CKSUM(1) User Commands CKSUM(1)

## NAME

cksum - write file checksums and sizes

cksum [file]...

## DESCRIPTION

The cksum command calculates and writes to standard output a cyclic
redundancy check (CRC) for each input file, and also writes to standard
output the number of octets in each file.

For each file processed successfully, cksum will write in the following
format:

"%u %d %s\n" <checksum>, <# of octets>, <path name>

If no file operand was specified, the path name and its leading space
will be omitted.

The CRC used is based on the polynomial used for CRC error checking in
the referenced Ethernet standard.

The encoding for the CRC checksum 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:

1. The 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 is used.

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

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

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

## OPERANDS

The following operand is supported:

file
A path name of a file to be checked. If no file operands are
specified, the standard input is used.

## USAGE

The cksum command is typically used to quickly compare a suspect file
against a trusted version of the same, such as to ensure that files
transmitted over noisy media arrive intact. However, this comparison
cannot be considered cryptographically secure. The chances of a damaged
file producing the same CRC as the original are astronomically small;
deliberate deception is difficult, but probably not impossible.

Although input files to cksum can be any type, the results need not be
what would be expected on character special device files. Since this
document does not specify the block size used when doing input, checksums
of character special files need not process all of the data in those
files.

The algorithm is expressed in terms of a bitstream divided into octets.
If a file is transmitted between two systems and undergoes any data
transformation (such as moving 8-bit characters into 9-bit bytes or
changing "Little Endian" byte ordering to "Big Endian"), identical CRC
values cannot be expected. Implementations performing such
transformations may extend cksum to handle such situations.

See largefile(5) for the description of the behavior of cksum when
encountering files greater than or equal to 2 Gbyte ( 2^31 bytes).

## ENVIRONMENT VARIABLES

See environ(5) for descriptions of the following environment variables
that affect the execution of cksum: LANG, LC_ALL, LC_CTYPE, LC_MESSAGES,
and NLSPATH.

## EXIT STATUS

The following exit values are returned:

0
All files were processed successfully.

>0
An error occurred.

## ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+