git blame

git blame <file> annotates every line of a file with the commit, author, and date that last changed it — the “who wrote this line, and when?” lookup.

Why it matters

Before touching unfamiliar code you need its provenance: which commit (and PR/ticket) introduced a line, who to ask, and whether a “weird” check guards a real edge case. Blame turns a single line into a thread you can pull — commit message, author, and date — making it indispensable for debugging, archaeology, and writing a bisect-style narrative of a regression.

How it works

For each line Git walks history backward and stops at the commit where that line reached its current form, printing ^?sha (author date line#) content. A leading ^ marks a line from the very first (boundary) commit.

Flag	Effect
-L 40,60	blame only lines 40–60 (or -L :funcName:file)
-w	ignore whitespace-only changes when assigning blame
-C / -M	detect lines copied/moved from other files or within the file
-e	show author email instead of name
<rev>	blame the file as of a past revision, e.g. git blame HEAD~5 -- f

• Skipping noise commits — a pure reformat or rename will claim every line. git blame --ignore-rev <sha> (or a .git-blame-ignore-revs file) skips listed commits so blame points at the real author.
• -C -C -C — escalating -C widens copy detection (same commit → any committed file), great for tracking code that moved during a refactor.
• Hosts (GitHub) render blame in the UI and let you “view blame prior to this change” to step back through rewrites.

Example

$ git blame -L 12,14 -w src/auth.py
a1b2c3d (Ada 2025-09-02 12) def verify(token):
e4f5a6b (Ben 2026-01-18 13)     if token is None:        # <- added in PR #812
a1b2c3d (Ada 2025-09-02 14)         raise AuthError()

Pitfalls

• Blame ≠ blame — it shows the last touch, not the original author; a whitespace or move commit can mask real history. Use -w, -M/-C, and --ignore-rev.
• Deleted lines are invisible — blame only annotates lines that still exist; to find when a line was removed, use git log -S<text> (the pickaxe) instead.
• Reformatting blame storms — without an ignore-revs file, a Prettier/gofmt pass makes one commit own the whole file.
• It’s per-line, not semantic — moving a function shows the move commit unless -M/-C is on; blame can’t tell you why, only which commit — read the commit for that.

See also

• git-show
• git-bisect