Você está usando
-f
Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background. This implies -n. The recommended way to start X11 programs at a remote site is with something like ssh -f host xterm.
e
-N
Do not execute a remote command. This is useful for just forwarding ports.
Que entra no fundo antes de executar um comando (que é suprimido). Por isso, fica em segundo plano. O sleep
nunca é executado e aguarda ser morto.