Sum (Unix)

sum is a legacy utility available on some Unix and Unix-like operating systems. This utility outputs a 16-bit checksum of each argument file, as well as the number of blocks they take on disk.^[1] Two different checksum algorithms are in use. POSIX abandoned sum in favor of cksum.

sum

Original author(s)	Ken Thompson
Developer(s)	AT&T Bell Laboratories
Initial release	November 3, 1971; 52 years ago (1971-11-03)
Operating system	Unix, Unix-like, Inferno
Platform	Cross-platform
Type	Command
License	coreutils: GPLv3+

Overview

The sum program is generally only useful for historical interest. It is not part of POSIX. Two algorithms are typically available: a BSD checksum and a SYSV checksum. Both are weaker than the already weak 32-bit CRC used by cksum.^[2]

The default algorithm on FreeBSD and GNU implementations is the BSD checksum. Switching between the two algorithms is done via command line options.^[2][1]

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 above algorithm appeared in Seventh Edition Unix.

The System V sum, -s in GNU sum and -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.

Example of use:

$ echo Hello > testfile
$ sum testfile
36978     1

Example of -s use in GNU sum:

$ echo Hello > testfile
$ sum -s testfile
510 1 testfile

Example of using standard input, -r and printf to avoid newline:

$ printf Hello | sum -r
08401     1

See also

• GNU Core Utilities
• UnxUtils port to native Win32

References

1. sum(1) — manual pages from GNU coreutils
2. sum(1) – FreeBSD General Commands Manual

External links

• sum in The Single UNIX ® Specification, Version 2, 1997, opengroup.org – does not specify checksum algorithm
• 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
• The Unix Tree, tuhs.org – the source code of old versions of Unix