TweetFollow Us on Twitter

Time-based Daemons

Volume Number: 19 (2003)
Issue Number: 9
Column Tag: Programming

Section 7

Time-based Daemons

by Rich Morin

at(1), batch(1), cron(1), etc.

Most daemons (i.e., background processes) are event-based, responding to an incoming packet, the appearance of a file, etc. Some daemons are time-based, however, occurring at a given time (or times).

BSD (and thereby OSX) makes it very easy to set up time-based daemons. In fact, there are several ways to do this, depending on your needs. Let's look at some of the options.

CRON

The cron(8) subsystem (see also crontab(1,5)) runs commands at times which are specified in one or more control files.. It is also, as described below, the basis for time-based services such as at(1) and periodic(8).

Note: The command "man cron" fails in Mac OS X 10.2.6, but you can view the man page by typing "more /usr/share/man/cat8/cron.8.gz".

Originally, there was only one crontab(8) file, located at /etc/crontab. This file was only editable by root, though it could run commands as other users. Later, individual users were given the ability to maintain their own control files, using the crontab(1) command.

The default version of /etc/crontab on Mac OS X looks like:

# /etc/crontab
SHELL=/bin/sh
PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin
HOME=/var/log
#
#minute hour mday month wday who  command
#
#*/5    *    *    *     *    root /usr/libexec/atrun
#
# Run daily/weekly/monthly jobs.
15      3    *    *     *    root periodic daily
30      4    *    *     6    root periodic weekly
30      5    1    *     *    root periodic monthly

Environment variables can be set (using Bourne shell syntax) for the scheduled commands. Thus, the commands listed in this file will have HOME, PATH, and SHELL set for them. Be sure to take these settings into account when writing scripts to be run under cron; if your script asks for a command that isn't found on the PATH, for example, it won't act as desired.

As in the case of most BSD control files (and many scripting languages), "#" can be used to indicate the start of a comment, extending through the end of the current line. This is often used to disable scheduling lines (such as the one for "atrun", in this example).

Each scheduling line has three parts, separated by white space. The first part may be a special string (e.g., @reboot, @daily), but more commonly it will be a set of five fields, also separated by white space. The second part is the username (e.g., root) under which the command will be run. The third part is the command (e.g., "periodic daily").

In the example, "periodic daily" is scheduled to be run (as root) at 3:15 AM every day. Similarly, "periodic weekly" and "periodic monthly" are scheduled for 4:30 AM each Saturday and 5:30 AM on the first day of each month, respectively.

The format of crontab files for individual users is almost identical to that for /etc/crontab. The only difference is that the "who" (username) field is not present. This makes sense; only the root account is able to set the user id under which a command will be run.

Although /etc/crontab can be edited in any desired manner, the individual crontab files must be edited by means of the crontab(1) command. This prevents race conditions, ensures that the cron daemon will notice any changes, etc.

Periodic

If you have a system maintenance command that needs to be run during off hours on a daily, weekly, or monthly basis, the periodic subsystem (periodic(8), periodic.conf(5)) may be exactly what you want.

The periodic(8) command is actually a shell script. I won't discuss it here, but you may wish to give it a look: "more /usr/sbin/periodic". Basically, however, the script runs every executable file found in the specified directory. For example, "periodic daily" runs any commands found in /etc/periodic/daily:

% wc -l /etc/periodic/daily/*
      56 /etc/periodic/daily/100.clean-logs
     131 /etc/periodic/daily/500.daily
     187 total

I wouldn't suggest modifying any of these files, as Apple may overwrite them in an update, but putting in your own files should be fairly safe. Just stuff an executable file into the appropriate subdirectory, picking the filename to sort into the desired execution order: For example, if you have a script that needs to run after "500.daily", you could name it something like "600.local.fooscript".

You may enjoy looking through these files to see what gets done while you're off snoozing: try "more /etc/periodic/*/*". The configuration files, described in periodic.conf(5), are also worth a look.

At, Batch, etc.

The at(1) and batch(1) commands act in a very similar manner to each other. Both commands schedule a file for execution at a specified time. The difference is that batch(1) also checks the system load level, ensuring that the command doesn't add work to an already-overloaded system.

In order to use either command, however, you'll have to uncomment the "atrun" line in /etc/crontab, causing the program to be run every five minutes:

*/5    *    *    *     *    root /usr/libexec/atrun

Actually, there's no particular reason why you couldn't schedule atrun to run every minute, if you wish. On a desktop machine, an occasional process start-up is unlikely to make a noticeable difference. To try this, just edit the line to:

*      *    *    *     *    root /usr/libexec/atrun

Note: The documentation and configuration of at(1) in OSX 10.2.6 are a bit deficient. Although the at(1) man page says that "Traditional access control to at and batch via the files /var/at/at.allow and /var/at/at.deny is not implemented", the program will fail unless (at least) one of these files is present. The spool directory (/var/at/spool) may also be missing, causing scheduled jobs to silently fail. Fortunately, the fixes are simple:

% su
Password:
# touch /var/at/at.deny
# mkdir /var/at/spool
# exit
exit
%

Having worked our way past the setup hassles, let's try running some at(1) jobs. Here's a short test script we can use:

:
# att - at(1) test script
(date; printenv | sort) > att.$$.out

For the shell-challenged, here's a rundown of what's going on here. The initial colon tells the kernel that the script should be interpreted by the Bourne shell. The real work is done by a single line which starts up a subshell (subsidiary copy of the shell), has it run "date" and "printenv | sort", and redirects the (concatenated) output into a file.

