comm

Additional Notes

comm compares two already-sorted files line by line and prints three columns: lines only in the first file, lines only in the second file, and lines in both. It is faster and simpler than diff when both inputs are sorted.

Both input files must be sorted the same way, or the output is unreliable. Use sort first when the files are not already sorted.

Syntax

comm [options] FILE1 FILE2

Parameters

• options: Flags that suppress columns.
• FILE1, FILE2: The two sorted files to compare.

Common Options

• -1: Suppress column 1 (lines unique to FILE1).
• -2: Suppress column 2 (lines unique to FILE2).
• -3: Suppress column 3 (lines common to both).
• --check-order: Warn if the input is not sorted.
• --nocheck-order: Do not warn about sort order.

Output Layout

• Column 1: lines only in FILE1.
• Column 2: lines only in FILE2.
• Column 3: lines in both files.

Examples

comm a.txt b.txt

Show all three columns.

comm -23 a.txt b.txt

Print lines only in a.txt (suppress columns 2 and 3).

comm -13 a.txt b.txt

Print lines only in b.txt.

comm -12 a.txt b.txt

Print only the lines common to both files.

sort list-a.txt > a.sorted
sort list-b.txt > b.sorted
comm -23 a.sorted b.sorted

Sort both files first, then show what is unique to the first.

Practical Notes

• Both files must be sorted identically; otherwise the result is wrong.
• Use sort on both inputs before comparing.
• comm -23 / comm -13 are handy for set-difference operations.
• comm -12 gives the set intersection of two sorted lists.
• For unsorted or patch-style comparison, use diff instead.