Strange Ctrl+C behavior Post: 302997359

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Top Forums Shell Programming and Scripting Strange Ctrl+C behavior Post 302997359 by bakunin on Friday 12th of May 2017 03:24:21 AM

05-12-2017

Registered User

Quote:

Originally Posted by JackK

I forgot to tell that "nohup my_script.sh..." is executed inside parent shell script, which was used to prepare my_script.sh
Does this change anything? Smilie

As it is i already suspected that, so: no, that doesn't change anything.

It is like this: when you start a login shell a "terminal" is created (by a process called getty or something analogous). This terminal is part of the process environment of your login session and is then "inherited" thoughout the process tree originating in your login session. That is, whatever you start from there - foreground processes, background processes, ... - will always use this terminal for any (not-redirected) output. As soon as this terminal ceases to exist (i.e. you log off) all the other processes using this terminal too will also be terminated, because they too lose their terminal.

Now, you might want to have processes continue to run without your session remaining - in fact, this is what most applications are supposed to do: they should continue to run even if the session where they are started ends. One needs a way of telling a process that "yes, there is a terminal, but don't care if it goes away".

For exactly this purpose there is nohup ("no termination on hangup") and this is what it does. Still: this only works if the program honours it (that is: there needs to be some part in the code dealing with it) and second: it doesn't matter if the process is started froma script or by hand: this influences only where in the process tree it is located. Since the purpose of nohup is solely to "disown" the process it doesn't matter how it would otherwise inherit something (the terminal) it now won't.

I hope this explains it.

bakunin

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LEARN ABOUT DEBIAN

tcgetpgrp

TCGETPGRP(3)						     Linux Programmer's Manual						      TCGETPGRP(3)

NAME

       tcgetpgrp, tcsetpgrp - get and set terminal foreground process group

SYNOPSIS

       #include <unistd.h>

       pid_t tcgetpgrp(int fd);

       int tcsetpgrp(int fd, pid_t pgrp);

DESCRIPTION

       The  function  tcgetpgrp() returns the process group ID of the foreground process group on the terminal associated to fd, which must be the
       controlling terminal of the calling process.

       The function tcsetpgrp() makes the process group with process group ID pgrp the foreground process group on the terminal associated to  fd,
       which  must  be	the  controlling  terminal  of	the  calling  process, and still be associated with its session.  Moreover, pgrp must be a
       (nonempty) process group belonging to the same session as the calling process.

       If tcsetpgrp() is called by a member of a background process group in its session, and the calling process  is  not  blocking  or  ignoring
       SIGTTOU, a SIGTTOU signal is sent to all members of this background process group.

RETURN VALUE

       When  fd refers to the controlling terminal of the calling process, the function tcgetpgrp() will return the foreground process group ID of
       that terminal if there is one, and some value larger than 1 that is not presently a process group ID otherwise.	When fd does not refer	to
       the controlling terminal of the calling process, -1 is returned, and errno is set appropriately.

       When successful, tcsetpgrp() returns 0.	Otherwise, it returns -1, and errno is set appropriately.

ERRORS

       EBADF  fd is not a valid file descriptor.

       EINVAL pgrp has an unsupported value.

       ENOTTY The  calling  process  does  not have a controlling terminal, or it has one but it is not described by fd, or, for tcsetpgrp(), this
	      controlling terminal is no longer associated with the session of the calling process.

       EPERM  pgrp has a supported value, but is not the process group ID of a process in the same session as the calling process.

CONFORMING TO

       POSIX.1-2001.

NOTES

       These functions are implemented via the TIOCGPGRP and TIOCSPGRP ioctls.

   History
       The ioctls appeared in 4.2BSD.  The functions are POSIX inventions.

SEE ALSO

       setpgid(2), setsid(2), credentials(7)

COLOPHON

       This page is part of release 3.44 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

GNU
								    2003-01-28							      TCGETPGRP(3)