GNU/Linux Desktop Survival Guide by Graham Williams
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SSH Agents

Entering the passphrase every time you want to do a remote copy or make a remote connection can be tiring. There is a simple mechanism provided to avoid this. All you need do is to start up a shell or other similar command that already knows what your passphrase is (by you telling it just once). This is done with ssh-agent to start the shell and ssh-add to store the passphrase for this shell:

  $ ssh-agent bash
  $ ssh-add             (or ssh-add .ssh/id_dsa)
  Need passphrase for .ssh/id_dsa
  Enter passphrase for .ssh/id_dsa 
  Identity added: .ssh/id_dsa (.ssh/id_dsa)
  $ ssh alpine

The ssh-add will prompt you for your passphrase but from then on all further interaction with the remote host will not require the passphrase again.

The -l option of ssh-add lists the key(s) currently kept in memory for this ssh-agent. The -d option removes an identity from memory.