Rock, Paper, Scissors in bash, based on @cassidoo's interview question from November 24, 2019

rps.sh

#!/usr/bin/env bash
set -eu -o pipefail
# shellcheck source=/Users/andrewrich/etc/functions.sh
. "$HOME/etc/functions.sh"
readonly R="ROCK" P="PAPER" S="SCISSORS"

about () {
cat <<EOF
$( basename "$0" ) by Andrew Rich <[email protected]>
Based on @cassidoo's interview question from https://buttondown.email/cassidoo/archive/be-careful-with-your-words-once-they-are-said/
Rock/Paper/Scissors: https://en.wikipedia.org/wiki/Rock_paper_scissors
Truth table: https://www.skeptic.com/wordpress/wp-content/uploads/Rock-paper-scisscors-510px.jpg

EOF
}
syntax () {
cat <<EOF
Usage: $( basename "$0" ) <THROW>
Possible values for THROW are ROCK, PAPER, SCISSORS
The first letter of the value is sufficient.

Examples:
$ rps ROCK
=> Computer throws PAPER: Computer wins!
$ rps SCISSORS
=> Computer throws PAPER: You win!
$ rps PAPER
=> Computer throws PAPER: It's a draw!

EOF
}

function get_player_throw () {
	# this function gets the player throw interactively if they didn't enter one
	read -e -r -n 1 -p "Rock, Paper, Scissors (or Quit): " THROW
	THROW=$( toUpper "${THROW}" )
	echo "${THROW}"
}
function player_opts () {
	# this function processes/validates the player-entered options
	# get the first letter and capitalize it
	# if it matches R P S Q, return; else call get_player_throw to get a new entry
	PLAYER_OPT="$( toUpper "$1" )"
	PLAYER_OPT="${PLAYER_OPT:0:1}"
	while ! [[ "${PLAYER_OPT}" =~ ^(R|P|S|Q)$ ]]; do
		PLAYER_OPT=$( get_player_throw )
	done
	echo "${PLAYER_OPT}"
}
function computer_throw () {
	# this function returns the computer's throw
	shuf -n 1 -e R P S
}
function play () {
	# this function plays PLAYER_THROW against COMP_THROW and returns the winner
	PLAYER_THROW=$1
	COMP_THROW=$2
	[ "$PLAYER_THROW" == "$COMP_THROW" ] && ( echo "TIE"; return )
	case $PLAYER_THROW in
		'R')
			case $COMP_THROW in
				'P') echo 'Computer';;
				'S') echo 'Player';;
			esac;;
		'P')
			case $COMP_THROW in
				'R') echo "Player";;
				'S') echo 'Computer';;
			esac;;
		'S')
			case $COMP_THROW in
				'R') echo "Computer";;
				'P') echo 'Player';;
			esac;;
		esac
}


if [ $# != 1 ]; then
	about; syntax; THROW="$( get_player_throw )"
else
	THROW="$1"
fi
PL="$( player_opts "$THROW" )"
while [ "${PL}" != "Q" ]; do
	CO="$( computer_throw )"
	PL_OUT=$( eval echo "\$${PL}" )
	CO_OUT=$( eval echo "\$${CO}" )
	echo -e "\n$PL_OUT vs $CO_OUT: $(play "$PL" "$CO")"
	THROW=""
	PL="$( player_opts "$THROW" )"
done
echo -e "\nGoodbye!\n"

Since you, like me, probably don't have a copy of functions.sh, you'll be wanting this or something like it:

function toUpper() {
  echo ${1^^}
}

Thanks @wgmyers, I blanked on that.