MacOS / Linux BASH Script to create and add SSH keys to a Linux server

use_ssh_keys.sh

#!/bin/bash
############################################################################################################################################################
############################################################################################################################################################
# https://github.com/1x24
# 1x24 AT tuta.io
# March 31, 2023
############################################################################################################################################################
# This BASH script checks if you have SSH keys on your iMac or MacOS laptop, or your Linux machine.
# If you do not have the SSH keys, it generates a modern one for you.
# In either case, it copies the key to your server and makes some configuration changes to make it easy to login with the keys.
# Save then use as follows: 
#   (1) Open Terminal and navigate to the folder where this script is saved.
#   (2) Enter the following command to make the script executable
#		chmod +x use_ssh_keys.sh
#   (3) Now run the script as follows, substituting in your server username (to replace USERNAME), server host (to replace MY.SERVER.HOST) 
#		    and the server port to replace SERVER_SSH_PORT (unless told otherwise, use the number 22)
#		./use_ssh_keys.sh USERNAME MY.SERVER.HOST SERVER_SSH_PORT
#   (4) From now on you can simply open terminal and run ssh MY.SERVER.HOST e.g. ssh fancy.server.com
#############################################################################################################################################################
#############################################################################################################################################################

if [ $# -ne 3 ]; then
  echo "Usage: $0 server_username server_host server_ssh_port"
  exit 1
fi

USERNAME="$1"
SERVER_HOST="$2"
SERVER_PORT="$3"

KEYS_FOUND=0
KEY_PATH=""

# Check for existing keys in the common locations
for key_type in ed25519 rsa; do
  KEY_PATH="$HOME/.ssh/id_${key_type}.pub"
  if [ -f "$KEY_PATH" ]; then
    KEYS_FOUND=1
    break
  fi
done

# Generate a new SSH key if none are found
if [ $KEYS_FOUND -eq 0 ]; then
  echo "No SSH keys found. Generating a new one..."
  ssh-keygen -t ed25519 -f "$HOME/.ssh/id_ed25519" -N ""
  KEY_PATH="$HOME/.ssh/id_ed25519.pub"
fi

# Copy the key to the remote server
echo "Copying SSH key to the remote server..."
PUB_KEY_CONTENT=$(cat "$KEY_PATH")
sshCommands="mkdir -p ~/.ssh && chmod 700 ~/.ssh && echo '$PUB_KEY_CONTENT' >> ~/.ssh/authorized_keys && chmod 600 ~/.ssh/authorized_keys"
ssh -o StrictHostKeyChecking=no -o PreferredAuthentications=password -o PubkeyAuthentication=no -p "$SERVER_PORT" "${USERNAME}@${SERVER_HOST}" "$sshCommands"

# Check the return status of the SSH command
if [ $? -eq 0 ]; then
  # Append configuration to the local SSH config file
  SSH_CONFIG="$HOME/.ssh/config"
  echo "Appending configuration to the local SSH config file..."
  {
    echo "Host $SERVER_HOST"
    echo "  Hostname $SERVER_HOST"
    echo "  User $USERNAME"
    echo "  Port $SERVER_PORT"
    echo "  IdentityFile ${KEY_PATH%*.pub}"
    echo "  PreferredAuthentications publickey"
    echo "  GSSAPIAuthentication no"
    echo "  ChallengeResponseAuthentication no"
    echo "  HashKnownHosts no"
    echo "  KbdInteractiveAuthentication no"
    echo "  TCPKeepAlive yes"
    echo "  ServerAliveInterval 25"
    echo "  ServerAliveCountMax 600"
  } >> "$SSH_CONFIG"

  echo "Done!"
else
  echo "SSH key copying failed. Please check the error message and try again."
fi