comm Command Cheat Sheet

comm compares two sorted files line by line. It is the standard tool for performing set operations (intersection, difference, union) on text files in the terminal.

Synopsis

comm [OPTION]... FILE1 FILE2

Description

comm reads FILE1 and FILE2 (which must be sorted) and produces output with three columns: 1. Lines only in FILE1 2. Lines only in FILE2 3. Lines in both files

If a file is not sorted, comm might complain or produce incorrect results.

Basic Usage

Compare Two Files

file1.txt:

apple
banana
cherry

file2.txt:

banana
date
elderberry

Command:

comm file1.txt file2.txt

Output:

apple
        banana
    date
    elderberry
cherry

(Note: The alignment above depends on tabs. Column 1 is unique to file1, Column 2 unique to file2, Column 3 common.)

Correct Output for Example Above

apple
        banana
cherry
    date
    elderberry

(Wait, if cherry is in file1 but not file2, it should be in col 1. If date is in file2 but not file1, col 2. banana in both, col 3).

Column Suppression (The Power of comm)

The most useful way to use comm is by suppressing columns you don't want.

Option	Suppresses	Result
`-1`	Column 1	Show unique to `FILE2` and common lines
`-2`	Column 2	Show unique to `FILE1` and common lines
`-3`	Column 3	Show only unique lines (Difference)
`-12`	Col 1 & 2	Show intersection (only common lines)
`-13`	Col 1 & 3	Show lines only in `FILE2`
`-23`	Col 2 & 3	Show lines only in `FILE1`

Practical Examples

Find lines common to both files (Intersection):

comm -12 file1.txt file2.txt
# Output: banana

Find lines in file1 but NOT in file2 (Difference):

comm -23 file1.txt file2.txt
# Output: apple, cherry

Find lines in file2 but NOT in file1:

comm -13 file1.txt file2.txt
# Output: date, elderberry

Print all unique lines from both files (Symmetric Difference):

comm -3 file1.txt file2.txt | sed 's/\t//g'
# Output: apple, cherry, date, elderberry

Advanced Usage

Sorting Inline

Since comm requires sorted input, you often use it with process substitution to sort on the fly.

comm -12 <(sort unsorted1.txt) <(sort unsorted2.txt)

Case Insensitive Comparison

Use sort -f (ignore case) inside the substitution, but comm lacks a native ignore-case flag in many versions (GNU comm has --nocheck-order but not case folding). Better to lowercase everything first if needed.

comm -12 <(tr 'A-Z' 'a-z' < file1 | sort) <(tr 'A-Z' 'a-z' < file2 | sort)

Changing Separator

By default, columns are separated by tabs. Use --output-delimiter to change this.

comm -12 --output-delimiter="," file1.txt file2.txt

Check Order

If you are unsure if files are sorted:

comm --check-order file1.txt file2.txt

It will report an error if files are not sorted.

Practical Scenarios

Compare Lists of Users

Find users who are in both the developers group and the admins group (assuming you have list files).

comm -12 dev_users.txt admin_users.txt

Validating a Migration

You migrated lines from a database. 1. Export source IDs to source.txt. 2. Export destination IDs to dest.txt. 3. Check for missing IDs:

comm -23 <(sort source.txt) <(sort dest.txt)
# Shows IDs that are in source but missing in dest

Server Inventory Check

Compare a list of expected packages vs installed packages.

comm -23 expected_packages.txt <(dpkg --get-selections | awk '{print $1}' | sort)
# Lists packages you expected but are NOT installed

comm vs diff

Feature	comm	diff
Input Requirement	Must be sorted	Can be unsorted
Output Style	3 Columns	Contextual patch format (<, >)
UseCase	Set logic (Intersection/Difference)	Content changes/Patching
Complexity	Simple filter	distinctive analysis

Exit Status

Code	Meaning
0	Success
1	Error (usually unsorted input if checking enabled)

Tips

Always sort first! comm relies entirely on lexical order. If "apple" comes after "banana" in your file, logic breaks.
Use man comm to check for --nocheck-order if you want to force a comparison on unsorted files (results undefined though).