How to check if ssh-agent is already running in bash?

I have a sample sh script on my Linux environment, which basically run's the ssh-agent for the current shell, adds a key to it and runs two git commands:

#!/bin/bash
eval "$(ssh-agent -s)"
ssh-add /home/duvdevan/.ssh/id_rsa

git -C /var/www/duvdevan/ reset --hard origin/master
git -C /var/www/duvdevan/ pull origin master

Script actually works fine, but every time I run it I get a new process so I think it might become a performance issue and I might end up having useless processes out there.

An example of the output:

Agent pid 12109
Identity added: /home/duvdevan/.ssh/custom_rsa (rsa w/o comment)

Also, along with all this, is it possible to find an existing ssh-agent process and add my keys into it?

No, really, how to check if ssh-agent is already running in bash?

Answers so far don't appear to answer the original question...

Here's what works for me:

if ps -p $SSH_AGENT_PID > /dev/null
then
   echo "ssh-agent is already running"
   # Do something knowing the pid exists, i.e. the process with $PID is running
else
eval `ssh-agent -s`
fi

This was taken from here

Also, along with all this, is it possible to find an existing ssh-agent process and add my keys into it?

Yes. We can store the connection info in a file:

# Ensure agent is running
ssh-add -l &>/dev/null
if [ "$?" == 2 ]; then
    # Could not open a connection to your authentication agent.

    # Load stored agent connection info.
    test -r ~/.ssh-agent && \
        eval "$(<~/.ssh-agent)" >/dev/null

    ssh-add -l &>/dev/null
    if [ "$?" == 2 ]; then
        # Start agent and store agent connection info.
        (umask 066; ssh-agent > ~/.ssh-agent)
        eval "$(<~/.ssh-agent)" >/dev/null
    fi
fi

# Load identities
ssh-add -l &>/dev/null
if [ "$?" == 1 ]; then
    # The agent has no identities.
    # Time to add one.
    ssh-add -t 4h
fi

This code is from pitfalls of ssh agents which describes both the pitfalls of what you're currently doing, of this approach, and how you should use ssh-ident to do this for you.

If you only want to run ssh-agent if it's not running and do nothing otherwise:

if [ $(ps ax | grep [s]sh-agent | wc -l) -gt 0 ] ; then
    echo "ssh-agent is already running"
else
    eval $(ssh-agent -s)
    if [ "$(ssh-add -l)" == "The agent has no identities." ] ; then
        ssh-add ~/.ssh/id_rsa
    fi

    # Don't leave extra agents around: kill it on exit. You may not want this part.
    trap "ssh-agent -k" exit
fi

However, this doesn't ensure ssh-agent will be accessible (just because it's running doesn't mean we have $SSH_AGENT_PID for ssh-add to connect to).

If you want it to be killed right after the script exits, you can just add this after the eval line:

trap "kill $SSH_AGENT_PID" exit

Or:

trap "ssh-agent -k" exit

$SSH_AGENT_PID gets set in the eval of ssh-agent -s.

You should be able to find running ssh-agents by scanning through /tmp/ssh-* and reconstruct the SSH_AGENT variables from it (SSH_AUTH_SOCK and SSH_AGENT_PID).