UNIX for Dummies Questions & Answers

When I do the following command where jxh777 is my userid to see my processes, I see a question mark in the return. Does that mean anything ?

ps -ef | grep jxh777

jxh777 9966 9965 0 16:47:46 pts/tb 0:00 -ksh
jxh777 11660 9966 1 17:05:02 pts/tb 0:00 grep jhabi0
jxh777 28162 28160 0 13:49:38 ? 0:00 perl ./Mob.pl

Thanks in advance for your insight

That field is the controlling terminal. When there is ? mark it means that process has no controlling terminal. A process with no controlling terminal is a deamon. It cannot open /dev/tty. And there is no terminal where you could type cntl-c to interrupt it. The process may have been started by another daemon such as cron.

i have some kind of clarification here..

can all the process without a controlling terminal be termed as a daemon ? ....
isnt that a daemon should necessarily be a session leader ... >>>

Good heavens, no!

A program like "cron" needs to be able to daemonize itself. That is, I should be able to start cron from a tty. If I do that, cron will have a controlling terminal when it first starts to run. To lose that controlling terminal (on a Posix compliant system), a sequence of steps is needed which does indeed include becoming a session leader. I believe that this may be what you are thinking of. However, this is just a transitory state. A correctly written modern daemon will then fork(). The parent (a session leader) will exit. The child (not a session leader) will then strive to run forever. Since it is not a session leader, it can open a tty device without acquiring it as a controlling terminal. In the old days, a daemon would need to fork() every time it wanted to write on /dev/console. The child would open /dev/console, write(), then exit().

And that brings up an interesting point. The session concept arose during the 90's. Until there were sessions, a process could not be a session leader. But early versions of unix, although they lacked sessions, did indeed have daemons. "No controlling terminal" is the definition of daemon that I learned in the early 80's. I have never seen anything to challenge that. But I don't think that "daemon" has its definition controlled by a standards body or anything like that.

1.7 How do I get my program to act like a daemon?

Just to illustrate what Perderabo has

Code:

(  tail -f /var/log/syslog & )
#exit from session

The tail -f will continue to run without a tty.

This is a different case than what I had in mind and more explanation is in order. When a session leader with a controlling terminal exits, the terminal is yanked away from the session. Posix demands that a HUP be delivered to the foreground process group. Also, suppose that a background job had been suspended because it wanted tty input. There will be no way for it to resume. So if any process a background process group is stopped, the entire group will be sent a HUP followed by a CONT. Some OS's will send a HUP to all background processes. Regardless, a process can ignore HUP and continue to function. Any process groups that continue to run after the session leader has exited are called orphaned. A member of a orphaned process group cannot read nor write to its controlling terminal nor will it recieve any signals from key presses on the terminal. However with most OS's, the controlling terminal remains open and this may show up in the ps output. The close() system call is supposed to succeed although any buffered data is discarded.

As this page says: Poor jxh461 probably is beginning to regret the question.

Actually, I was only reponding to this bit

Certianly worth your effort though, it's great to have this information available on the site.