mcookie(1) — Linux manual page


MCOOKIE(1)                      User Commands                     MCOOKIE(1)

NAME         top

       mcookie - generate magic cookies for xauth

SYNOPSIS         top

       mcookie [options]

DESCRIPTION         top

       mcookie generates a 128-bit random hexadecimal number for use with
       the X authority system.  Typical usage:

              xauth add :0 . `mcookie`

       The "random" number generated is actually the MD5 message digest of
       random information coming from one of the sources getrandom() system
       call, /dev/urandom, /dev/random, or the libc pseudo-random functions,
       in this preference order. See also the option --file.

OPTIONS         top

       -f, --file file
              Use this file as an additional source of randomness (for
              example /dev/urandom).  When file is '-', characters are read
              from standard input.

       -m, --max-size number
              Read from file only this number of bytes.  This option is
              meant to be used when reading additional randomness from a
              file or device.

              The number argument may be followed by the multiplicative
              suffixes KiB=1024, MiB=1024*1024, and so on for GiB, TiB, PiB,
              EiB, ZiB and YiB (the "iB" is optional, e.g., "K" has the same
              meaning as "KiB") or the suffixes KB=1000, MB=1000*1000, and
              so on for GB, TB, PB, EB, ZB and YB.

       -v, --verbose
              Inform where randomness originated, with amount of entropy
              read from each source.

       -V, --version
              Display version information and exit.

       -h, --help
              Display help text and exit.

FILES         top


BUGS         top

       It is assumed that none of the randomness sources will block.

SEE ALSO         top

       md5sum(1), X(7), xauth(1), rand(3)

AVAILABILITY         top

       The mcookie command is part of the util-linux package and is
       available from Linux Kernel Archive 

COLOPHON         top

       This page is part of the util-linux (a random collection of Linux
       utilities) project.  Information about the project can be found at 
       ⟨⟩.  If you have a
       bug report for this manual page, send it to  This page was obtained from the
       project's upstream Git repository
       ⟨git://⟩ on
       2020-07-14.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-07-14.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

util-linux                      December 2014                     MCOOKIE(1)