How to check if ssh-agent is already running in ba

Question

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?

Answer 1

Thanks to all the answers here. I've used this thread a few times over the years to tweak my approach. Wanted to share my current ssh-agent.sh checker/launcher script that works for me on Linux and OSX.

The following block is my $HOME/.bash.d/ssh-agent.sh

function check_ssh_agent() {
  if [ -f $HOME/.ssh-agent ]; then
    source $HOME/.ssh-agent > /dev/null
  else
    # no agent file
    return 1
  fi

  if [[ ${OSTYPE//[0-9.]/} == 'darwin' ]]; then
    ps -p $SSH_AGENT_PID > /dev/null  
    # gotcha: does not verify the PID is actually an ssh-agent
    # just that the PID is running
    return $?
  fi

  if [ -d /proc/$SSH_AGENT_PID/ ]; then
    # verify PID dir is actually an agent
    grep ssh-agent /proc/$SSH_AGENT_PID/cmdline  > /dev/null  2> /dev/null; 
    if [ $? -eq 0 ]; then
      # yep - that is an agent
      return 0
    else
      # nope - that is something else reusing the PID
      return 1
    fi
  else
    # agent PID dir does not exist - dead agent
    return 1
  fi 
}

function launch_ssh_agent() {
  ssh-agent > $HOME/.ssh-agent
  source $HOME/.ssh-agent
  # load up all the pub keys
  for I in $HOME/.ssh/*.pub ; do
    echo adding ${I/.pub/}
    ssh-add ${I/.pub/}
  done
}

check_ssh_agent
if [ $? -eq 1 ];then
  launch_ssh_agent
fi

I launch the above from my .bashrc using:

if [ -d $HOME/.bash.d ]; then
  for I in $HOME/.bash.d/*.sh; do
    source $I  
  done
fi

Hope this helps others get up and going quickly.

Created a public gist if you want to hack/improve this with me: https://gist.github.com/dayne/a97a258b487ed4d5e9777b61917f0a72

Answer 2

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).