store / remove ssh key (git passphrase)

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*
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$ eval $(ssh-agent)
## The output without the eval will look like this:
$ ssh-agent
ssh-agent
SSH_AUTH_SOCK=/tmp/ssh-hvcwJQnSOHOi/agent.125894; export SSH_AUTH_SOCK;
SSH_AGENT_PID=125895; export SSH_AGENT_PID;
echo Agent pid 125895;

After the ssh-agent is started (with the eval). You can add SSH keys with ssh-add

$ ssh-add ~/.ssh/google
$ ssh-add ~/.ssh/id_rsa

With current versions of SSH you can also add the option AddKeysToAgent to the ~/.ssh/config file:

## ~/.ssh/config
AddKeysToAgent yes


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* Remove known host
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ssh-keygen -R 192.168.123.254

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*
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~/.bashrc
SSH_ENV=$HOME/.ssh/environment
# start the ssh-agent
function start_agent {
    echo "Initializing new SSH agent..."
    # spawn ssh-agent
    /usr/bin/ssh-agent | sed 's/^echo/#echo/' > ${SSH_ENV}
    echo succeeded
    chmod 600 ${SSH_ENV}
    . ${SSH_ENV} > /dev/null
    /usr/bin/ssh-add
}

if [ -f "${SSH_ENV}" ]; then
     . ${SSH_ENV} > /dev/null
     ps -ef | grep ${SSH_AGENT_PID} | grep ssh-agent$ > /dev/null || {
        start_agent;
    }
else
    start_agent;
fi

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* MacBook
********************

As of macOS Sierra 10.12.2 Apple added an ssh_config option called UseKeychain which allows a 'proper' resolution to the problem. Add the following to your ~/.ssh/config file: