De acordo com ps
manual:
t ttylist
Select by tty. Nearly identical to -t and --tty, but can also
be used with an empty ttylist to indicate the terminal
associated with ps. Using the T option is considered cleaner
than using t with an empty ttylist.
Aqui " select by tty " acaba por significar " selecione por controlando tty ". E ssh -f
seria executado em segundo plano como um daemon que não possui um controle de tty . Veja a saída do passo 111 no seguinte exemplo:
[STEP 107] # tty
/dev/pts/28
[STEP 108] # ssh -D :7070 -f -n -N 127.0.0.1
[STEP 109] # pgrep -lf 7070
72531 ssh
[STEP 110] # ls -l /proc/72531/fd
total 0
lrwx------ 1 root root 64 2016-11-16 17:47 0 -> /dev/pts/28
lrwx------ 1 root root 64 2016-11-16 17:47 1 -> /dev/pts/28
lrwx------ 1 root root 64 2016-11-16 17:47 2 -> /dev/pts/28
lrwx------ 1 root root 64 2016-11-16 17:47 3 -> socket:[3572721]
lrwx------ 1 root root 64 2016-11-16 17:47 4 -> socket:[3572770]
[STEP 111] # ps p 72531 j
PPID PID PGID SID TTY TPGID STAT UID TIME COMMAND
1 72531 72531 72531 ? -1 Ss 0 0:00 ssh -D :7070 -f -n -N 127.0.0.1
[STEP 112] #