Content-type: text/html Man page of cksum

cksum

Section: User Commands (1)
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NAME

cksum - Displays the checksum and byte count of a file  

SYNOPSIS

cksum [file...]


 

STANDARDS

Interfaces documented on this reference page conform to industry standards as follows:

cksum:  XPG4, XPG4-UNIX

Refer to the standards(5) reference page for more information about industry standards and associated tags.
 

OPTIONS

None
 

OPERANDS

The path name of a file to be checked.

If no file is specified, standard input is used.
 

DESCRIPTION

The cksum command reads the files specified by the file argument and calculates a 32-bit checksum Cyclic Redundancy Check (CRC) and the byte count for each file. If no files are specified, standard input is read. The checksum, number of bytes, and file name are written to standard output. If standard input is used, no path name is printed.

The cksum command can be used to compare a suspect file copied or communicated over noisy transmission lines against an exact copy of a trusted file. The comparison made by the cksum command may not be cryptographically secure; however, it is unlikely that an accidentally damaged file will produce the same checksum as the original file.

[Compaq]  The cksum command uses a different algorithm than the sum command to calculate the 32-bit checksum CRC. The cksum command uses a CRC algorithm based on the Ethernet standard frame check. In addition, the sum block count is an octet count in cksum.

[Compaq]  The IEEE Std 1003.2-1992 and X/Open CAE specifications require a slightly different method of calculating checksum values. To obtain these new standards, set the environment variable CMD_ENV to xpg4 as follows: export CMD_ENV=xpg4

[Compaq]  To maintain compatibility if the environment variable CMD_ENV is not set, the default action of the cksum command reports the traditional checksum values.

The CRC checksum is obtained in the following way:

The 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: 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. 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 less than or equal to 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.
 

EXIT STATUS

The following exit values are returned: All files were processed successfully. An error occurred.
 

EXAMPLES

To display the checksum and the size, in bytes, of file1 and file2, enter: cksum file1 file2 3995432187 1390 file1 3266927833 20912 file2

This output shows that the checksum of the file1 file is 3995432187 and contains 1390 bytes, and that the checksum of the file2 file is 3266927833 and contains 20912 bytes.
 

ENVIRONMENT VARIABLES

The following environment variables affect the execution of cksum: [Compaq]  If this variable is set to xpg4, the method of calculating checksum values specified by IEEE Std 1003.2-1992 (POSIX.2) and X/Open CAE specifications is used. Provides a default value for the internationalization variables that are unset or null. If LANG is unset or null, the corresponding value from the default locale is used. If any of the internationalization variables contain an invalid setting, the utility behaves as if none of the variables had been defined. If set to a non-empty string value, overrides the values of all the other internationalization variables. Determines the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multibyte characters in arguments). Determines the locale for the format and contents of diagnostic messages written to standard error. Determines the location of message catalogues for the processing of LC_MESSAGES.
 

SEE ALSO

Commands:  sum(1), wc(1)

Standards:  standards(5)


 

Index

NAME
SYNOPSIS
STANDARDS
OPTIONS
OPERANDS
DESCRIPTION
EXIT STATUS
EXAMPLES
ENVIRONMENT VARIABLES
SEE ALSO

This document was created by man2html, using the manual pages.
Time: 02:42:55 GMT, October 02, 2010