How to Check If Ssh-Agent Is Already Running in Bash

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

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

Running SSH Agent when starting Git Bash on Windows

2013: In a git bash session, you can add a script to ~/.profile or ~/.bashrc (with ~ being usually set to %USERPROFILE%), in order for said session to launch automatically the ssh-agent.

If the file doesn't exist, just create it.

This is what GitHub describes in "Working with SSH key passphrases".

The "Auto-launching ssh-agent on Git for Windows" section of that article has a robust script that checks if the agent is running or not.

Below is just a snippet, see the GitHub article for the full solution.

# This is just a snippet. See the article above.
if ! agent_is_running; then
    agent_start
    ssh-add
elif ! agent_has_keys; then
    ssh-add
fi

Other Resources:

"Getting ssh-agent to work with git run from windows command shell" has a similar script, but I'd refer to the GitHub article above primarily, which is more robust and up to date.

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hardsetting adds in the comments (2018):

If you want to enter the passphrase the first time you need it, and not when opening a shell, the cleanest way to me is:

• removing the ssh-add from the .bash_profile, and
• adding "AddKeysToAgent yes" to your .ssh/config file (see "How to make ssh-agent automatically add the key on demand?").

This way you don't even have to remember running ssh-add.

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And Tao adds in the comments (2022):

It's worth noting why this script makes particular sense in Windows, vs (for example) the more standard linuxey script noted by @JigneshGohel in another answer:

By not relying on the SSH_AGENT_PID at all, this script works across different msys & cygwin environments.

An agent can be started in msys2, and still used in git bash, as the SSH_AUTH_SOCK path can be reached in either environment.

The PID from one environment cannot be queried in the other, so a PID-based approach keeps resetting/creating new ssh-agent processes on each switch.

How to run ssh-add on windows?

One could install Git for Windows and subsequently run ssh-add:

Step 3: Add your key to the ssh-agent

To configure the ssh-agent program to use your SSH key:

If you have GitHub for Windows installed, you can use it to clone repositories and not deal with SSH keys. It also comes with the Git Bash tool, which is the preferred way of running git commands on Windows.

1. Ensure ssh-agent is enabled:

   • If you are using Git Bash, turn on ssh-agent:

     # start the ssh-agent in the background
     ssh-agent -s
     # Agent pid 59566
     

   • If you are using another terminal prompt, such as msysgit, turn on ssh-agent:

     # start the ssh-agent in the background
     eval $(ssh-agent -s)
     # Agent pid 59566
     

2. Add your SSH key to the ssh-agent:

   ssh-add ~/.ssh/id_rsa
   

ssh-agent in bash script causes many dead processes

Your script probably shouldn't start ssh-agent; it should make use of an ssh-agent that's already running. That way, the user is responsible for starting a single agent that can be used by multiple invocations of the script.

The simplest thing you can do, though, is simply add either

kill $SSH_AGENT_PID

or

ssh-agent -k

to the end of your script to kill the agent that was just started. One of the things the eval command does is sets the value of SSH_AGENT_PID to the process ID of the just-started agent.

(The former is useful if you have, for whatever reason, multiple concurrent agents, so that you kill the correct agent.)

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