Linux diff Command: Compare Text Files and Directories

The diff command compares two text files or directory trees and reports how they differ. Unified output from diff -u includes surrounding context and is widely used for patches, code review, and configuration comparison.

This guide explains the basic syntax, key options, practical examples, related commands, common cautions, and frequently asked questions for diff.

What Is the diff Command?

Diff computes line-oriented changes that transform the first input into the second. No output normally means no reportable difference. Exit status 0 means equal, 1 means differences were found, and 2 means trouble, so status 1 is a valid comparison result rather than a generic execution error.

Basic Syntax

diff [OPTION]... FILE1 FILE2
diff [OPTION]... DIRECTORY1 DIRECTORY2

Options and edge cases can vary by implementation and version. Check diff --help and man diff on the target system before relying on implementation-specific behavior.

Basic diff Usage

Compare two files

The default format describes line changes using traditional diff notation.

diff old.conf new.conf

Produce unified output

The unified format shows changed lines with surrounding context and is easier to review.

diff -u old.conf new.conf

Compare directories recursively

Add -r to descend into corresponding subdirectories.

diff -ru project-old project-new

Reading Change Indicators and Exit Status

diff -u shows the differing lines of two files in unified format. In the example below, old from the first file is changed to new in the second file.

printf 'old\n' > before.txt
printf 'new\n' > after.txt
diff -u before.txt after.txt

Key parts of the output (file header times are omitted):

-old
+new

- indicates lines only in the first file, + indicates lines only in the second file. If the two files differ, the exit status is 1, so distinguish this from execution failure status 2.

Key Options

Practical Examples

Create a patch file

Use stable relative paths when the patch must be applied in another working tree.

diff -u original.conf modified.conf > config.patch

Summarize directory differences

The -qr combination reports differing and one-sided files without printing every changed line.

diff -qr release-a release-b

Ignore only spacing-width changes

Use this only when whitespace is not semantically important; indentation-sensitive languages require extra care.

diff -u -b before.txt after.txt

diff and Related Commands

Important Cautions

Exit status 1 means different

Treat it as a normal comparison outcome; status 2 indicates an actual problem.

Detailed binary diffs are not line-friendly

Use cmp or a checksum tool when byte equality is the main question.

Whitespace options can hide meaningful changes

Do not ignore indentation or spacing when the file format gives it semantic meaning.

FAQ

What does empty diff output mean?

The inputs have no reportable differences under the selected options.

Which output format is easiest to read?

Unified output from diff -u is usually the most convenient.

Can diff compare entire directories?

Yes. Use -r for recursive comparison.

Is exit code 1 an error?

No. It normally means differences were found; code 2 indicates trouble.

Related Resources

• Essential Linux Commands: A Practical Reference by Task
• GNU Diffutils diff documentation