In Section 24.5, we pointed out that the ps command needs special options (-x for
BSD-derived versions and -e for System V-type) to list
processes without a controlling terminal.
But just what is a controlling terminal? Just what it sounds like: the terminal from which the process was started. In the ps listing, this is usually given as a tty, or terminal ID. That ps entry usually corresponds to a serial port, or a pty. A pty or "pseudo-terminal" is a construct that makes a window or network login (Section 1.21) look to the operating system just like a terminal.
In the ps listing, a tty might appear as
t1 for /dev/tty1,
p3 for /dev/ttyp3,
or as some other designation, such as co for
/dev/console, the full-screen display of a workstation
before any window system is started. Processes without a controlling terminal
show a question mark (?).
How does a process "lose" its controlling terminal? Easy. Some processes, such as system "daemons" (Section 1.10) never had one — they were started by system scripts that weren't started from any terminal, or they disconnected themselves from their controlling terminals. But it's also possible that you started a process running in the background, logged out, and logged back on later or on another terminal to find it still running without a controlling terminal. Disowned processes (Section 23.11) fit this category too.
The tty command can be used to report which "terminal" you're currently connected to. For example:
% tty
/dev/ttyp2Running tty without a controlling terminal
gives the message not
a
tty.
— TOR