Generate random text of arbirary length using selectable character sets in POSIX shells

randomchars

#!/bin/sh
_bsdl='
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'

usage() {
	echo Generate random strings
	echo Usage:
	echo "randomchars [-l <length>] [-f <format string>]"
	echo "format string options:"
	printf "\ta\tinclude lowercase letters a-z\n"
	printf "\tA\tinclude uppercase letters A-Z\n"
	printf "\td\tinclude digits 0-9\n"
	printf "\ts\tinclude symbols\n"
	printf "\th\thexadecimal (lowercase) - cannot be combined with \"H\" or \"b\"\n\t\toverrides all other inclusions\n"
	printf "\tH\thexadecimal (uppercase) - cannot be combined with \"h\" or \"b\"\n\t\toverrides all other inclusions\n"
	printf "\tb\tbase64 (character set) - cannot be combined with \"h\" or \"H\"\n\t\toverrides all other inclusions\n"
	printf "\tB\tbase32 (character set) - cannot be combined with \"h\" or \"H\"\n\t\toverrides all other inclusions\n"
	echo Default options are: -l 32 -f aAd
	exit 1
}

parseargs() {
	unset _length _formatstr
	while [ $# -gt 0 ]; do
		_arg="$1"a
		case $_arg in
			-l)
				[ "$_length" ] && usage
				if ( [ "$2" ] && [ "$2" -gt 0 ] 2>/dev/null ); then
					_length=$2
				else
					usage
				fi
				shift 2
				;;
			-f)
				[ "${_formatstr}" ] && usage
				[ "$2" ] || usage
				_formatstr=$2
				shift 2
				;;
			*)
				usage
				;;
		esac
	done
	[ "$_length" ] || _length=32
	[ "$_formatstr" ] || _formatstr=aAd
}

parse_format() {
	_formatstr=$1

	unset do_a do_A do_d do_s do_h do_H do_b
	OPTIND=1
	while getopts "aAdshHbB" _opt
	do
		case $_opt in
			a)	do_a=y;;
			A)	do_A=y;;
			d)	do_d=y;;
			s)	do_s=y;;
			h)	do_h=y;;
			H)	do_H=y;;
			b)	do_b=y;;
			B)	do_B=y;;
			[?])	usage
				;;
		esac
	done
	[ "$do_b" ] && ( [ "$do_B" ] || [ "$do_h" ] || [ "$do_H" ] ) && usage
	[ "$do_B" ] && ( [ "$do_h" ] || [ "$do_H" ] ) && usage
	( [ "$do_h" ] && [ "$do_H" ] ) && usage
	( [ "$do_h" ] || [ "$do_H" ] || [ "$do_b" ] || [ "$do_B" ] ) && unset do_a do_A do_d do_s
}

mkcharset() {
	_value=$1
	while [ $_value -le $2 ]; do
		printf "%3.3o " $_value
		_value=$(($_value + 1))
	done
	printf "\n"
}

buildcharset() {
	# Enumerate character sets as space-separated octal values
	# eliminating the need to deal with special characters
	[ "$do_a" ] && a_set=$(mkcharset 97 122)
	[ "$do_A" ] && A_set=$(mkcharset 65 90)
	[ "$do_d" ] && d_set=$(mkcharset 48 57)
	[ "$do_s" ] && s_set="$(mkcharset 33 47)$(mkcharset 58 64)$(mkcharset 91 96)$(mkcharset 123 126)"
	echo ${a_set}${A_set}${d_set}${s_set}
}

countargs() {
	echo $#
}

getrandchar() {
	_index=$1
	shift $_index
	echo $1
}

parseargs "$@"
parse_format -$_formatstr

if [ "$do_b" ]; then
	dd if=/dev/urandom bs=1 count=$_length 2</dev/null | base64 | \
		cut -b 1-$_length
elif [ "$do_B" ]; then
	dd if=/dev/urandom bs=1 count=$_length 2</dev/null | base32 | \
		cut -b 1-$_length
elif [ "$do_h" ]; then
	od -vAn -x -N $(((_length + 1)-2)) </dev/urandom | \
		tr -d '[:space:]' | cut -b 1-$_length
elif [ "$do_H" ]; then
	od -vAn -x -N $(((_length + 1)-2)) </dev/urandom | \
		tr -d '[:space:]' | tr '[:lower:]' '[:upper:]' | \
		cut -b 1-$_length
else
	char_set=$(buildcharset)
	set_size=$(countargs $char_set)
	# Our largest possible output character set is n=94 characters. 256 modulus n
	# can be too unfairly weighted against characters later in the set.
	# 2-byte numbers ( 65536 % n ) should balance that out a bit more
	randombytes=$(od -vAn -x -N $((_length * 2)) </dev/urandom)
	
	for _randval in $randombytes; do
		_char=$(getrandchar $((0x$_randval % $set_size + 1)) $char_set)
		printf "\\$_char"
	done
	printf "\n"
fi

Probably not much novel here but I got tired of looking up online generators in a web browser whenever I needed a random string in a particular format.

• Settable output lenght
• Settable output charsets
  • lowercase letters
  • UPPERCASE LETTERS
  • digits 0 - 9
  • All other ASCII printable symbols (non-whitespace)
  • OR hexadecimal lowercase
  • OR hexadecimal uppercase
  • OR base64 character set
  • OR base32 character set

Defaults (no args) outputs 32-characters of mixed-case alphanumeric text.
Largest output character set includes mixed-case, decimals and symbols (base94 charset)