cmp Command Cheat Sheet

cmp (compare) is a utility for comparing two files of any type (text or binary) byte by byte. It is faster and simpler than diff when you just want to know if files differ or where the first difference is.

Synopsis

cmp [OPTION]... FILE1 [FILE2 [SKIP1 [SKIP2]]]

Description

cmp compares two files byte by byte. - If files are identical, it is silent (prints nothing) and returns exit code 0. - If they differ, it prints the byte and line number of the first difference and returns exit code 1. - If one file is a prefix of the other (one ends early), it prints a message to stderr.

Basic Usage

Compare Two Files

cmp file1.bin file2.bin

Output if different:

file1.bin file2.bin differ: byte 9, line 1

Compare File with Standard Input

echo "hello" | cmp - file.txt

Options

-b, --print-bytes

Print the differing bytes.

cmp -b file1 file2

Output:

file1 file2 differ: byte 9, line 1 is 150 h 145 e

-l, --verbose

Output all differing bytes (byte number and octal values).

cmp -l file1 file2

Output:

9 150 145
10 145 154

This is extremely useful for binary file analysis.

-s, --quiet, --silent

Suppress all normal output. Use only the exit code.

cmp -s file1 file2

Perfect for if statements in scripts.

-i, --ignore-initial=SKIP

Skip text/data at the beginning of files.

# Skip first 100 bytes of both
cmp -i 100 file1 file2

# Skip 100 bytes of file1 and 50 bytes of file2
cmp -i 100:50 file1 file2

-n, --bytes=LIMIT

Compare at most LIMIT bytes.

# Compare only the first 1KB
cmp -n 1024 file1 file2

Exit Codes

cmp exit codes are standardized and reliable for scripting.

Practical Examples

Check if ISO Download is Correct

After downloading image.iso, compare it against a verified copy or check if a burned disk matches the image.

cmp /dev/cdrom image.iso

Compare Header of Files

Check if two files have the same 10-byte header.

cmp -n 10 file1. header file2.header

Conditional Script Execution

Run a command only if configuration has changed.

if ! cmp -s config.conf config.conf.bak; then
    echo "Config changed, restarting service..."
    systemctl restart myservice
    cp config.conf config.conf.bak
fi

Locating Binary Corruption

If two binaries should be the same but have a different checksum, finding where they differ can give a clue (e.g., specific offset might correspond to a metadata field).

cmp -l bad_file good_file | head -n 5

cmp vs diff

Notes

• Byte counts are 1-based.
• Input can be from pipes/stdin (- as filename).
• cmp essentially answers "Are these the same?" while diff answers "How are these different?".