$ Linux Command

Files

sum

Compute and verify file checksums.

checksumfileintegritysumverify

Additional Notes

sum computes a simple checksum and block count for one or more files. It uses a traditional Unix CRC algorithm (either BSD or System V style) that is much weaker than modern hash algorithms like MD5, SHA-256, or SHA-512. The checksum is computed from the file contents and the output includes the checksum value, the number of blocks (in 512-byte or 1024-byte units), and the filename.

The command is defined by POSIX but is rarely used today. For integrity verification, modern tools like md5sum, sha256sum, sha1sum, or cksum are preferred.

Syntax

sum [options] [file...]

Parameters

  • file: One or more files to compute checksums for. If omitted, reads from standard input.

Common Options

  • -r: Use BSD-compatible checksum algorithm (default). Output: checksum, block count (512-byte blocks), filename.
  • -s: Use System V-compatible checksum algorithm. Output: checksum, block count (1024-byte blocks), filename.
  • --help: Show usage information.
  • --version: Show version information.

Examples

sum file.txt

Compute the BSD checksum and block count for file.txt.

sum -s file.txt

Compute the System V checksum and block count for file.txt.

sum * > checksums.txt

Compute checksums for all files in the current directory and save.

cat file.txt | sum

Compute the checksum of data piped from stdin.

Practical Notes

  • The BSD algorithm (-r) computes a 16-bit checksum. Collisions are very easy to produce intentionally.
  • The System V algorithm (-s) computes a different 16-bit checksum with 1024-byte blocks.
  • Neither algorithm is cryptographically secure.
  • Use md5sum, sha256sum, or sha1sum for reliable integrity checking.
  • cksum uses a more robust CRC-32 algorithm and is also POSIX-specified.
  • sum is mainly encountered in legacy scripts and educational contexts.