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cmp – Compare two files for differences

The cmp command is used to compare two files and check if they are identical or different. It is a useful tool for verifying that two files contain the same data. The command compares the contents of two files byte by byte and reports the first byte that differs between the two files.

Overview

The basic syntax for using cmp is as follows:

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

Here, FILE1 and FILE2 are the two files that need to be compared. If FILE2 is not specified, cmp compares FILE1 with the standard input. SKIP1 and SKIP2 are the number of bytes to skip from the beginning of FILE1 and FILE2, respectively.

If cmp finds a difference between the files, it prints the byte offset and the differing byte values in octal format. If the files are identical, cmp produces no output.

Examples

  1. Compare two files:
$ cmp file1.txt file2.txt

This command will compare the contents of file1.txt and file2.txt. If the files differ, cmp will output the byte offset and the differing byte values.

  1. Compare files with different byte offsets:
$ cmp file1.txt file2.txt 10 20

This command will compare the contents of file1.txt and file2.txt, skipping the first 10 bytes in file1.txt and the first 20 bytes in file2.txt.

  1. Compare a file with standard input:
$ cmp file1.txt -

This command will compare the contents of file1.txt with the standard input. The hyphen (-) represents the standard input.

Options

The following table lists the available options for the cmp command:

Option Description
-b, –print-bytes Print differing bytes in octal format
-i, –ignore-initial=NUM Ignore the first NUM bytes in both files
-l, –verbose Print all differing bytes
-n, –bytes=NUM Compare up to NUM bytes
-s, –quiet, –silent Suppress output

Troubleshooting tips

  • If cmp produces no output, the files are identical.
  • If cmp reports a difference, check the byte offset and the differing byte values to identify the location and nature of the difference.
  • If cmp hangs or takes a long time to complete, try specifying a smaller number of bytes to compare using the -n option.
  • If cmp reports an error message, check that the files exist and that you have permission to read them.

Notes

  • The cmp command is not designed to compare binary files. Use the diff command instead.
  • The cmp command is case-sensitive. To perform a case-insensitive comparison, use the --ignore-case option.
  • The cmp command can be used to compare files of different sizes. If one file is shorter than the other, cmp reports a difference at the first differing byte.

Related commands