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Alternatives to cron
As useful as cron is, there are a couple of alternatives
that you should be aware of.
If your system is often off or in hibernation—for example, if you use a
UNIX laptop—consider adding anacron
to your system. Anacron is similar to
cron in that it schedules jobs to run in the future;
but unlike cron, anacron
run jobs even if the job's scheduled time has passed.
For example, if you scheduled a file-system backup to run Sunday but the system
is switched off from Friday to Monday, anacron
runs the Sunday job as soon as the system is reactivated on Monday. In contrast,
cron merely checks whether a job is to run right
now; hence, if the system is off when the job is scheduled, the job doesn't run.
Anacron has far fewer scheduling options than
cron. It can only schedule jobs in whole-day
intervals, such as one, seven, or 30 days, but it's a better choice for jobs that
must run frequently and reliably.
Also, you must launch anacron from cron.
Each time anacron runs, it reads its own
configuration file consisting of pairs, where each pair is a job and its
frequency expressed in days. If a job hasn't run in its period, anacron
runs the job and notes the time the job ran. When all jobs finish running,
anacron exits.
Anacron is available on most Linux distributions,
but you can also easily download and build the source code yourself. Visit the
anacron project page
to get the latest release.
The anacron primary configuration file can be found
in /etc/anacron. You can set environment variables just as you do with
cron, but its entries are simpler:
SHELL=/bin/zsh PATH=/usr/bin:/bin:/usr/local/bin # format: frequency delay name job 1 10 day-to-day daily.chores.sh |
The first number is the period, so 1 means run once
every day. A 7 would mean run once per seven days,
and so on. The second number is the delay, which is the number of minutes to wait
after anacron launches to start this job. The delay field,
if set to distinct values, prevents all jobs from starting at the same time. The name
day-to-day is just a helpful nickname. The rest of the line specifies the job;
here, the shell script daily.chores.sh, found in one of the directories in the named
path, runs every day.
Anacron has good documentation in the form of man pages,
and you can find very good tips on anacron on the Web.
(Check out Rod Smith's Linux Magazine
article, which I edited in October 2007.) Anacron is
ideal for UNIX road warriors or for any systems administrator who wants a bit of extra
insurance.
Launchd: A modern alternative to cron
Cron is certainly a capable and venerable utility, as
evidenced by its widespread usage. Recent additions in Vixie cron,
such as shorthand for @reboot, make it even easier
to administer. However, cron does have some
shortcomings:
- Although
cronjobs are defined in crontab files, you cannot start and stop acronjob from the command line. Moreover, you cannot create an ad hoc job at the command line and submit it to the calendar. Crondoes not enforce resource limits. A job can consume innumerable cycles and memory if run as root. You may, instead, want to damp down a job so that it cannot interfere with othercronjobs and the overall quality of system operations.Cronjobs adhere rigidly to a schedule. There is no way, for instance, to have a job launch only when an event occurs, such as the creation of a file.- In a larger context, UNIX-like systems have many core components capable
of launching other programs on demand, including
cron,xinetd(orinetd) for networking daemons, andinit, the progenitor of all system processes. Each core component has its own set of configuration files, making it difficult to know which components to tailor to make a change.
To address these shortcomings, Apple Computer created a unified launch facility, aptly
named launchd, to start processes on boot, on
demand, and at specified intervals. In fact, launchd replaced
cron (and init and several
other system utilities used to boot and initialize the system) in Mac OS 10.4 Tiger.
(Apple left cron on the system, though as a convenience
and because Vixie cron has more flexible scheduling
options.) Indeed, the phenomenal boot speed of Mac OS X can be attributed to
launchd: It enumerates what to launch at boot but
executes the programs only when first needed.
The code for launchd is available as open source from its
home page on Mac OS Forge.
To date, launchd has been ported to FreeBSD but not
to other UNIX or Linux systems. However, various projects are actively implementing
the equivalent of launchd, so a brief survey of its
features is necessary:
- Rather than create a job to poll a directory for new files,
launchdcan automatically monitor a directory for new files or monitor an empty directory for any files and launch your job on demand.Launchddoes not poll; instead, it uses the kqueues facility to have the kernel alert it when a directory is changed. (Linux has a similar event facility called inotify, which will be covered in a separate developerWorks article in the coming months.) - If specified,
launchduseschrootto send your job to a new directory. Pronounced "cha-root,"chrootis a system call to change the directory that the forward slash (/) and the root directory point to. Thus, if you usechrootto send the file to /opt/root, all files outside /opt/root are inaccessible—after all, /opt/root is now /, the top-level directory of the file system—and all directories within /opt/root become top-level directories. You most commonly usechrootto secure jobs so that code cannot wander into the larger file system to wreak havoc. - You can set resource limits for a job. Resources you can constrain include memory, stack size, and the maximum number of open files.
- When a task is defined and loaded into
launchd, you can start and stop the job by name from the command line.
Launchd is made of three components: the launchd
daemon itself; the launchctl utility used to add, alter, and remove jobs and
affect launchd; and one or more configuration files, where
each file defines one or more jobs. Given its origin on Mac OS X, launchd
configuration files are simply properties files, which can be expressed as
Extensible Markup Language (XML).
Briefly, here is how you would use launchd on Mac OS X—say,
to monitor a directory for incoming files and run a job on demand:
- Create a properties file to express the job and all its attributes.
You can use the Mac's Property Editor, or you can edit the XML by hand. In either case, the resulting file looks something like Listing 1.
Listing 1. A sample launchd job to monitor a file system directory for changes<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> <plist version="1.0"> <dict> <key>Label</key> <string>com.example.processor</string> <key>OnDemand</key> <true/> <key>Program</key> <string>/Users/strike/bin/processor</string> <key>ProgramArguments</key> <array> <string>processor</string> </array> <key>WatchPaths</key> <array> <string>/Users/strike/data/incoming</string> </array> </dict> </plist>
In a nutshell, this file runs the utility script found in /Users/strike/bin/processor whenever the contents of the directory /Users/strike/data/incoming changes. Setting
OnDemandto True tellslaunchdto start this job as needed. Save the file to ~/Library/LaunchAgents/com.example.processor.plist. - Load the job into
launchdwith launchctl:% launchctl load ~/Library/LaunchAgents/com.example.processor.plist
If you want to verify the last operation or see your list of saved jobs, simply type
launchctl list. - To remove a job, again use launchctl with
unload:% launchctl unload -w ~/Library/LaunchAgents/com.example.processor.plist
What does
-wdo? It removes the job fromlaunchdcompletely. Without it, the job would re-load automatically at login (because the job is in the per-user collection of launch agents).
The launchd man pages have lots of information, and if
you're a Mac OS X user, you can find any number of applications for
launchd. Hopefully, some clever developer will port
launchd more widely.
