diff: Comparing Files

Spot every change between two files

diff a b shows the lines that differ between two files. The default output is terse and old-school. Add -u for the unified format git, GitHub, and code reviewers all use.

The flags you'll need

• -u — unified diff with 3 lines of context (the standard).
• -r — recursive across two directories.
• -q — only say if files differ; don't show changes.
• -i — ignore case.
• -w — ignore all whitespace.
• -B — ignore blank-line changes.
• -y — side-by-side display.
• --color=auto — color the diff (GNU diff 3.4+).

Reading a unified diff

Lines starting with - are in a only, + are in b only. The hunk header @@ -10,4 +10,5 @@ means "4 lines starting at line 10 in a, 5 lines starting at line 10 in b." Same format git diff uses.

Patches

Save a diff to a file: diff -u a b > change.patch. Apply it later: patch < change.patch. This is how Linux kernel patches travel by email.

Better than diff for code

git diff uses the same unified format but adds smart rename detection and color. delta (modern-tools track) makes diffs human-readable. For directory comparison, diff -rq dir1 dir2 + manual review beats most GUI tools.