Add user with SSH key, first argurment must be a valid username and second argument should be a valid SSH key. It also adds user to sudoers file, enabling the user to run super user command without password.

add_user.sh

#!/usr/bin/env bash

NEWUSER="$1"
SSHKEY="$2"

# run script as superuser
if [ "$EUID" -ne 0 ]; then
    echo "Please run as root"
    exit 1
fi

if [ -z "$NEWUSER" ]; then
    echo "No username provided"
    exit 2
fi

if [ -z "$SSHKEY" ]; then
    echo "No SSH key provided"
    exit 3
fi

# check if user exists
if id -u "$NEWUSER" >/dev/null 2>&1; then
    echo "User $NEWUSER already exists"
    exit 4
fi

# check if adduser command exists
if ! command -v adduser >/dev/null 2>&1; then
    echo "adduser command not found"
    exit 5
fi

# check if usermod command exists
if ! command -v usermod >/dev/null 2>&1; then
    echo "usermod command not found"
    exit 6
fi

adduser "$NEWUSER" \
    && usermod -aG wheel "$NEWUSER" \
    && mkdir /home/"$NEWUSER"/.ssh \
    && touch /home/"$NEWUSER"/.ssh/authorized_keys \
    && echo "$SSHKEY" | tee /home/"$NEWUSER"/.ssh/authorized_keys > /dev/null \
    && chmod 700 /home/"$NEWUSER"/.ssh/ \
    && chmod 600 /home/"$NEWUSER"/.ssh/authorized_keys \
    && chown -R "$NEWUSER":"$NEWUSER" /home/"$NEWUSER"/.ssh/ \
    && dig +short myip.opendns.com @resolver1.opendns.com
    
# Add user to sudoers
echo "$NEWUSER ALL=(ALL) NOPASSWD:ALL" | tee -a /etc/sudoers > /dev/null

# Add user to sshd_config
echo "AllowUsers $NEWUSER" | tee -a /etc/ssh/sshd_config > /dev/null

# Restart sshd
systemctl restart sshd