## NAME

`cksum`

, `sum`

— display file checksums and
block counts

## SYNOPSIS

`cksum` |
[`-bpqrtx` ]
[`-a` algorithms]
[`-c` [checklist ...]]
[`-o` 1 | 2]
[`-s` string]
[file ...] |

`sum` |
[`-bpqrtx` ] [`-a`
algorithms] [`-c`
[checklist ...]] [`-o`
1 | 2]
[`-s` string]
[file ...] |

## DESCRIPTION

The `cksum`

utility writes to the standard
output a single line for each input file. The format of this line varies
with the algorithm being used as follows:

- cksum
- The output line consists of three whitespace separated fields: a CRC checksum, the number of octets in the input, and name of the file or string. If no file name is specified, the standard input is used and no file name is written.
- sum
- The output line consists of three whitespace separated fields: a CRC checksum, the number of kilobytes in the input, and name of the file or string. If no file name is specified, the standard input is used and no file name is written.
- sysvsum
- The output line consists of three whitespace separated fields: a CRC checksum, the number of 512-byte blocks in the input, and name of the file or string. If no file name is specified, the standard input is used and no file name is written.
- all others
- The output line consists of four whitespace separated fields: the name of the algorithm used, the name of the file or string in parentheses, an equals sign, and the cryptographic hash of the input. If no file name is specified, the standard input is used and only the cryptographic hash is output.

The `sum`

utility is identical to the
`cksum`

utility, except that it defaults to using
historic algorithm 1, as described below. It is provided for compatibility
only.

The options are as follows:

`-a`

`algorithms`- Use the specified algorithm(s) instead of the default (cksum). Supported
algorithms include
`cksum`,`md4`,`md5`,`rmd160`,`sha1`,`sha256`,`sha384`,`sha512`,`sum`, and`sysvsum`. Multiple algorithms may be specified, separated by a comma or whitespace. Additionally, multiple`-a`

options may be specified on the command line. Case is ignored when matching algorithms. The output format may be specified on a per-algorithm basis by using a single-character suffix, e.g. “sha256b”. If the algorithm has a ‘b’ suffix, the checksum will be output in base64 format. If the algorithm has an ‘x’ suffix, the checksum will be output in hex format. If an algorithm with the same output format is repeated, only the first instance is used. Note that output format suffixes are not supported for the`cksum`,`sum`and`sysvsum`algorithms. `-b`

- Output checksums in base64 notation, not hexadecimal by default. A
‘b’ or ‘x’ suffix on the algorithm will
override this default. This option is ignored for the
`cksum`,`sum`and`sysvsum`algorithms, which do not use hexadecimal output. `-c`

[`checklist ...`]- Compares all checksums contained in the file
`checklist`with newly computed checksums for the corresponding files. Output consists of the digest used, the file name, and an OK or FAILED for the result of the comparison. This will validate any of the supported checksums. If no file is given, stdin is used. The`-c`

option may not be used in conjunction with more than a single`-a`

option. `-o`

`1`|`2`- Use historic algorithms instead of the (superior) default one (see below).
`-p`

- Echoes stdin to stdout and appends the checksum to stdout.
`-q`

- Only print the checksum (quiet mode) or if used in conjunction with the
`-c`

flag, only print the failed cases. `-r`

- Reverse the format of the hash algorithm output, making it match the checksum output format.
`-s`

`string`- Prints a checksum of the given
`string`. `-t`

- Runs a built-in time trial. Specifying
`-t`

multiple times results in the number of rounds being multiplied by 10 for each additional flag. `-x`

- Runs a built-in test script.

Algorithm 1 (aka `sum`) is the algorithm used
by historic BSD systems as the
`sum`

algorithm and by historic
AT&T System V UNIX systems as the
`sum`

algorithm when using the
`-r`

option. This is a 16-bit checksum, with a right
rotation before each addition; overflow is discarded.

Algorithm 2 (aka `sysvsum`) is the algorithm
used by historic AT&T System V UNIX
systems as the default `sum`

algorithm. This is a
32-bit checksum, and is defined as follows:

s = sum of all bytes; r = s % 2^16 + (s % 2^32) / 2^16; cksum = (r % 2^16) + r / 2^16;

Both algorithm 1 and 2 write to the standard output the same fields as the default algorithm, except that the size of the file in bytes is replaced with the size of the file in blocks. For historic reasons, the block size is 1024 for algorithm 1 and 512 for algorithm 2. Partial blocks are rounded up.

The default CRC used is based on the polynomial used for CRC error checking in the networking standard ISO 8802-3: 1989. The CRC checksum encoding is defined by the generating polynomial:

G(x) = x^32 + x^26 + x^23 + x^22 + x^16 + x^12 + x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1

Mathematically, the CRC value corresponding to a given file is defined by the following procedure:

`n`bits to be evaluated are considered to be the coefficients of a mod 2 polynomial M(x) of degree

`n`-1. These

`n`bits are the bits from the file, with the most significant bit being the most significant bit of the first octet of the file and the last bit being the least significant bit of the last octet, padded with zero bits (if necessary) to achieve an integral number of octets, followed by one or more octets representing the length of the file as a binary value, least significant octet first. The smallest number of octets capable of representing this integer are used.

M(x) is multiplied by x^32 (i.e., shifted left 32 bits) and divided by G(x) using mod 2 division, producing a remainder R(x) of degree <= 31.

The coefficients of R(x) are considered to be a 32-bit sequence.

The bit sequence is complemented and the result is the CRC.

The other available algorithms are described in their respective man pages in section 3 of the manual.

## EXIT STATUS

The `cksum`

and `sum`

utilities exit 0 on success or >0 if an error occurred.

## SEE ALSO

The default calculation is identical to that given in pseudo-code in the following ACM article:

Dilip V. Sarwate,
Computation of Cyclic Redundancy Checks Via Table
Lookup, *Communications of the ACM*,
August 1988.

## STANDARDS

The `cksum`

utility is compliant with the
IEEE Std 1003.1-2008 (“POSIX.1”)
specification.

All the flags are extensions to that specification.

## HISTORY

A `sum`

command appeared in
Version 2 AT&T UNIX. The
`cksum`

utility appeared in
4.4BSD.

## CAVEATS

Do not use the `cksum`,
`md4`, `md5`,
`sum`, or `sysvsum` algorithms to
verify file integrity. An attacker can trivially produce modified payload
that has the same checksum as the original version. Use a cryptographic
checksum instead.