Generic solution to prevent a long cron job from running in parallel?
Take a look at the run-one
package. From the manpage for the run-one
command :
run-one is a wrapper script that runs no more than one unique instance of some command with a unique set of arguments.
This is often useful with cronjobs, when you want no more than one copy running at a time.
Like time
or sudo
, you just prepend it to the command. So a cronjob could look like:
*/60 * * * * run-one rsync -azP $HOME example.com:/srv/backup
For more information and background, check out the blog post introducing it by Dustin Kirkland.
A very simple way of settup a lock:
if mkdir /var/lock/mylock; then
echo "Locking succeeded" >&2
else
echo "Lock failed - exit" >&2
exit 1
fi
A scripts which want to run needs te create the lock. If the lock exists, another script is busy, so the first script can't run. If the file don't exists, no script has acquired the lock. So the current script acquires the lock. When the script has finished the lock needs to be realeased by removeing the lock.
For more information about bash locks, check this page
See also Tim Kay's solo
, which performs locking by binding a port on a loopback address unique to the user:
http://timkay.com/solo/
In case his site goes down:
Usage:
solo -port=PORT COMMAND
where
PORT some arbitrary port number to be used for locking
COMMAND shell command to run
options
-verbose be verbose
-silent be silent
Use it like this:
* * * * * solo -port=3801 ./job.pl blah blah
Script:
#!/usr/bin/perl -s
#
# solo v1.7
# Prevents multiple cron instances from running simultaneously.
#
# Copyright 2007-2016 Timothy Kay
# http://timkay.com/solo/
#
# It is free software; you can redistribute it and/or modify it under the terms of either:
#
# a) the GNU General Public License as published by the Free Software Foundation;
# either version 1 (http://dev.perl.org/licenses/gpl1.html), or (at your option)
# any later version (http://www.fsf.org/licenses/licenses.html#GNUGPL), or
#
# b) the "Artistic License" (http://dev.perl.org/licenses/artistic.html), or
#
# c) the MIT License (http://opensource.org/licenses/MIT)
#
use Socket;
alarm $timeout if $timeout;
$port =~ /^\d+$/ or $noport or die "Usage: $0 -port=PORT COMMAND\n";
if ($port)
{
# To work with OpenBSD: change to
# $addr = pack(CnC, 127, 0, 1);
# but make sure to use different ports across different users.
# (Thanks to www.gotati.com .)
$addr = pack(CnC, 127, $<, 1);
print "solo: bind ", join(".", unpack(C4, $addr)), ":$port\n" if $verbose;
$^F = 10; # unset close-on-exec
socket(SOLO, PF_INET, SOCK_STREAM, getprotobyname('tcp')) or die "socket: $!";
bind(SOLO, sockaddr_in($port, $addr)) or $silent? exit: die "solo($port): $!\n";
}
sleep $sleep if $sleep;
exec @ARGV;