display file checksums and block
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:
- 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.
- 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 options are as follows:
- Use the specified algorithm(s) instead of the default (cksum). Supported
algorithms include cksum, md5, rmd160, sha1, sha224, sha256, sha384,
sha512/256, and sha512. Multiple algorithms may be specified, separated by
a comma or whitespace. Additionally, multiple
-aoptions 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 algorithm.
- 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 algorithm.
- Compare the checksum of each file against the checksums in the checklist. Any specified file that is not listed in the checklist will generate an error.
- If this option is specified, the file options become
checklists. Each checklist should contain hash results in the normal
format, which will be verified against the specified paths. Output
consists of the digest used, the file name, and an OK, FAILED, or MISSING
for the result of the comparison. This will validate any of the supported
checksums. If no file is given, stdin is used. The
-coption may not be used in conjunction with more than a single
- Place the checksum into hashfile instead of stdout.
- Echoes stdin to stdout and appends the checksum to stdout.
- Only print the checksum (quiet mode) or if used in conjunction with the
-cflag, only print the failed cases.
- Reverse the format of the hash algorithm output, making it match the checksum output format.
- Prints a checksum of the given string.
- Runs a built-in time trial. Specifying
-tmultiple times results in the number of rounds being multiplied by 10 for each additional flag.
- Runs a built-in test script.
The default CRC used is based on the polynomial used for CRC error checking in the networking standard ISO/IEC 8802-3:1996. The other available algorithms are described in their respective man pages in section 3 of the manual.
cksum utility exits 0 on
success, and >0 if an error occurs.
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.
cksum utility is compliant with the
IEEE Std 1003.1-2008 (“POSIX.1”)
All the flags are extensions to that specification.
cksum utility appeared in
Do not use the cksum or md5 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.