add passphrase to ssh agent?

#!/usr/bin/expect -f
spawn ssh-add /home/user/.ssh/id_rsa
expect "Enter passphrase for /home/user/.ssh/id_rsa:"
send "passphrase\n";
expect "Identity added: /home/user/.ssh/id_rsa (/home/user/.ssh/id_rsa)"
interact
#!/usr/local/bin/expect
spawn  sftp  -b cmdFile [email protected]
expect "password:"
send "shhh!\n";
interact