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Full Discussion: [KSH/Bash] Starting a parent process from a child process?
Top Forums Shell Programming and Scripting [KSH/Bash] Starting a parent process from a child process? Post 302408512 by alister on Monday 29th of March 2010 05:51:50 PM
Old 03-29-2010
The death of a process, even a parent process, does not trigger the kernel to send SIGHUP to any processes. That only happens when the controlling terminal/pseudo terminal is hung up/closed. If SIGHUP is not the signal being sent, nohup will not prevent a process dying if that is the default action for whatever signal it is actually being sent (a SIGINT or SIGTERM will still kill a process, unless they're explicitly trapped by the receiving process).

Default behavior when a SIGHUP signal is sent to a process is for the kernel to terminate the process. nohup simply sets the SIGHUP signal to be ignored and then execs its argument (the command you want to run, which then inherits this signal handling modification from nohup ... all other singal handling is unmodified).

The heart of OpenBSD's nohup implementation:
Code:
	/*
	 * The nohup utility shall take the standard action for all signals
	 * except that SIGHUP shall be ignored.
	 */
	(void)signal(SIGHUP, SIG_IGN);

	execvp(argv[1], &argv[1]);

from src/usr.bin/nohup/nohup.c - view - 1.14

Regards,
Alister
Last edited by alister; 03-29-2010 at 07:00 PM..
 

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LEARN ABOUT BSD

nohup

NICE(1) 						      General Commands Manual							   NICE(1)

NAME

       nice, nohup - run a command at low priority (sh only)

SYNOPSIS

       nice [ -number ] command [ arguments ]

       nohup command [ arguments ]

DESCRIPTION

       Nice  executes  command	with low scheduling priority.  If the number argument is present, the priority is incremented (higher numbers mean
       lower priorities) by that amount up to a limit of 20.  The default number is 10.

       The super-user may run commands with priority higher than normal by using a negative priority, e.g. `--10'.

       Nohup executes command immune to hangup and terminate signals from the controlling terminal.  The priority  is  incremented  by	5.   Nohup
       should  be invoked from the shell with `&' in order to prevent it from responding to interrupts by or stealing the input from the next per-
       son who logs in on the same terminal.

FILES

       nohup.out standard output and standard error file under nohup

SEE ALSO

       csh(1), setpriority(2), renice(8)

DIAGNOSTICS

       Nice returns the exit status of the subject command.

BUGS

       Nice and nohup are particular to sh(1).	If you use csh(1), then commands executed with ``&'' are automatically immune  to  hangup  signals
       while  in  the  background.   There  is	a builtin command nohup which provides immunity from terminate, but it does not redirect output to
       nohup.out.

       Nice is built into csh(1) with a slightly different syntax than described here.	The form ``nice +10'' nices to positive nice,  and  ``nice
       -10'' can be used by the super-user to give a process more of the processor.

4th Berkeley Distribution					    May 8, 1986 							   NICE(1)
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