cmp Command in Linux: Compare Files Byte by Byte (Examples & Cheat Sheet)

The cmp command is a lightweight Linux utility used to compare two files byte by byte. Unlike diff, which works line by line, cmp focuses on low-level comparison and reports the exact byte and line number where the first difference occurs.

If the files are identical, cmp produces no output, making it ideal for scripts and automated checks.

cmp Command - Quick Cheat Sheet

Basic cmp Command Usage

Compare two files byte by byte

Use the cmp command without any options to compare two files from the beginning.

cmp file1 file2

If a difference is found, cmp reports the byte and line number of the first mismatch:

file1 file2 differ: byte 145, line 3

If the files are identical, no output is displayed and the command exits silently.

Compare symbolic links

The cmp command can also be used to compare symbolic links. In this case, cmp compares the contents of the files the links point to, not the symbolic link paths themselves.

cmp symlink1 symlink2

You can also compare a symbolic link directly with its target file:

cmp symlink1 actual_file

If both resolve to the same underlying file content, cmp produces no output, indicating that the files are identical. This is useful for validating deployment symlinks and versioned configuration files.

Viewing Differences with cmp

The cmp command provides multiple options to control how differences between files are displayed. By default, it stops at the first mismatch, but with specific options you can inspect the actual byte values or view all differences across the files.

Print differing bytes (-b)

Use the -b or --print-bytes option to display the actual differing byte values when a mismatch is found.

cmp -b file1 file2

Sample output:

file1 file2 differ: byte 145, line 3 is 162 r 151 i

In this output:

• 145 is the byte number where the difference occurs
• 162 and 151 are the byte values
• r and i are the corresponding character representations

This option is useful when debugging text or binary files where the exact byte difference matters.

Display all differing bytes (-l)

By default, cmp stops after the first difference. Use the -l or --verbose option to list all differing bytes between two files.

cmp -l file1 file2

Sample output:

145 162 151
146 145 163
147 160 40
148 157 141
...

Each line represents:

• The byte number
• The byte value from the first file
• The byte value from the second file

Skipping and Limiting Bytes in cmp

The cmp command allows you to skip a specific number of bytes or limit how much of the files are compared. This is useful when files share a common header or when only part of the content needs to be validated.

Skip initial bytes (-i)

Use the -i or --ignore-initial option to skip the first N bytes in both files before comparison begins.

cmp -i 150 file1 file2

After skipping the specified number of bytes, cmp starts comparing from the next byte and reports the first difference found.

Skip different bytes per file

You can skip a different number of bytes in each file by specifying two values separated by a colon.

cmp -i 150:180 file1 file2

This means:

• The first 150 bytes of file1 are skipped
• The first 180 bytes of file2 are skipped

You can also skip bytes without using the -i option by passing them as positional arguments.

cmp file1 file2 150
cmp file1 file2 150 180

Compare limited bytes (-n)

Use the -n or --bytes option to limit the comparison to a specific number of bytes.

cmp -n 160 file1 file2

Only the first 160 bytes of both files are compared. This is useful when validating file headers, magic numbers, or partial file content without comparing the entire file.

Using cmp in Shell Scripts

Silent comparison using exit codes (-s)

Use the -s, --quiet, or --silent option to suppress all normal output and rely entirely on the exit status.

cmp -s file1 file2

You can then check the exit code using $? to determine the result.

cmp -s file1 file2
if [ $? -eq 0 ]; then
    echo "Files are identical"
elif [ $? -eq 1 ]; then
    echo "Files are different"
else
    echo "Comparison failed"
fi

Exit status meanings:

• 0 – Files are identical
• 1 – Files differ
• >1 – An error occurred

This approach avoids unnecessary output and keeps scripts clean.

Handle errors and suppress stderr

The -s option suppresses successful output but does not suppress error messages. To hide errors, redirect standard error to /dev/null.

cmp file1 file2 extra_file 2>/dev/null

cmp Exit Codes and Version Info

cmp exit status explained

The cmp command returns a numeric exit code instead of verbose output, making it ideal for conditional logic in shell scripts.

Exit status values:

• 0 – The files are identical
• 1 – The files are different
• >1 – An error occurred (missing file, permission issue, invalid arguments)

Example:

cmp -s file1 file2
echo $?

Check cmp version

To display the installed version of the cmp command, use the --version option.

cmp --version

Sample output:

cmp (GNU diffutils) 3.10
Copyright (C) 2023 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Torbjörn Granlund and David MacKenzie.

Summary

The cmp command is a simple yet powerful tool for comparing files at the byte level in Linux. Unlike line-based tools, cmp focuses on detecting exact binary differences and reports precise byte and line positions.

Key takeaways:

• Compares files byte by byte with minimal overhead
• Produces no output when files are identical
• Supports advanced options to view, skip, or limit comparisons
• Provides reliable exit codes for scripting and automation
• Ideal for binary files, configuration validation, and CI checks

When accuracy and automation matter, cmp is the preferred choice over line-oriented comparison tools.

Further Reading

• man page for cmp command