As noted in UnixShellPatterns daemons need to be isolated from their terminals.

'''Forces'''

One problem with daemons is stopping them.  Another is avoiding running two
of them at one time.   You also need to be able to stop and restart a daemon
without thinking.

'''Solution'''
A simple solution is
	1 Make the daemon '''generate its own stop script'''.
	1 Write a '''start''' script

You can use the data in the stop script to test whether the daemon is still running. 

'''Example'''
I run a simple Mailing list for  students in some of my classes.  For
historical reasons it is called 'bbs'.  Once upon a time
The crontab was a bit disfunctional on the system so I used a daemon was called
	bbs.daemon
I would leave it running, sorting my Email, and redistributing it to
other people in the class:
	: script of daemon that sorts bbs messages
	echo kill -9 $$ > /u/faculty/dick/bin/stop.bbs
	while sleep 30
	do
	        /u/faculty/dick/bin/post.bbs
	done

When I run start.bbs the script checks to see if the
daemon is already running, and if not spawns the
deamon:
	: script to start an agent that proceses personal and BBS mail
	cd $HOME
	default=180
	if /u/faculty/dick/bin/psg bbs|egrep bbs.daemon
	then
	        echo is already running
	        /u/faculty/dick/bin/psg bbs|egrep bbs.daemon|awk '{print $2}'|
	                sed 's/^/kill -9 /' > /u/faculty/dick/bin/stop.bbs
	        exit 1
	fi
	pwd
	echo Starting Bulletin Board with delay ${1:-$default} seconds....
	cat /dev/null >nohup.out
	nohup nice bbs.daemon ${1:-$default} &
	sleep 1; echo                                                                        


(psg is another script that does a PS and a Grep).

--DickBotting
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any comments?
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I've seen a somewhat different approach (Linux's gpm mouse daemon), where the program is written so that it check for an instance of itself running, and if found, exits, unless invoked with a specific argument (-k), where if there is another running copy, kills that copy before exiting. --PeteHardie
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CategoryUnixShellPattern