Has my script finished? Post: 20573

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Top Forums UNIX for Dummies Questions & Answers Has my script finished? Post 20573 by Perderabo on Monday 29th of April 2002 01:06:56 PM

04-29-2002

Administrator Emeritus

There is no perfect solution to this problem. Someone else on the system might be running an identically named script. And pids do recycle.

Here is a quick script that illustrates my solution:

Code:

#! /usr/bin/ksh

if [[ -f lockout.pid ]] ; then
        if kill -0 $(cat lockout.pid) ; then
                echo still running
                exit 0
        fi
fi
echo $$ > lockout.pid
trap "rm lockout.pid" 0
sleep 30
exit

Is the script is functioning properly, it will create and delete the lockout.pid file correctly. If the lockout.pid file exists, then the process should also exist.

If the script is abruptly killed and cannot remove the lockout.pid file then the next instance of the script will detect it. The new instance will try to send signal zero to the old pid. If the script is not running as root, it will not be able to signal another user's process.

If the previous instance of the script finished normally, the new instance of the script cannot be fooled. If the previous instance of the script is still running, the new instance of the script cannot be fooled. These are the normal conditions we expect to find.

So to fool the script, first it must abort, second the pid must have recycled and the pid must now be in use by a new, unrelated process, and third, the new process be belong to the same user that is running the script. The third condition is lifted if root runs the script.

Perderabo

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file::pid

File::Pid(3pm)						User Contributed Perl Documentation					    File::Pid(3pm)

NAME

       File::Pid - Pid File Manipulation

SYNOPSIS

	 use File::Pid;

	 my $pidfile = File::Pid->new({
	   file => '/some/file.pid',
	 });

	 $pidfile->write;

	 if ( my $num = $pidfile->running ) {
	     die "Already running: $num
";
	 }

	 $pidfile->remove;

DESCRIPTION

       This software manages a pid file for you. It will create a pid file, query the process within to discover if it's still running, and remove
       the pid file.

       new

	 my $pidfile = File::Pid->new;

	 my $thisfile = File::Pid->new({
	   file => '/var/run/daemon.pid',
	 });

	 my $thisfileandpid = File::Pid->new({
	   file => '/var/run/daemon.pid',
	   pid	=> '145',
	 });

       This constructor takes two optional paramters.

       "file" - The name of the pid file to work on. If not specified, a pid file located in "File::Spec->tmpdir()" will be created that matches
       "(File::Basename::basename($0))[0] . '.pid'". So, for example, if $0 is ~/bin/sig.pl, the pid file will be /tmp/sig.pl.pid.

       "pid" - The pid to write to a new pidfile. If not specified, $$ is used when the pid file doesn't exist. When the pid file does exist, the
       pid inside it is used.

       file

	 my $pidfile = $pidfile->file;

       Accessor/mutator for the filename used as the pid file.

       pid

	 my $pid = $pidfile->pid;

       Accessor/mutator for the pid being saved to the pid file.

       write

	 my $pid = $pidfile->write;

       Writes the pid file to disk, inserting the pid inside the file.	On success, the pid written is returned. On failure, "undef" is returned.

       running

	 my $pid = $pidfile->running;
	 die "Service already running: $pid
" if $pid;

       Checks to see if the pricess identified in the pid file is still running. If the process is still running, the pid is returned. Otherwise
       "undef" is returned.

       remove

	 $pidfile->remove or warn "Couldn't unlink pid file
";

       Removes the pid file from disk. Returns true on success, false on failure.

       program_name

       This is a utility method that allows you to determine what "File::Pid" thinks the program name is. Internally this is used when no pid file
       is specified.

SEE ALSO

       perl.

AUTHOR

       Casey West, <casey@geeknest.com>.

COPYRIGHT

	 Copyright (c) 2005 Casey West.  All rights reserved.
	 This module is free software; you can redistribute it and/or modify it
	 under the same terms as Perl itself.

perl v5.8.8							    2008-04-05							    File::Pid(3pm)