blame & bisect

git blame src/auth/middleware.ts

Prints every line of middleware.ts with the commit hash, author, timestamp, and line content. You can trace any line back to its origin.

git blame -L 10,20 src/auth/middleware.ts

Restricts blame output to lines 10 through 20. Use this when you only care about a specific function or block, not the entire file.

git blame -w src/auth/middleware.ts

Reports the commit that introduced the meaningful content, ignoring whitespace-only changes. A line reformatted by a linter will still show the original author.

git blame -C src/auth/middleware.ts

Tracks lines across commits to find the original commit that introduced them, even if they were moved by a later commit. Useful for squashed commits where the blame commit is the squash, not the original work.

git blame --porcelain src/auth/middleware.ts | head -20

Outputs machine-readable blame data. Each commit is followed by key-value pairs: author, time, summary. Useful for scripting or building custom blame tools.

git blame -L 15,15 src/auth/middleware.ts

Blame a single line, then use the commit hash to view the full commit: git show <hash>. This takes you from a line of code to the exact change that introduced it.

git bisect start
git bisect bad                 # current commit is broken
git bisect good v1.2.0         # this tag was working

Starts the bisect session. Git checks out a commit halfway between good and bad. You test the code and mark each checkout as good or bad until Git identifies the exact breaking commit.

# Test the current checkout
npm test

# If tests pass:
git bisect good

# If tests fail:
git bisect bad

# Git checks out the next commit to test

Each good/bad answer halves the search space. After ~10 steps, you've searched 1000 commits. After ~20 steps, you've searched a million. Git reports the exact commit that introduced the regression.

# When bisect finishes, it prints the bad commit
git bisect reset

Resets HEAD back to the branch you were on before starting bisect. Always run this when you're done, whether bisect completed or you need to abort.

git bisect start HEAD v1.0.0
git bisect run npm test

Runs npm test at each commit. If the test passes (exit 0), that commit is marked good. If it fails (exit 1), it's marked bad. Git drives the entire process — you just wait for the result.

#!/bin/bash
# test-build.sh - returns 0 if build succeeds
npm run build 2>/dev/null
if [ $? -eq 0 ]; then
  exit 0    # good
else
  exit 1    # bad
fi

git bisect start HEAD v2.0.0
git bisect run ./test-build.sh

A custom script can test anything: build success, runtime behavior, performance thresholds. The script must exit with 0 (good), 1 (bad), or 125 (skip) for bisect to work automatically.

git bisect run sh -c "npm install && npm test"

Wraps the test in a shell command that installs dependencies first. If npm install fails (exit non-zero, non-1), bisect skips that commit. Useful when some commits have broken lockfiles or missing dependencies.

git log -S "calculateDiscount" --oneline

Searches commit history for changes that add or remove the string calculateDiscount. Combined with blame, this gives you the full story of when and why a function appeared.