comm

What it does

comm compares two sorted files and outputs three columns:

• Column 1 — lines only in file1
• Column 2 — lines only in file2
• Column 3 — lines in both files

How it works (mechanical)

comm walks both sorted inputs simultaneously (like merge-sort comparison). Because of this, both files must be sorted first.

10 Practical Examples

# 1) Basic comparison
comm file1.txt file2.txt

# 2) Show only lines unique to file1
comm -23 file1.txt file2.txt

# 3) Show only lines unique to file2
comm -13 file1.txt file2.txt

# 4) Show only common lines
comm -12 file1.txt file2.txt

# 5) Compare sorted mount user lists
sort inFSTAB > f1
sort inu0users > f2
comm f1 f2

# 6) One-liner comparison
comm >(sort file1) >(sort file2)

# 7) Find missing accounts
comm -23 >(sort expected.txt) >(sort actual.txt)

# 8) Intersection set operation
comm -12 >(sort groupA.txt) >(sort groupB.txt)

# 9) Suppress column 3 (common lines)
comm -3 file1 file2

# 10) Compare pipeline output
comm >(ls dir1 | sort) >(ls dir2 | sort)

Notes & Gotchas

• Both inputs must be sorted.
• Whitespace differences count as differences.
• Columns are separated by tabs.
• Use process substitution <( ) for elegant comparisons.

Historical Context

comm is one of the classic Unix set-comparison tools. It pairs naturally with sort and reflects Unix’s “text as data” philosophy.

Related Commands

• diff — detailed differences
• sort — required pre-step
• uniq — remove duplicates
• join — relational-style join