Git Blame

git blame annotates a file line by line, telling you which commit last touched each line and who wrote it. It answers "why is this here" with a commit hash rather than a guess.

The basic form

git blame src/index.ts

Each line is prefixed with the abbreviated commit hash, the author, a timestamp, and the line number. The pointer format is configurable, which matters because the default is hard to read in a wide line.

Better output

git blame -w src/index.ts            # ignore whitespace-only changes
git blame -w -C -C -C src/index.ts   # also try moving lines between files
git blame -L 20,40 src/index.ts      # only lines 20 to 40
git blame -C src/index.ts            # who last wrote this much of the file

-w is the one that makes blame useful. Without it, a file reformatted by a formatter appears to be entirely rewritten, and you learn nothing. With it, and -C to follow lines that moved, blame points at the commit that actually introduced the logic.

git blame -w -C -C -C is the common incantation, and it is worth having in your fingers.

From a blame to the change

The hash in the margin is a commit, so the next command is usually git show:

git show a1b2c3d

Why a line is there

git blame answers who last changed a line, not why it exists. A line nobody remembers writing is usually a fix for a bug nobody wrote down, which is what git log -S "<string>" is for — it finds the commits that added or removed a literal string:

git log -S "retryCount" --oneline

Only your changes

git blame --author="Vicky" src/index.ts

Interactive: git gui blame

Some terminals have a mode that walks the history of a single line as you move through the file. In VS Code, the GitLens extension provides the same thing. Both are far quicker than reading hashes and running git show on each.

A caveat

Blame attributes the last commit to touch a line, not the person who wrote the original logic. A one-character fix on a line makes the fix's author the blame for the whole line, which is why -w and -C exist — and why reading the commit is part of using the output. See Git Show and History.