From my question, it was clear that I cannot easily decide the features of a daemon.I have read in different articles and from different forums that service --status-all command can be used to list all the daemons in my system. But I do not think that the command is listing all daemons because NetworkManager,a daemon which is currently running in my Ubuntu 14.04 system, is not listed by the command. Is there some command to list the running daemons or else is there some way to find the daemons from the filesystem itself?

  • Are you sure it's not listed? How are you checking? I can see it on my Debian. Note that the name is network-manager, not NetworkManager. – terdon Dec 22 '14 at 10:50
  • Yes. I am sure. Nothing related to the term network is listed. Also it lists anacron which is mentioned as not a daemon in its init script. – Jackzz Dec 22 '14 at 10:52
  • Anacron not being a daemon is more a question of semantics because it is not run constantly. It is still run as a service which is what you normally refer to as daemons. Please edit your question and i) tell us which Ubuntu you are running and ii) what exactly you mean by "daemon". What is your final objective here? – terdon Dec 22 '14 at 10:56
  • I suppose any service running in the background is a daemon. I mentioned anacron because it was said in /etc/init.d/anacron that it is not a daemon. My objective is to write a C++ program to list all daemons running in my system. For that I need to know which files to parse to get the details. – Jackzz Dec 22 '14 at 11:01
  • 1
    Well, if you define daemons as services, service --status-all is what you need. Ubuntu seems to treat NetworkManager differently. I get both networking and network-manager in the output of services --status-all on Debian but only networking on Ubuntu. I think you need to define what exactly you mean by "daemon". – terdon Dec 22 '14 at 11:04
up vote 40 down vote accepted

The notion of daemon is attached to processes, not files. For this reason, there is no sense in "finding daemons on the filesystem". Just to make the notion a little clearer : a program is an executable file (visible in the output of ls) ; a process is an instance of that program (visible in the output of ps).

Now, if we use the information that I gave in my answer, we could find running daemons by searching for processes which run without a controlling terminal attached to them. This can be done quite easily with ps:

$ ps -eo 'tty,pid,comm' | grep ^?

The tty output field contains "?" when the process has no controlling terminal.

The big problem here comes when your system runs a graphical environment. Since GUI programs (i.e. Chromium) are not attached to a terminal, they also appear in the output. On a standard system, where root does not run graphical programs, you could simply restrict the previous list to root's processes. This can be achieved using ps' -U switch.

$ ps -U0 -o 'tty,pid,comm' | grep ^?

Yet, two problems arise here:

  • If root is running graphical programs, they will show up.
  • Daemons running without root privileges won't. Note that daemons which start at boot time are usually running as root.

Basically, we would like to display all programs without a controlling terminal, but not GUI programs. Luckily for us, there is a program to list GUI processes : xlsclients! This answer from slm tells us how to use it to list all GUI programs, but we'll have to reverse it, since we want to exclude them. This can be done using the --deselect switch.

First, we'll build a list of all GUI programs for which we have running processes. From the answer I just linked, this is done using...

$ xlsclients | cut -d' ' -f3 | paste - -s -d ','

Now, ps has a -C switch which allows us to select by command name. We just got our command list, so let's inject it into the ps command line. Note that I'm using --deselect afterwards to reverse my selection.

$ ps -C "$(xlsclients | cut -d' ' -f3 | paste - -s -d ',')" --deselect

Now, we have a list of all non-GUI processes. Let's not forget our "no TTY attached" rule. For this, I'll add -o tty,args to the previous line in order to output the tty of each process (and its full command line) :

$ ps -C "$(xlsclients | cut -d' ' -f3 | paste - -s -d ',')" --deselect -o tty,args | grep ^?

The final grep captures all lines which begin with "?", that is, all processes without a controlling tty. And there you go! This final line gives you all non-GUI processes running without a controlling terminal. Note that you could still improve it, for instance, by excluding kernel threads (which aren't processes)...

$ ps -C "$(xlsclients | cut -d' ' -f3 | paste - -s -d ',')" --ppid 2 --pid 2 --deselect -o tty,args | grep ^?

... or by adding a few columns of information for you to read:

$ ps -C "$(xlsclients | cut -d' ' -f3 | paste - -s -d ',')" --ppid 2 --pid 2 --deselect -o tty,uid,pid,ppid,args | grep ^?

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