cksum - write file checksums and sizes
cksum [file ...]
The cksum utility 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. 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) = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + 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 x32 (that is, 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 following operand is supported:
- A pathname of a file to be checked. If no file operands are specified, the standard input is used.
The standard input is used only if no file operands are specified. See the INPUT FILES section.
The input files can be any file type.
The following environment variables affect the execution of cksum:
- Provide a default value for the internationalisation variables that are unset or null. If LANG is unset or null, the corresponding value from the implementation-dependent default locale will be used. If any of the internationalisation variables contains an invalid setting, the utility will behave as if none of the variables had been defined.
- If set to a non-empty string value, override the values of all the other internationalisation variables.
- Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single- as opposed to multi-byte characters in arguments).
- Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
- Determine the location of message catalogues for the processing of LC_MESSAGES .
For each file processed successfully, the cksum utility will write in the following format:
"%u %d %s\n", <checksum>, <# of octets>, <pathname>
If no file operand was specified, the pathname and its leading space will be omitted.
Used only for diagnostic messages.
The following exit values are returned:
- All files were processed successfully.
- An error occurred.
The cksum utility 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 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 or on file types not described by the XSH specification. Since this specification 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.