Comments on: Daemonizing Processes

By: James Cape

James Cape — Sat, 18 Oct 2008 01:30:43 +0000

Jeff: Sorry, I see daemon(3) and go into "library call, can't use" mode. Rudd-O was referring to upstart. Alastair: Yeah, it doesn't actually daemonize the process for you, though: $nice /bin/bash -c "$corelimit >/dev/null 2>&1 ; $*" It just launches it after setting a bunch of things, then checks the return value and prints the [OK]/[FAILED] messages.

By: Alastair

Alastair — Fri, 17 Oct 2008 15:25:10 +0000

Red Hat have long used a function in the /etc/init.d/functions script called daemon to daemonise processes. It allows passing optional nice level, a pidfile to track the resulting pid and a user to run as.

By: Jeff Schroeder

Jeff Schroeder — Fri, 17 Oct 2008 14:57:46 +0000

James,

I wasn’t referring to upstart at all, I was referring to setsid. setsid is part of util-linux and installed by default on EVERY RHEL box.

Here is one of our internal dns servers in asia:
root@ns1.tyo01.mtt:~ # cat /etc/redhat-release
Red Hat Enterprise Linux Server release 5 (Tikanga)
root@ns1.tyo01.mtt:~ # rpm –qf ‘which setsid‘
util-linux-2.13–0.44.el5
root@ns1.tyo01.mtt:~ # setsid
usage: setsid program [arg …]

Also, disown requires you to background the process. Something like:
./somecommand & disown –h

Note the –h to disown, that keeps it from dying in the event of a sighup.

By: James Cape

James Cape — Fri, 17 Oct 2008 12:48:21 +0000

Rudd-O/Jeff Schroeder:

Unfortunately, my platform is RHEL5, so there’s no upstart, and the app I’m trying to deal with is Java, so I can’t call daemon(3).

Luke:

That’s actually a better solution, I had heard that was only in ksh, but apparently that’s not the case. Thanks for that.

By: Adam Williamson

Adam Williamson — Fri, 17 Oct 2008 09:13:37 +0000

As well as daemon(3) there’s daemonize:

[root@lenovo VTK]# urpmq –i daemonize
Name : daemonize
Version : 1.5.2
Release : 2mdv2009.0
Group : System/Servers
Size : 18177 Architecture: i586
Source RPM : daemonize-1.5.2-2mdv2009.0.src.rpm
URL : http://www.clapper.org/software/daemonize/
Summary : Run a command as a Unix daemon

By: Götz Waschk

Götz Waschk — Fri, 17 Oct 2008 06:24:26 +0000

There’s also daemonize: http://www.clapper.org/software/daemonize/

By: Luke

Luke — Fri, 17 Oct 2008 03:02:03 +0000

Don’t forget about the ‘disown’ bash builtin:

disown [-ar] [-h] [jobspec …]
Without options, each jobspec is removed from the table of
active jobs. If the –h option is given, each jobspec is not
removed from the table, but is marked so that SIGHUP is not sent
to the job if the shell receives a SIGHUP. If no jobspec is
present, and neither the –a nor the –r option is supplied, the
current job is used. If no jobspec is supplied, the –a option
means to remove or mark all jobs; the –r option without a job–
spec argument restricts operation to running jobs. The return
value is 0 unless a jobspec does not specify a valid job.

In other words, after invoking disown, if you close the associated terminal it doesn’t kill the job.

By: Jeff Schroeder

Jeff Schroeder — Fri, 17 Oct 2008 02:43:32 +0000

Damn, Rudd-O beat me to it. Take a look at daemon(3). setsid it the easiest way to do what you want. It does everything you want.

By: Rudd-O

Rudd-O — Fri, 17 Oct 2008 01:44:37 +0000

oh, look into upstart events, you no longer need to write daemons if you use them.

By: Rudd-O

Rudd-O — Fri, 17 Oct 2008 01:42:10 +0000

Closing stdin is not enough. There is the chdir / and the double fork t
plus setsid to set the daemonized process as session leader, otherwise some signals sent to the parent process reach the “daemon”. I dunno if start stop daemon implements all of this correctly, but your sh trick is doing it wrong.