diff shows exactly what changed between two files; patch applies such a description to a target. The pair is older than every modern editor's git integration — and still the cleanest vocabulary for "here are my changes" in plain text.
Reading a diff #
$ diff -u config.old config.new
--- config.old
+++ config.new
@@ -12,4 +12,4 @@
-timeout 30
+timeout 60The unified format (almost always what you want) shows three lines of context around each change: - lines were removed, + lines added, @@ marks the position. Skim any pull request and you are reading exactly this.
For directories, one flag compares whole trees:
$ diff -ru old-release/ new-release/ > release.patchUseful flags #
| Flag | Effect |
|---|---|
-u | Unified format, the readable default |
-r | Recurse into directories |
-q | Report only which files differ |
-w | Ignore whitespace changes |
-y | Side-by-side columns |
-q is the fast audit — which files differ at all — before digging into any single pair.
Applying changes #
patch reads a diff and applies it elsewhere, which is how fixes travelled by email for decades and still do in some projects:
$ patch --dry-run -p1 < fix.patch
$ patch -p1 < fix.patchThe -p1 strips one leading path component — the convention when patches were made from inside a source tree. --dry-run previews without writing; it is the look-before-you-delete habit, translated.
Compare whole directories with find feeding the candidates, and inspect single files with stat, file, and which. More in the terminal notes series.