Start SSH agent only once and add identity information without password prompt

ssh_add_identity.sh

SSH_ENV="$HOME/.ssh/environment"
KEY="$HOME/.ssh/id_rsa"

function start_agent {
    # Starts the ssh-agent
    # and adds its env vars into $SSH_ENV file.

    echo "Initializing new SSH agent..."
    # spawn ssh-agent
    ssh-agent | sed 's/^echo/#echo/' > "$SSH_ENV"
    echo succeeded
    chmod 600 "$SSH_ENV"
    . "$SSH_ENV" > /dev/null
}

function add_identities {
    # Adds $KEY identities to the agent,
    # if the agent has no identities associated
    ssh-add -l | grep "The agent has no identities" > /dev/null
    if [ $? -eq 0 ]; then ssh-add $KEY; fi
}

function is_ssh_agent_pid_valid () {
    # Accepts ssh agent's pid as the first argument. 
    # Returns 0 if ssh-agent process is running,
    # otherwise returns non-zero.
   
    ps -ef | grep $1 | grep -v grep | grep ssh-agent > /dev/null
}

# check for running ssh-agent with proper $SSH_AGENT_PID
if [ -n "$SSH_AGENT_PID" ] && is_ssh_agent_pid_valid $SSH_AGENT_PID; then 
    add_identities; 
else
    # if $SSH_AGENT_PID is not properly set, we might be able to load one from $SSH_ENV
    if [ -f "$SSH_ENV" ]; then . "$SSH_ENV" > /dev/null; fi
    if ! is_ssh_agent_pid_valid $SSH_AGENT_PID; then start_agent; fi
    add_identities 
fi

I had an annoying problem. Whenever I wanted to do some git operations with a remote repository, such as git pull, it asked me to input a password, which I had set when creating ~/.ssh/id_rsa for security reasons. The first solution I found was to create a file ak with the following contents and source it:

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

It asks for a password, I input it and then git doesn't ask me for a password anymore. True, only if you invoke git in the same terminal session. But I use multiple sessions via tmux terminal multiplexer. Thus in every new window where I wanted to use git I had to type source ~/bin/ak and type a password. Still annoying. Also behind the scenes it would have as many ssh-agent processes running as the number of times I sourced that file.

Then after a year of struggling, I browsed the web again and found how to solve this problem once and for all. The original solution can be found here. I fixed one issue there and slightly improved it, the result is the script above. Copy it into your '.bashrc' and I hope it will make your life easier too.