my-docs/Linux/Basic-Administration/30-diff.md

diff Command Reference

The diff command is a standard Unix/Linux utility used to compare files line by line. It is commonly used in development, DevOps, and system administration to identify changes between configuration files, source code, logs, or generated outputs.

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1. Basic File Comparison

Compare two files line by line:

diff file1 file2

Output Behavior

• Shows only the lines that differ

• Uses symbols to indicate changes:

  • < line from file1
  • > line from file2
  • c change
  • a addition
  • d deletion

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2. Side-by-Side Comparison

Display files next to each other:

diff -y file1 file2

Notes

• Useful for human-readable comparison
• Differences are shown in two columns
• Change indicators appear in the middle

Limit output width:

diff -y --width=120 file1 file2

Suppress common lines:

diff -y --suppress-common-lines file1 file2

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3. Unified Diff Format (Most Common)

Generate a unified diff:

diff -u file1 file2

Why Unified Diff

• Standard format used by Git, patch, and code reviews
• Shows context before and after changes
• Easier to read and apply

Example output markers:

• + added lines
• - removed lines
• @@ line numbers and context

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4. Save Diff Output to a File

Redirect diff output to a file:

diff -u file1 file2 > different.diff

Common use cases:

• Code reviews
• Patch creation
• Change tracking
• CI/CD artifact storage

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5. Recursive Directory Comparison

Compare directories:

diff -r dir1 dir2

Unified recursive diff:

diff -ru dir1 dir2

Ignore missing files:

diff -rq dir1 dir2

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6. Ignore Differences

Ignore whitespace:

diff -w file1 file2

Ignore blank lines:

diff -B file1 file2

Ignore case differences:

diff -i file1 file2

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7. Apply a Diff as a Patch

Create a patch:

diff -u oldfile newfile > change.patch

Apply patch:

patch < change.patch

Dry-run patch:

patch --dry-run < change.patch

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8. diff vs Git diff

diff	git diff
Compares any files	Compares Git-tracked files
Works without Git	Requires Git repository
Produces patch files	Integrated with version control

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9. Best Practices

• Use -u format for readability and compatibility
• Store diff files with .diff or .patch extensions
• Avoid committing generated diff files unless required
• Use diff for system configuration audits
• Use git diff inside Git repositories

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10. Quick Reference

diff file1 file2
diff -y file1 file2
diff -u file1 file2
diff -ru dir1 dir2
diff -u file1 file2 > change.diff