ssh agent remember passphrase

$ ssh-add -l

$ ssh-add .ssh/id_rsa 
Enter passphrase for .ssh/id_rsa: 
Identity added: .ssh/id_rsa (.ssh/id_rsa)

$ SSH_AUTH_SOCK=/tmp/ssh-hZQhwQlxahPX/agent.1833; export SSH_AUTH_SOCK; 
$ SSH_AGENT_PID=1834; export SSH_AGENT_PID; 

$ ssh-agent 
SSH_AUTH_SOCK=/tmp/ssh-hZQhwQlxahPX/agent.1833; export SSH_AUTH_SOCK; 
SSH_AGENT_PID=1834; export SSH_AGENT_PID; 
echo Agent pid 496;