westrock2000 has asked for the wisdom of the Perl Monks concerning the following question:

Consider the following 
system("kedit &"); #or `kedit &`
chomp(input=<>);
exit;

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If I execute the command it will open a kedit and continue to a user input prompt. If I hit enter it will exit the script and the kedit will continue to stay open.

But if I were to close out the xterm while it was waiting for $input, the kedit application will also close out.

How can I prevent the kedit from being closed if the script is prematurely exited?

Replies are listed 'Best First'.
Re: Keep a "system" process running if script is prematurely exited?
by ikegami (Patriarch) on Jun 17, 2010 at 04:28 UTC

    I believe the child needs to disconnect STDIN, STDOUT and STDERR from the terminal. 

    use IPC::Open3 qw( open3 );
open(local *TO_CHLD, '<', '/dev/null') or die;
open(local *FR_CHLD, '>', '/dev/null') or die;
open3('>&TO_CHLD', '<&FR_CHLD', '<&FR_CHLD', 'kedit');

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    It might also need to call POSIX's setsid. I'm not sure what it does ( but I think it's what later-mentioned disown does ).

    Update: It's the other way around. setsid is required, but replacing STDIN, STDOUT and STDERR is probably not.

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      I'm not sure what it does

      setsid(2) creates a new session and connection to the controlling terminal is lost. The calling process becomes the new session and process group leader. This is commonly used to disconnect from a terminal when writing a daemon.

      I doubt it is relevant here. The editor being launched is an X-Windows application which does not use STDIN/OUT/SHAKEITALLABOUT. Is it Friday yet?
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        Copying the docs isn't very useful. I still have no idea what "disconnecting from a terminal" means.
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Re: Keep a "system" process running if script is prematurely exited?
by cdarke (Prior) on Jun 17, 2010 at 07:37 UTC
    UNIX shells vary, but most send a SIGHUP to the child when the process ends. There are several ways of getting your child process to ignore SIGHUP. A shell type method is to use the nohup(1) program,:
    system("nohup kedit &"); #or `kedit &`

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    another is to set 
    $SIG{HUP} = 'IGNORE';

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    in your perl before the system command.

    However there might be a simpler way. You can supress this default action in bash using 
    shopt -u huponexit

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    Tested using CentOS 5.2, Perl 5.12.0, bash 3.2.25(1) (where the default for huponexit is off), KEdit 1.3.
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      The nohup command worked very easily. Thanks.
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Re: Keep a "system" process running if script is prematurely exited?
by Anonymous Monk on Jun 17, 2010 at 04:14 UTC
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Re: Keep a "system" process running if script is prematurely exited?
by sflitman (Hermit) on Jun 17, 2010 at 05:44 UTC
    Under bash, you can use the disown command to detach it from the process and make it owned by the top-level process init. From bash: 
    # gedit &
# disown $!

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    Where $! is the last process id. To do this in Perl, I tried this: 
    # perl -e 'system q{gedit &}'

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    It starts gedit in its own window on my Ubuntu system and returns to the shell prompt. Then I close the terminal, and voila! gedit is still running. Can anyone tell us why disown is not needed in this case?

    HTH,
    SSF

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      Probably because bash shell option huponexit is set to off (see shopt for option settings).
      By the way, disown -h will supress HUP for a specific job, see help disown
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      Probably because the perl process that launched gedit is finished (isn't connected to the shell anymore ie STDIN/STDOUT/STDERR are closed)

      You can confirm by comparing strace/truss output with 

      perl -e 'system q{gedit &} scalar <>;'

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Re: Keep a "system" process running if script is prematurely exited?
by JavaFan (Canon) on Jun 17, 2010 at 14:28 UTC
    You may want to read the Complete Dissociation of Child from Parent section in perlipc.
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Re: Keep a "system" process running if script is prematurely exited?
by Argel (Prior) on Jun 17, 2010 at 19:31 UTC
    The following code to create a daemon may be useful (though e.g. you may want to leave your PATH alone). 
    use IO::Socket;
use POSIX qw(WNOHANG setsid);

sub daemonize {
        $SIG{CHLD} = 'IGNORE';        # Configure to autoreap zombies
        die "Can't fork" unless defined ( my $child = fork );  # FORK 
+<<<<<<<<<<<<
        CORE::exit(0) if $child;      # Parent exits
        setsid();                     # Become session leader
        open( STDIN,  "</dev/null" ); # Detach STDIN from shell
        open( STDOUT, ">/dev/null" ); # Detach STDOUT from shell
        open( STDERR, ">&STDOUT"   ); # Detach STDERR from shell
        chdir '/tmp';                 # Change working directory
        umask(0);                     # Reset umask
        $ENV{PATH} = '/bin:/sbin:/usr/sbin';  # Reset PATH
        }

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    WARNING: Verify your system supports autoreaping of zombies!!!!!

    You can read more in these threads: See also Re: Perl daemon that runs another daemon, Running Perl scripts from ssh, how to make a demon in perl?, and Persistent perl.

    Elda Taluta; Sarks Sark; Ark Arks
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