Simple alphanumeric password generation.

passgen.sh

#! /bin/sh

# Simple alphanumeric password generation.

DEFAULT_LENGTH=10

if [ "$1" = "-h" -o "$1" = "--help" ]
then
    echo "Usage: $0 [length]" 1>&2
    echo "Generate a random alphanumeric password." 1>&2
    echo -n "If a length is given, output that many characters. Otherwise, "
    echo "output $DEFAULT_LENGTH characters." 1>&2
    exit 1
fi

length=$DEFAULT_LENGTH
if [ $# -gt 0 -a ! `echo "$1" | tr -d [:digit:] | wc -c` -gt 1 ]
then
    length=$1
fi

# We make a horrible assumption that a random string 16 * $length will contain
# at least $length alphanumeric characters.
read_length=`expr $length \* 16`

head -c $read_length < /dev/urandom | tr -dc [:alnum:] | head -c $length && echo