A process is opening a tty but "ps t tty" cannot see it

Question

See following example:

[STEP 101] # tty
/dev/pts/3
[STEP 102] # ssh -f -n -N -D :7070 127.0.0.1
[STEP 103] # pgrep -lf 7070
33109 ssh
[STEP 104] # ls -l /proc/33109/fd/
total 0
lrwx------ 1 root root 64 2016-11-08 10:45 0 -> /dev/pts/3
lrwx------ 1 root root 64 2016-11-08 10:45 1 -> /dev/pts/3
lrwx------ 1 root root 64 2016-11-08 10:45 2 -> /dev/pts/3
lrwx------ 1 root root 64 2016-11-08 10:45 3 -> socket:[3328390]
lrwx------ 1 root root 64 2016-11-08 10:45 4 -> socket:[3328441]
[STEP 105] # ps t pts/3
   PID TTY      STAT   TIME COMMAND
 33115 pts/3    R+     0:00 ps t pts/3
 66586 pts/3    Ss     0:08 /root/bin/bash
[STEP 106] #

In step 104 it shows the ssh process is opening pts/3 but why the ps t pts/3 command in step 105 cannot see it?

Answer 1

According to 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.

Here "select by tty" turns out to mean "select by controlling tty". And ssh -f would run in background as a daemon which does not have a controlling tty. See output of step 111 in following example:

[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] #