shellrc/rc.d/02_ssh_agent.sh

# Ref: https://stackoverflow.com/a/34332776
#      https://docs.github.com/en/authentication/connecting-to-github-with-ssh/working-with-ssh-key-passphrases#auto-launching-ssh-agent-on-git-for-windows
#      https://web.archive.org/web/20210506080335/https://mah.everybody.org/docs/ssh

SSH_ENV="${HOME}/.ssh/.agent_env"

function start_agent {
    echo -n "Initialising new SSH agent..."

    eval `/usr/bin/ssh-agent`
    echo 'export SSH_AUTH_SOCK'=${SSH_AUTH_SOCK} >  ${SSH_ENV}
    echo 'export SSH_AGENT_PID'=${SSH_AGENT_PID} >> ${SSH_ENV}

    echo "succeeded"
    chmod 600 "${SSH_ENV}"
    . "${SSH_ENV}" > /dev/null
    add_key
}

function list_not_loaded_keys {
  KEYS=$(list_keys)
  KEYS_LOADED=$(ssh-add -l | awk '{ print $3}')
  KEYS_NOT_LOADED=""
  for key in ${KEYS}
  do
    if ! grep -q "${key}" <<< ${KEYS_LOADED}; then
      KEYS_NOT_LOADED="${KEYS_NOT_LOADED} ${key}"
    fi
  done

  echo "${KEYS_NOT_LOADED}"
}

function add_key {
  /usr/bin/ssh-add -t 86400 $(list_keys)
}

function update_keys {
  KEYS=$(list_not_loaded_keys)
  [[ -n ${KEYS} ]] && /usr/bin/ssh-add -t 86400 ${KEYS}
}

function list_keys {
  PRIV_KEYS=""
  for file in ${HOME}/.ssh/id_*
  do
    if grep -q 'PRIVATE' ${file}; then
      PRIV_KEYS="${PRIV_KEYS} ${file}"
    fi
  done

  echo "${PRIV_KEYS}"
}

# Source SSH settings, if applicable
if [ -f "${SSH_ENV}" ]; then
    . "${SSH_ENV}" > /dev/null
    # agent_run_state: 0=agent running w/ key; 1=agent w/o key; 2=agent not running
    agent_run_state=$(/usr/bin/ssh-add -l &> /dev/null; echo $?)

    if [ $agent_run_state -eq 2 ]; then
      start_agent
    elif [ $agent_run_state -eq 1 ]; then
      add_key
    elif [ $agent_run_state -eq 0 ]; then
      update_keys
    fi
else
    start_agent;
fi

# Create our own hardlink to the socket (with random name)
MYSOCK=/tmp/ssh_agent.${RANDOM}.sock
ln --force --no-target-directory ${SSH_AUTH_SOCK} ${MYSOCK}
export SSH_AUTH_SOCK=${MYSOCK}

end_agent()
{
    # if we are the last holder of a hardlink, then kill the agent
    nhard=$(ls -l ${SSH_AUTH_SOCK} | awk '{print $2}')
    if [[ "${nhard}" -eq 2 ]]; then
        rm ${SSH_ENV}
        /usr/bin/ssh-agent -k
    fi
    rm ${SSH_AUTH_SOCK}
}

trap end_agent EXIT SIGKILL SIGTERM