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 Manpage icon:

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;

Tags:

Bash

Cron

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