When bash is called non-interactively, PS1 is unset. However, if it's a login shell, ~/.bash_profile is still sourced. Therefore, testing PS1 tells us if this is a login but non-interactive shell (such as: ssh host 'uptime'). Testing PS1 before setting it preserves that state for later tests if desired.

#if [ "$SSH_AGENT_PID" -a "$SHLVL" = "1" ] ; then
#   eval `/usr/bin/ssh-agent -k`
#fi