How to Compare Files Line by Line in Linux | diff Command

The diff command in Linux is a powerful tool used to compare files line by line and identify differences between them. It provides a clear and structured output, showing the lines that need to be changed, added, or removed to make the files identical.

Syntax of diff Command

diff [options] file1 file2

• file1: The first file to compare.
• file2: The second file to compare.
• [options]: Optional flags to modify the behavior or output format.

Basic Usage of diff

1. Compare Two Text Files:

   diff file1.txt file2.txt
   

   Output Explanation:

   • Lines starting with < are from file1.
   • Lines starting with > are from file2.
   • Contextual indicators (a, c, d) explain whether lines are added, changed, or deleted.
2. Side-by-Side Comparison:

   diff -y file1.txt file2.txt
   

   This displays the differences side by side:

   • The | symbol indicates differing lines.
   • < and > indicate missing or extra lines in one file.

Options with diff

1. Show Context Lines:

   diff -c file1.txt file2.txt
   

   Displays several lines of context around the differences for better clarity.

2. Unified Output:

   diff -u file1.txt file2.txt
   

   Produces a unified format, often used in patches.

3. Ignore Whitespace Differences:

   diff -w file1.txt file2.txt
   

   Ignores differences in spaces or tabs.

4. Suppress Identical Files:

   diff -q file1.txt file2.txt
   

   Displays only whether the files differ, without showing the actual differences.

5. Limit Output to a Specific Number of Lines:

   diff -C 3 file1.txt file2.txt
   

   Displays only 3 lines of context around the differences.

Example

Consider two files:

file1.txt:

apple
banana
cherry
date

file2.txt:

apple
banana
grape
date

Command:

diff file1.txt file2.txt

Output:

3c3
< cherry
---
> grape

Graphical Tools for Comparison

For users who prefer graphical tools, utilities like Meld or KDiff3 provide an intuitive interface for comparing files line by line.

Install Meld:

sudo apt install meld

Launch Meld:

meld file1.txt file2.txt

Conclusion

The diff command is a versatile tool for comparing files line by line in Linux. Whether you prefer minimalistic output, side-by-side comparison, or graphical interfaces, diff and its options provide flexibility and precision for file comparison tasks.