git blame --porcelain and -L: Machine-Readable Blame
git blame --porcelain emits a stable, parseable format (one block per line with author, time, and SHA), while -L<start>,<end> scopes blame to a line range.
Automated ownership and regression tooling parses blame, and the human format is not stable. --porcelain (and --line-porcelain) give a documented machine format; -L, -C/-M, and --ignore-revs-file refine what it reports.
What it does
git blame annotates each line of a file with the commit that last changed it. --porcelain outputs a stable format suitable for scripts: a header line with SHA and line numbers, followed by author/committer/summary fields. -L limits to a line range. -C/-M detect copied/moved lines. --ignore-revs-file skips noise commits (like a mass reformat).
Common usage
git blame --porcelain -L 40,60 src/app.js
# follow code moved between files
git blame -C -M src/app.js
# one record per line (easiest to parse)
git blame --line-porcelain src/app.js
# ignore a bulk-reformat commit
git blame --ignore-revs-file .git-blame-ignore-revs src/app.jsOptions
| Flag | What it does |
|---|---|
| --porcelain | Stable machine-readable output (dedupes commit headers) |
| --line-porcelain | Like --porcelain but full header per line |
| -L <start>,<end> | Restrict to a line range |
| -C / -M | Detect copied / moved lines across or within files |
| --ignore-revs-file <f> | Skip commits listed in the file |
| -w | Ignore whitespace changes when assigning blame |
In CI
Blame needs history: on a shallow clone, lines from before the boundary are attributed to the boundary commit, giving wrong owners, so fetch-depth: 0 for ownership tooling. Commit a .git-blame-ignore-revs file and reference it (or set blame.ignoreRevsFile) so mass-reformat commits do not pollute blame in every job.
Common errors in CI
"fatal: no such path '<file>' in HEAD" means the path is wrong or absent at that commit. "fatal: file <x> has only N lines" means an -L range exceeds the file. Blame pointing everything at one recent commit is the classic shallow-clone symptom: unshallow the clone.