32-bit uniform random number generation in the shell in the range of [1, n]

0-README.md

The following shell scripts pass POSIX compiance with posh(1) on Debian. They have also been tested in bash(1), ksh(1), and dash(1) (which provides /bin/sh) on Debian.

Feedback of testing with other shells on other systems to target portability is greatly appreciated. I don't get notified of Gist comments, so reach out to me on Twitter instead.

Execute with an argument as a maximum random number, no bigger than 4294967296:

$ ./insecure-random.sh 100
81
$ ./secure-random.sh 100
17

insecure-random.sh

#!/bin/sh
# POSIX compliant

m="$((4294967296 - (4294967296 % $1)))"
while
    r="$( (date +"%s"; who -a; ps -aef; df -P; env) 2>&1 | cksum | awk '{print $1}')"
    [ "$r" -gt "$m" ]
do :; done
echo "$((($r % $1) + 1))"

secure-random.sh

#!/bin/sh
# Not POSIX compliant, due to /dev/urandom
# I don't know how to get to a CSPRNG in POSIX

m="$((4294967296 - (4294967296 % $1)))"
while
    r="$(dd if=/dev/urandom count=4 2>&1 | cksum | awk '{print $1}')"
    [ "$r" -gt "$m" ]
do :; done
echo "$((($r % $1) + 1))"