SSH Key-based authentication (also known as public-key authentication) allows for password-less authentication and it is a more secure and a much better solution than password authentication. One major advantage of SSH password-less login, let alone security is that it allows for automation of various kinds of cross-server processes.

In this article, we will demonstrate how to create an SSH key pair and copy the public key to multiple remote Linux hosts at once, with a shell script.

Create a New SSH Key in Linux

First, generate the SSH key pair (the private/identity key that an SSH client uses to authenticate itself when logging into a remote SSH server and the public key stored as an authorized key on a remote system running an SSH server) using the ssh-keygen command as follows:

# ssh-keygen

Create a Shell Script for Mulitple Remote Logins

Next, create a shell script that will help in copying a public key to multiple remote Linux hosts.

# vim ~/.bin/ssh-copy.sh

Copy and paste the following code in the file (replace the following variables accordingly USER_NAME – the username to connect with, HOST_FILE – a file which contains the list of hostnames or IP addresses, and ERROR_FILE – a file to store any ssh command errors).

#!/bin/bash
USER_NAME="root"
HOST_FILE="/root/hosts"
ERROR_FILE="/tmp/ssh-copy_error.txt"
PUBLIC_KEY_FILE="$1"

if [ ! -f $PUBLIC_KEY_FILE ]; then
 echo "File '$PUBLIC_KEY_FILE' not found!"
 exit 1
fi

if [ ! -f $HOST_FILE ]; then
 echo "File '$HOST_FILE' not found!"
 exit 2
fi

for IP in `cat $HOST_FILE`; do
 ssh-copy-id -i $PUBLIC_KEY_FILE $USER_NAME@$IP 2>$ERROR_FILE
 RESULT=$?
 if [ $RESULT -eq 0 ]; then
 echo ""
 echo "Public key successfully copied to $IP"
 echo ""
 else
 echo "$(cat $ERROR_FILE)"
 echo 
 exit 3
 fi
 echo ""
done

Save the file and close it.

Then make the script executable with the chmod command as shown.

# chmod +x ssh-copy.sh

Now run the ssh-copy.sh script and specify your public key file as the first argument as shown in the screenshot:

# ./ssh-copy.sh /root/.ssh/prod-rsa.pub

Next, use ssh-agent to manage your keys, which holds your decrypted private key in memory and uses it to authenticate logins. After starting the ssh-agent, add your private key to it as follows:

# eval "$(ssh-agent -s)"
# ssh-add ~/.ssh/prod_rsa

Login to Remote Linux Server without Password

Now you can log into any of your remote hosts without providing a password for SSH user authentication. This way, you can automate cross-server processes.

# ssh [email protected]

That’s all we had for you! If you have any contribution(s) to make particularly towards improving the shell script, let us know via the feedback form below.