Bash diff Command - Compare Files

Using the diff Command

The diff command compares two files line by line and shows which lines are different.

The examples below use these two files:

colors1.txt

red
green
blue

colors2.txt

red
yellow
blue

Basic Usage

Give diff two files. Lines that start with < are only in the first file, and lines that start with > are only in the second:

diff colors1.txt colors2.txt
2c2
< green
---
> yellow

2c2 means "line 2 in the first file was changed into line 2 in the second file". You will also see a (added) and d (deleted).

If the files are the same, diff prints nothing.

Unified Format (-u)

Use -u to show the changes with some surrounding lines. Removed lines start with - and added lines start with +.

This is the format used by Git and most code review tools:

diff -u colors1.txt colors2.txt
--- colors1.txt	2026-11-04 10:00:00.000000000 +0100
+++ colors2.txt	2026-11-04 10:05:00.000000000 +0100
@@ -1,3 +1,3 @@
 red
-green
+yellow
 blue

Only Say If Files Differ (-q)

Use -q when you only want to know if the files are different, not how:

diff -q colors1.txt colors2.txt
Files colors1.txt and colors2.txt differ

Ignore Case and Spaces

Use -i to ignore upper and lower case, and -w to ignore all spaces:

echo "Hello" > a.txt
echo "HELLO" > b.txt
diff -i a.txt b.txt

No output means the files are the same when case is ignored.

Use diff in Scripts

diff returns an exit code: 0 if the files are the same, and 1 if they are different. You can use this in an if statement:

if diff -q colors1.txt colors2.txt > /dev/null; then echo "Same"; else echo "Different"; fi
Different

Options Overview

• -u: Unified format, with lines marked + or -
• -q: Only report whether the files differ
• -i: Ignore upper and lower case
• -w: Ignore all spaces
• -r: Compare folders, including sub-folders