comm

Additional Notes

comm compares two sorted files and prints three columns: lines only in the first file, lines only in the second file, and lines common to both.

Use it when comparing sets of lines, such as package lists, usernames, hostnames, or file inventories.

Syntax

comm [options] file1 file2

Parameters

• file1: First sorted file.
• file2: Second sorted file.
• options: Controls for suppressing output columns.

Common Options

• -1: Suppress lines unique to file1.
• -2: Suppress lines unique to file2.
• -3: Suppress lines common to both files.
• --check-order: Check that input is sorted.
• --nocheck-order: Do not check sort order.

Examples

comm old.txt new.txt

Compare two sorted files in three columns.

comm -12 old.txt new.txt

Show only lines common to both files.

comm -23 old.txt new.txt

Show lines only in old.txt.

sort a.txt > a.sorted
sort b.txt > b.sorted
comm -13 a.sorted b.sorted

Show lines only in b.txt after sorting.

Practical Notes

• Inputs must be sorted using the same collation rules.
• Set LC_ALL=C for bytewise predictable sorting in scripts.
• Use diff when order and changed blocks matter.