git blame

Who broke this and when. Be kind in the comments.

🎯 What & why

git blame shows who last touched each line and in which commit — the first stop when you need to ask why does this line exist? before you ask who do I yell at?

🧠 Mental model

A reverse history walk per line: for every line in the current file, Git follows it backwards through diffs until it finds the commit that introduced it (optionally chasing through renames, moves, and copies).

🛠️ Synopsis

git blame [-c] [-b] [-l] [--root] [-t] [-f] [-n] [-s] [-e] [-p] [-w]
          [--incremental] [-L <range>] [-S <revs-file>]
          [-M[<num>]] [-C[<num>]] [-C] [-C]
          [--since=<date>] [--ignore-rev <rev>]
          [--ignore-revs-file <file>] [--color-lines] [--color-by-age]
          [<rev> | --contents <file> | --reverse <rev>..<rev>]
          [--] <file>

🎚️ Switches & options

Flag	What it does
`-L <start>,<end>`	Restrict to a line range. Accepts `:funcname` and regex forms — `-L :main:src/app.c`.
`-w`	Ignore whitespace when comparing — kills noise from reindents.
`-M`	Detect moved lines within the same file.
`-C`	Detect lines copied from other files in the same commit. Repeat `-C -C` to search across all files in the commit; `-C -C -C` searches the whole tree.
`--show-email` / `-e`	Show committer email instead of name.
`--ignore-rev <rev>`	Skip a specific commit (e.g. a mass-format commit) so blame walks past it.
`--ignore-revs-file <file>`	Read multiple revs to ignore from a file — typically `.git-blame-ignore-revs`.
`-p` / `--porcelain`	Machine-readable output for tooling.

💡 Use cases

Tracking down why a confusing line exists before changing it.
Finding the commit (and PR) that introduced a regression in a specific line range.
Auditing license headers or copyright lines across renames with -M -C.
Skipping noise commits (formatter runs, license bumps) via .git-blame-ignore-revs.

🧪 Examples

Blame a function

git blame -L :handle_request:server.py

Ignore whitespace and moves

git blame -w -M src/parser.c

Show emails for lines 10-30

git blame -e -L 10,30 README.md

Honour the ignore-revs file

git blame --ignore-revs-file .git-blame-ignore-revs app.js

🎓 Recommendations

Commit a .git-blame-ignore-revs and set git config blame.ignoreRevsFile .git-blame-ignore-revs — GitHub honours it too.
Use -w -M -C as a default trio when chasing logic across refactors.
Be kind in the commit message — blame is a conversation with future readers; explain why, not what.

🪤 Common pitfalls

Blame credits the last toucher, not the original author — a whitespace fix can rewrite the apparent history. Use -w and ignore-revs to compensate.
-C -C -C can be very slow on large repos; reach for it only when needed.
Without --ignore-revs-file, mass-format or mass-rename commits drown out the real signal.

🔗 Related modules

📝 Quiz

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