Simple two string solution from this answer:
For sh, bash, etc:
# ~/.profile
if ! pgrep -q -U `whoami` -x 'ssh-agent'; then ssh-agent -s > ~/.ssh-agent.sh; fi
. ~/.ssh-agent.sh
For csh, tcsh, etc:
# ~/.schrc
sh -c 'if ! pgrep -q -U `whoami` -x 'ssh-agent'; then ssh-agent -c > ~/.ssh-agent.tcsh; fi'
eval `cat ~/.ssh-agent.tcsh`