Random Password Generator for Bash

rpg.sh

#!/bin/bash
# thanks to StackOverflow: https://stackoverflow.com/questions/101362/how-do-you-generate-passwords

while [ -n "$1" ]; do
    case "$1" in
        '-n')
            PASSWORD_LENGTH="$2"
            shift
            ;;
        '-p')
            PUNCTUATION_OPT='[:punct:]'
            ;;
        '-h')
            cat <<END
rpg.sh: Random Password Generator for bash!
usage:
    -n <num>  specify length of password
    -p        enable punctuation characters in generated password
    -h        print this
END
            exit 0
            ;;
        *)
            echo "Unrecognized option $1" 1>&2
            exit 1
            ;;
    esac
    shift
done

if [ -z "$PASSWORD_LENGTH" ]
then
    PASSWORD_LENGTH='12'
elif ! [[ $PASSWORD_LENGTH =~ ^[1-9][0-9]*$ ]]
then
    echo "Invalid length: $PASSWORD_LENGTH" 1>&2
    exit 1
fi

# Explanation:
# 1. use cat to read random bytes from /dev/urandom
# 2. use tr to filter so as to only include alphanumeric (optionally, also punctuation) characters
# 3. use fold to split the stream into lines of the given length
# 4. use head to only print the first line
cat /dev/urandom | tr -dc "[:alnum:]${PUNCTUATION_OPT}" | fold -w "$PASSWORD_LENGTH" | head -n 1