diff Command Guide

Basic Usage

diff shows differences between two files.

bash

diff file1.txt file2.txt

No output means the files are identical.

Common Options

diff Options

-u	Unified format (most common)
-c	Context format
-y	Side-by-side comparison
-i	Ignore case differences
-w	Ignore all whitespace
-b	Ignore whitespace changes
-r	Recursively compare directories
-q	Report only if files differ
--color	Colorize output

Unified Format (-u)

The most common format, used for patches and version control.

bash

diff -u file1.txt file2.txt

diff

--- file1.txt    2024-01-15 10:00:00
+++ file2.txt    2024-01-15 11:00:00
@@ -1,4 +1,4 @@
 line 1
-line 2 old
+line 2 new
 line 3
 line 4

Lines starting with - are removed, + are added.

Side-by-Side (-y)

bash

diff -y file1.txt file2.txt

# With specific width
diff -y -W 80 file1.txt file2.txt

text

line 1                           line 1
line 2 old                     | line 2 new
line 3                           line 3

Ignoring Whitespace

bash

# Ignore all whitespace
diff -w file1.txt file2.txt

# Ignore changes in whitespace amount
diff -b file1.txt file2.txt

# Ignore blank lines
diff -B file1.txt file2.txt

Comparing Directories

bash

# Recursive comparison
diff -r dir1/ dir2/

# Only report if different
diff -rq dir1/ dir2/

# Exclude patterns
diff -r --exclude="*.log" dir1/ dir2/

Colorized Output

bash

diff --color file1.txt file2.txt
diff --color=always file1.txt file2.txt | less -R

Creating and Applying Patches

bash

# Create a patch file
diff -u original.txt modified.txt > changes.patch

# Apply a patch
patch original.txt < changes.patch

# Reverse a patch
patch -R original.txt < changes.patch

Tip

Use -u format for patches as it provides context and is the standard format.

Practical Examples

Compare configuration files

bash

diff -u /etc/nginx/nginx.conf nginx.conf.new

Check if files are different

bash

if diff -q file1.txt file2.txt > /dev/null; then
    echo "Files are identical"
else
    echo "Files differ"
fi

Compare with git version

bash

diff file.txt <(git show HEAD:file.txt)

Compare command outputs

bash

diff <(ls dir1) <(ls dir2)

Show context around changes

bash

# Show 5 lines of context
diff -U5 file1.txt file2.txt

Alternative Tools

Diff Alternatives

colordiff	Wrapper for colored output
vimdiff	Visual diff in Vim
meld	GUI diff tool
delta	Modern pager for diff output
diff3	Compare three files

bash

# Use colordiff for colors
colordiff file1.txt file2.txt

# Visual diff in vim
vimdiff file1.txt file2.txt

# Three-way diff
diff3 mine.txt original.txt yours.txt

Exit Codes

0	Files are identical
1	Files are different
2	Error occurred

Summary

diff is essential for comparing files. Key takeaways:

Use diff -u for unified format (patches)
Use diff -y for side-by-side view
Use diff -r for directories
Use diff -q to only check if different
Use --color for colored output

Quick Reference

Basic

Options

Directory

Patch

Downloadable Image Preview

Basic Usage

Common Options

diff Options

Unified Format (-u)

Side-by-Side (-y)

Ignoring Whitespace

Comparing Directories

Colorized Output

Creating and Applying Patches

Practical Examples

Compare configuration files

Check if files are different

Compare with git version

Compare command outputs

Show context around changes

Alternative Tools

Diff Alternatives

Exit Codes

Exit Codes

Summary

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