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Answer: How do I run a script in the background? How should I run a script as daemon?

by jeroenes (Priest)
on Nov 16, 2000 at 21:28 UTC ( #42001=categorized answer: print w/replies, xml ) Need Help??

Q&A > input and output > How do I run a script in the background? How should I run a script as daemon? - Answer contributed by jeroenes

If you run a script in a the background with exec, things go nasty after a while when you kill your terminal. You can prevent this (I tried it only under linux) by setting SIG{INT}='IGNORE'.

Actually, a better solution is the Proc::daemon module, or the Net::daemon module if you need to claim a socket.

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Re: Answer: How do I run a script in the background? How should I run a script as daemon?
by Fastolfe (Vicar) on Nov 17, 2000 at 00:54 UTC
    If you disassociate the process from its parent (via setsid), this won't happen. When you kill your terminal (or exit it any way other than by typing "exit" or "logout"), a HUP signal is sent to the terminal's process group. This means every process that currently is in the shell's process group receives the HUP signal, which effectively kills everything started during that shell session.

    The approved way of disassociating a process from its process group is by changing its process group ID (pgid), such as via setsid. See perlipc and my other comment at Answer: Is it possible to background a perl script from within itself? for a method to cleanly do this.

      Why yes, you're right, but our friend, the late W. Richard Stevens goes farther and explains that the best method is as following:
      #untested code conversion from C sources exit if fork()!=0; setsid(); $SIG{'HUP'}='IGNORE'; exit if fork()!=0; chdir('/'); #prevents path-specific hax umask(0); #clear file mode creation mask openlog(...); #standard syslog call
      With the second call to fork, we are guaranteed that the child is not a session leader and therefore cannot acquire a controlling terminal. We ignore SIGHUP since the session leader is soon to be terminated. Just an expansion on what you said. I certainly admire and trust the late Mr. Stevens very much!
      AgentM Systems nor Nasca Enterprises nor Bone::Easy nor Macperl is responsible for the comments made by AgentM. Remember, you can build any logical system with NOR.
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