diff for file comparison in Linux CLI - Commands & Configuration

Introduction

Sometimes you need to see what changed between two files. The diff command helps by showing line-by-line differences so you can quickly spot changes.

When you want to compare two versions of a configuration file to find what changed.

When you need to check differences between two text documents before merging them.

When you want to verify if two files are exactly the same or have differences.

When you want to review code changes manually without using a graphical tool.

When you want to create a patch file showing changes between two files.

Commands

This command compares file1.txt and file2.txt and shows the differences line by line.

Terminal

diff file1.txt file2.txt

Expected Output

1c1

< Hello world

---

> Hello World!

The -u flag shows differences in a unified format, which is easier to read and commonly used for patches.

Terminal

diff -u file1.txt file2.txt

Expected Output

--- file1.txt	2024-06-01 12:00:00.000000000 +0000

+++ file2.txt	2024-06-01 12:01:00.000000000 +0000

@@ -1 +1 @@

-Hello world

+Hello World!

→

-u - Show differences in unified format

The -q flag tells diff to only report if files differ, without showing the details.

Terminal

diff -q file1.txt file2.txt

Expected Output

Files file1.txt and file2.txt differ

→

-q - Report only if files differ

Key Concept

If you remember nothing else from diff, remember: it shows exactly what lines changed between two files.

Common Mistakes

Running diff without specifying both file names

diff needs two files to compare; missing one causes an error.

Always provide two file names like diff file1.txt file2.txt

Expecting diff to show differences when files are identical

diff shows no output if files are exactly the same, which can be confusing.

Use diff -q to get a clear message if files differ or not.

Not using -u flag when creating patches

Without -u, the output is harder to read and not suitable for patch tools.

Use diff -u for unified format when sharing or applying patches.

Summary

Use diff file1.txt file2.txt to see line-by-line differences between two files.

Use diff -u for a unified, easier-to-read difference format.

Use diff -q to quickly check if files differ without details.