make a user, give them key access

make_user.sh

#!/bin/bash

# consume the first argument so we can deal with the key as one variable
username=$1
shift

sudo useradd $username
sudo -u $username mkdir /home/$username/.ssh/
sudo -u $username touch /home/$username/.ssh/authorized_keys
sudo chmod 640 /home/$username/.ssh/authorized_keys

# $@ means 'all of the arguments'. since we consumed the first one, all that's left is this. 
# this approach lets us avoid worrying about whether or not the pubkey was quoted
sudo -u $username sh -c "echo $@ >> /home/$username/.ssh/authorized_keys"