cmp — Byte-wise comparison

Linux / Basic Linux commands / cmp.html

Compares two files byte by byte and reports the first difference.

Basic Information

What is this command?

cmp compares two files byte by byte from the start and reports the first differing offset and values. This differs from diff, which shows text differences by line.

The -s option returns only an exit status, which suits scripts that test equality. It is especially useful for binary or compressed files.

What problems does it solve?

• Verifying a copied file matches the original
• Comparing binaries or compressed files
• Testing equality in scripts
• When line-based diff is not decisive

Key features

First difference

Reports the offset and values of the first difference.

• When to use: Finding where files diverge
• What you get: It pinpoints the byte precisely.
• Limitations: It does not list every difference at once.

Quiet comparison

The -s flag suppresses output and returns a status.

• When to use: When scripts only need equality
• What you get: Branching becomes simple.
• Limitations: You cannot see what differs.

Length checks

It also reports when lengths differ.

• When to use: Detecting truncated files
• What you get: Transfer problems surface quickly.
• Limitations: Equal content with different length is rare.

Installation and usage

Check whether it is installed

cmp --version

sudo apt install diffutils # Debian/Ubuntu sudo dnf install diffutils # Fedora

Real-world examples

Byte comparison

Show the first difference.

cmp a.bin b.bin

Quiet comparison

Get equality via exit status only.

cmp -s a.bin b.bin && echo SAME || echo DIFF

Compare archives

Check whether archives match.

cmp -s out1.tar.gz out2.tar.gz; echo $?