diff

Overview

The diff command compares files line by line and displays the differences. It’s essential for version control, code review, and file comparison tasks.

Syntax

diff [options] file1 file2
diff [options] directory1 directory2

Common Options

Option Description
-u Unified diff format
-c Context diff format
-i Ignore case differences
-w Ignore whitespace
-b Ignore changes in whitespace
-B Ignore blank lines
-r Recursive directory comparison
-q Brief output (only if files differ)
-s Report identical files
-y Side-by-side comparison
--color Colorize output

Output Formats

Format Description
Normal Default format with line numbers
Unified (-u) Git-style format
Context (-c) Shows context around changes
Side-by-side (-y) Two-column comparison

Key Use Cases

  1. Compare file versions
  2. Code review
  3. Configuration changes
  4. Backup verification
  5. Patch creation

Examples with Explanations

Example 1: Basic Comparison

diff file1.txt file2.txt

Shows differences between two files

Example 2: Unified Format

diff -u original.txt modified.txt

Shows differences in unified format (like Git)

Example 3: Directory Comparison

diff -r dir1/ dir2/

Recursively compares two directories

Understanding Output

Normal format symbols: - a - Added lines - d - Deleted lines - c - Changed lines - < - Lines from first file - > - Lines from second file

Example:

2c2
< old line
---
> new line

Common Usage Patterns

  1. Ignore whitespace:

    diff -w file1.txt file2.txt

  2. Side-by-side view:

    diff -y file1.txt file2.txt

  3. Quick check if files differ:

    diff -q file1.txt file2.txt

Advanced Options

Option Description
--exclude=pattern Exclude files matching pattern
--exclude-from=file Exclude patterns from file
-x pattern Exclude files matching pattern
-N Treat absent files as empty
-a Treat all files as text
--strip-trailing-cr Strip carriage returns

Patch Creation

Create patches for later application:

diff -u original.txt modified.txt > changes.patch

Apply patches:

patch original.txt < changes.patch

Performance Analysis

  • Efficient for text files
  • Memory usage scales with file size
  • Good for moderate-sized files
  • Consider alternatives for binary files
  • Fast for small differences

Additional Resources

Best Practices

  1. Use unified format for patches
  2. Ignore irrelevant whitespace
  3. Use recursive mode for directories
  4. Consider binary file handling
  5. Use with version control systems

Directory Comparison

  1. Compare structures:

    diff -r --brief dir1/ dir2/

  2. Exclude files:

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

  3. Show only differences:

    diff -r -q dir1/ dir2/

Integration Examples

  1. With git:

    git diff > changes.patch

  2. With find:

    diff <(find dir1 -type f | sort) <(find dir2 -type f | sort)

  3. Configuration management:

    diff -u /etc/config.orig /etc/config

Scripting Applications

  1. Backup verification:

    if diff -q original.txt backup.txt > /dev/null; then
    echo "Backup is identical"
fi

  2. Configuration monitoring:

    diff /etc/passwd /etc/passwd.bak || echo "Password file changed"

  3. Automated testing:

    diff expected_output.txt actual_output.txt || exit 1

Special Cases

  1. Binary files:

    diff -q binary1 binary2

  2. Large files:

    diff --speed-large-files file1 file2

  3. Case-insensitive:

    diff -i file1.txt file2.txt

Troubleshooting

  1. Binary file warnings
  2. Memory issues with large files
  3. Character encoding problems
  4. Permission denied errors
  5. Directory structure differences

Output Redirection

  1. Save differences:

    diff file1.txt file2.txt > differences.txt

  2. Suppress output:

    diff file1.txt file2.txt > /dev/null

  3. Error handling:

    diff file1.txt file2.txt 2>&1 | tee diff.log

Color Output

Modern diff versions support color:

diff --color=always file1.txt file2.txt

Environment variable:

export DIFF_COLORS="old=31:new=32:hunk=36"