diff command

diff - compare files line by line

The diff command in Linux is a utility used to compare two files and display their differences. It’s very useful for spotting changes in code, configuration files, or any text data.

Usage: diff [OPTION]... [FILES]...

• OPTION: Flags which enhances the diff abilities.
• FILES: The file(s) to compare.

Common Options

Option	Description
-u	Unified output with context
-y	Side-by-side output
-i	Ignore case differences
-w	Ignore all whitespace
-b	Ignore changes in whitespace amount
-r	Recursively compare directories
-q	Brief mode (just say if files differ)

---

Examples

• Basic Comparison

  Run diff with two files to see how they differ.

  $ diff file1.txt file2.txt
  • Sample files:
    • file1.txt: apple\nbanana\ncherry
    • file2.txt: apple\norange\ncherry
  • Output:

    2c2
    < banana
    ---
    > orange
  • Explanation:
    • 2c2: Line 2 in file1 changed to line 2 in file2.
    • < banana: Line from file1.
    • > orange: Line from file2.

  If the files are identical, there’s no output.

• Unified Output Format

  Use -u for a more readable "unified" format, showing context around changes.

  $ diff -u file1.txt file2.txt
  • Output:

    --- file1.txt   2025-03-29 10:00:00
    +++ file2.txt   2025-03-29 10:01:00
    @@ -1,3 +1,3 @@
    apple
    -banana
    +orange
    cherry
  • Explanation:
    • - lines are removed from file1.
    • + lines are added in file2.
    • Context lines (unchanged) are shown for reference.

• Side-by-Side Output

  Use -y to display differences in two columns.

  $ diff -y file1.txt file2.txt
  • Output:

    apple           apple
    banana        | orange
    cherry          cherry
  • Explanation:
    • | indicates a difference.
    • Identical lines align without markers.

• Ignoring Case

  Use -i to ignore case differences.

  $ diff -i file1.txt file2.txt
  • If file1.txt has "Apple" and file2.txt has "apple", -i treats them as the same.

• Ignoring Whitespace

  Use -w to ignore all whitespace differences or -b to ignore changes in the amount of whitespace.

  $ diff -w file1.txt file2.txt
  • Ignores spaces or tabs (e.g., "hello world" vs. "helloworld").

• Recursive Directory Comparison

  Use -r to compare directories and their contents.

  $ diff -r dir1/ dir2/
  • Compares all files in dir1 and dir2, reporting differences.

• Brief Output

  Use -q to only report whether files differ, not how.

  $ diff -q file1.txt file2.txt
  • Output: Files file1.txt and file2.txt differ (or nothing if identical).

To get help related to the diff command use --help option

$ diff --help