authorship intact, which is what shows up in `git log --format='%an %cn'` and in per-author statistics. ## am versus apply `git apply` is the lower-level tool: it takes a diff, changes the working tree (and the index with `--index`), and stops. No commit, no message, no authorship. `git apply --check` tests whether a patch would apply at all without touching anything. Use `apply` for a raw diff someone pasted; use `am` for anything produced by `format-patch`, because only `am` carries the metadata. ## When it does not apply cleanly If the tree has moved on, `am` refuses with a message saying the patch does not apply and stops mid-series. `git am -3` (`--3way`) retries as a three-way merge using the `index` lines: if the pre-image blobs are present in the repository, Git can reconstruct the original context and merge, possibly leaving conflict markers for you to resolve, after which `git am --continue` proceeds. `git am --skip` drops the current patch, and `git am --abort` restores the branch to where it started. `git am --show-current-patch=diff` prints the patch that is stuck so you can inspect it. ## Interview framing The point of the question is rarely the flags. It is whether you understand that a Git commit is more than a diff — it carries an author identity, an author date and a message — and that `format-patch`/`am` is the pair that preserves all of that across a transport with no network connection to the original repository.