sum (Unix)

sum

Original author(s)	Ken Thompson
Developer(s)	AT&T Bell Laboratories
Initial release	November 3, 1971 (1971-11-03)
Operating system	Unix, Unix-like, Inferno
Platform	Cross-platform
Type	Command
License	coreutils: GPLv3+

sum is a legacy utility available on some Unix and Unix-like operating systems. This utility outputs the checksum of each argument file, as well as the number of blocks they take on disk.

Overview

The sum program is generally only useful for historical interest. It is not part of POSIX. Two algorithms are typically available: a 16-bit BSD checksum and a 32-bit SYSV checksum. Both are weaker than the (already weak) CRC32 used by cksum.

The default algorithm on FreeBSD and GNU implementations is the weaker BSD checksum. Switching between the two algorithms is done via command line options.

The two commonly used algorithms are as follows.

The BSD sum, -r in GNU sum and -o1 in FreeBSD cksum:

• Initialize checksum to 0
• For each byte of the input stream
  • Perform 16-bit bitwise right rotation by 1 bit on the checksum
  • Add the byte to the checksum, and apply modulo 2 ^ 16 to the result, thereby keeping it within 16 bits
• The result is a 16-bit checksum

The System V sum, -s in GNU sum -o2 in FreeBSD cksum:

• checksum0 = sum of all bytes of the input stream modulo 2 ^ 32
• checksum1 = checksum0 modulo 2 ^ 16 + checksum0 / 2 ^ 16
• checksum = checksum1 modulo 2 ^16 + checksum1 / 2 ^ 16
• The result is a 16-bit checksum calculated from the initial 32-bit plain byte sum

Syntax

The sum utility is invoked from the command line according to the following syntax:

sum [OPTION]... [FILE]...

with the possible option parameters being:

• -r
  • use BSD checksum algorithm, use 1K blocks (defeats -s)
• -s, --sysv
  • use SYSV checksum algorithm, use 512 bytes blocks
• --help
  • display the help screen and exit
• --version
  • output version information and exit

When no file parameter is given, or when FILE is -, the standard input is used as input file.

See also

• GNU Core Utilities
• UnxUtils port to native Win32

External links

• sum in The Single UNIX ® Specification, Version 2, 1997, opengroup.org
• C.4 Utilities in The Open Group Base Specifications Issue 7, 2018 edition, opengroup.org – indicates sum as removed
• sum(1) – Version 7 Unix Programmer's Manual
• sum(1) – manual pages from GNU coreutils
• sum(1) – FreeBSD General Commands Manual – states the sum algorithms in options -o1 and -o2 to cksum
• sum(1) – Linux User Manual – User Commands
• sum(1) – Solaris 11.4 User Commands Reference Manual