diff

Additional Notes

diff compares files line by line and prints the differences. It supports several output formats: normal, context (-c), and unified (-u). Unified diffs are the most common because they are compact and can be applied with patch.

diff compares text. To compare directories recursively, add -r; to produce a reusable patch, use -u and redirect the output to a file.

Syntax

diff [options] FILE1 FILE2

Parameters

• options: Flags that change how diff behaves.
• FILE1, FILE2: The files (or directories, with -r) to compare.

Common Options

• -u: Unified diff output (the standard patch format).
• -c: Context diff output with surrounding lines.
• -r: Compare directories recursively.
• -q: Report only whether files differ, not the details.
• -y: Side-by-side output.
• -i: Ignore case differences.
• -w: Ignore all whitespace.
• -B: Ignore blank lines.
• --color: Colorize output when supported.
• --from-file=FILE: Compare FILE against every following argument.

Examples

diff old.txt new.txt

Show normal-format differences between two files.

diff -u old.txt new.txt

Show a unified diff with a few context lines.

diff -u old.txt new.txt > changes.patch

Save a unified diff as a patch file.

diff -qr dir-a dir-b

Recursively report which files differ between two directories without showing the content.

diff -y file1 file2

Show the two files side by side.

diff -uw script.sh script.new

Compare while ignoring whitespace differences.

Applying a Patch

patch < changes.patch

Apply a unified diff created earlier with diff -u.

patch -R < changes.patch

Reverse an already-applied patch.

Practical Notes

• Unified diffs from diff -u are readable and are what git diff produces.
• Use -r to diff whole directory trees; combine with -q for a quick "what changed" list.
• patch applies diff -u output; keep the original file name for patch to find it.
• Use -w/-i when you only care about real logic changes, not formatting.