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comm

file comparisonLinux/Unix

The comm command is one of the most frequently used commands in Linux/Unix-like operating systems. comm Compare two sorted files line by line

See Examples View Syntax

Quick Reference

Command Name:

comm

Category:

file comparison

Platform:

Linux/Unix

Basic Usage:

comm [options] [arguments]

Common Use Cases

1
Set operations
Perform set operations on sorted files
2
Data analysis
Analyze common and unique lines between files
3
File merging
Identify common lines for file merging operations
4
Data deduplication
Find duplicate lines across multiple files

Syntax

comm [OPTION]... FILE1 FILE2

Options

Option	Description
`-1`	Suppress lines unique to FILE1
`-2`	Suppress lines unique to FILE2
`-3`	Suppress lines that appear in both files
`--check-order`	Check that the input is correctly sorted, even if all input lines are pairable
`--nocheck-order`	Do not check that the input is correctly sorted
`--output-delimiter=STR`	Separate columns with STR
`--total`	Output a summary
`-z, --zero-terminated`	Line delimiter is NUL, not newline
`--help`	Display help message and exit
`--version`	Output version information and exit

Examples

How to Use These Examples

The examples below show common ways to use the comm command. Try them in your terminal to see the results. You can copy any example by clicking on the code block.

Basic Examples:

Show lines unique to file1, unique to file2, and common to both

comm file1.txt file2.txt

Show only lines unique to file1

comm -23 file1.txt file2.txt

Show only lines unique to file2

comm -13 file1.txt file2.txt

Show only lines common to both files

comm -12 file1.txt file2.txt

Advanced Examples:

Use with sort to ensure input is sorted

sort file1.txt > sorted1.txt

sort file2.txt > sorted2.txt comm sorted1.txt sorted2.txt

Compare files with case insensitivity

sort -f file1.txt | comm -12 - <(sort -f file2.txt)

Combine with other utilities

sort file1.txt file2.txt | uniq -d | comm -12 - <(sort file1.txt)

Compare word lists and redirect outputs to separate files

comm -1 <(sort wordlist1.txt) <(sort wordlist2.txt) > only_in_wordlist2.txt

comm -2 <(sort wordlist1.txt) <(sort wordlist2.txt) > only_in_wordlist1.txt

Try It Yourself

Practice makes perfect! The best way to learn is by trying these examples on your own system with real files.

Understanding Syntax

Pay attention to the syntax coloring: commands, options, and file paths are highlighted differently.

Notes

The comm command compares two sorted files line by line and outputs three columns: lines unique to the first file, lines unique to the second file, and lines common to both files.

Key points about comm:

Input files MUST be sorted for comm to work correctly
By default, the output is in three columns separated by tabs
Column 1: Lines unique to FILE1
Column 2: Lines unique to FILE2
Column 3: Lines common to both files
Options -1, -2, and -3 suppress the display of the respective columns
Can use - for standard input instead of a file

Common use cases:

Finding differences and similarities between two lists
Comparing text files that have been sorted
Identifying common elements between two sets of data
Creating set operations (union, intersection, difference) on text files
Finding unique or duplicate lines between files

Tips for using comm effectively:

Always sort input files first: sort file1.txt > sorted1.txt
Use process substitution in bash for inline sorting: comm <(sort file1.txt) <(sort file2.txt)
Combine options to isolate specific data: comm -23 = only lines unique to first file
Use --output-delimiter to make output more readable or easier to parse
For binary files or non-text data, consider using cmp or diff instead

Unlike diff which shows how to change the first file to match the second file, comm simply shows which lines are unique or common to both files.

Common Use Cases

Set operations

Perform set operations on sorted files

Data analysis

Analyze common and unique lines between files

File merging

Identify common lines for file merging operations

Data deduplication

Find duplicate lines across multiple files

Text processing

Process and compare text files systematically

Related Commands

These commands are frequently used alongside comm or serve similar purposes:

Explore All Commands

Use Cases

1

Set operations

Perform set operations on sorted files

2

Data analysis

Analyze common and unique lines between files

3

File merging

Identify common lines for file merging operations

4

Data deduplication

Find duplicate lines across multiple files

5

Text processing

Process and compare text files systematically

Learn By Doing

The best way to learn Linux commands is by practicing. Try out these examples in your terminal to build muscle memory and understand how the comm command works in different scenarios.

See Examples Try Lab Exercises

$ comm

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