Because "$$" evaluates to the process ID of the interpreting shell, the name of the file will look something like "att.12345.out". After you have edited the file, make it executable, run it, and examine the results:

% chmod +x att
% att
% more att.*.out
Fri Jul  4 18:46:38 PDT 2003
HOME=/Users/rdm
PATH=/Users/rdm/bin:...
PWD=/Users/rdm/...
SHELL=/bin/tcsh
...

The output shows us the date and time that the command was run, as well as the settings for any environment variables. Now, let's try scheduling the script via at(1), waiting for it to get run, and comparing the output with that of our first (manual) run:

% at -f att +1 minute
Job a010ce73c.000 will be executed using /bin/sh
% atq
Date                    Owner   Queue   Job#
19:04:00 07/04/03       rdm     a       a010ce73c.000
...
% atq
% _d_i_f_f_ _a_t_t_._*_._o_u_t_
__1_c_1_
_<_ _F_r_i_ _J_u_l_ _ _4_ _1_9_:_0_0_:_4_8_ _P_D_T_ _2_0_0_3_
_-_-_-_
_>_ _F_r_i_ _J_u_l_ _ _4_ _1_9_:_0_5_:_0_0_ _P_D_T_ _2_0_0_3_
_2_3_,_2_5_c_2_3_
_<_ _S_H_L_V_L_=_2_
_<_ _T_E_R_M_=_v_t_1_0_0_
_<_ _T_E_R_M_C_A_P_='"'"'"_
_-_-_-_
_>_ _S_H_L_V_L_=_1_

Not too many changes, really. The time changed, of course, but most of the environment variables stayed the same. SHLVL (the shell level) is lower for the at(1) run, because no interactive shell was involved. The TERM and TERMCAP variables aren't set for the at(1) run, because no terminal is attached to the process.

Rolling your own

If none of these facilities is quite what you need, consider creating your own time-based daemon. Simply putting a process to sleep for a specified period is quite simple; making a process wake up at a specified time is a bit trickier, but still quite possible.

If you take this approach, however, you may want to look at the source code for existing routines that perform similar services. The Darwin source code (www.opendarwin.org) has the source code for the commands described in this column. The CPAN (cpan.perl.org) is a good place to look for relevant Perl modules.


Rich Morin has been using computers since 1970, Unix since 1983, and Mac-based Unix since 1986 (when he helped Apple create A/UX 1.0). When he isn't writing this column, Rich runs Prime Time Freeware (www.ptf.com), a publisher of books and CD-ROMs for the Free and Open Source software community. Feel free to write to Rich at rdm@ptf.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

calibre 4.17.0 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
MacPilot 11.1.4 - $15.96
MacPilot gives you the power of UNIX and the simplicity of Macintosh, which means a phenomenal amount of untapped power in your hands! Use MacPilot to unlock over 1,200 features, and access them all... Read more
Transmission 3.00 - Popular BitTorrent c...
Transmission is a fast, easy, and free multi-platform BitTorrent client. Transmission sets initial preferences so things "just work", while advanced features like watch directories, bad peer blocking... Read more
Doom 3 1.3.1 - First-person shooter acti...
A massive demonic invasion has overwhelmed the Union Aerospace Corporation's (UAC) Mars Research Facility, leaving only chaos and horror in its wake. As one of only a few survivors, you must fight... Read more
Box Sync 4.0.8004 - Online synchronizati...
Box Sync gives you a hard-drive in the Cloud for online storage. Note: You must first sign up to use Box. What if the files you need are on your laptop -- but you're on the road with your iPhone? No... Read more
LibreOffice 6.4.4.2 - Free, open-source...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
Day One 4.14 - Maintain a daily journal.
Day One is an easy, great-looking way to use a journal / diary / text-logging application. Day One is well designed and extremely focused to encourage you to write more through quick Menu Bar entry,... Read more
Doxie 2.12.2 - Scan, share, and store do...
Introducing Doxie, the new, modern paper scanner that's so simple, it'll revolutionize the way you think about sharing and storing docs and photos forever. Doxie is ultra-portable, fully automatic,... Read more
Tinderbox 8.7.0 - Store and organize you...
Tinderbox is a personal content management assistant. It stores your notes, ideas, and plans. It can help you organize and understand them. And Tinderbox helps you share ideas through Web journals... Read more
Viber 13.0.0 - Send messages and make fr...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device, so... Read more

Latest Forum Discussions

See All

SINoALICE, Yoko Taro and Pokelabo's...
Yoko Taro and developer Pokelabo's SINoALICE has now opened for pre-registration over on the App Store. It's already amassed 1.5 million Android pre-registrations, and it's currently slated to launch on July 1st. [Read more] | Read more »
Masketeers: Idle Has Fallen's lates...
Masketeers: Idle Has Fallen is the latest endeavour from Appxplore, the folks behind Crab War, Thor: War of Tapnarok and Light A Way. It's an idle RPG that's currently available for Android in Early Access and will head to iOS at a later date. [... | Read more »
Evil Hunter Tycoon celebrates 2 million...
Evil Hunter Tycoon has proved to be quite the hit since launching back in March, with its most recent milestone being 2 million downloads. To celebrate the achievement, developer Super Planet has released a new updated called Darkness' Front Yard... | Read more »
Peak's Edge is an intriguing roguel...
Peak's Edge is an upcoming roguelike puzzle game from developer Kenny Sun that's heading for both iOS and Android on June 4th as a free-to-play title. It will see players rolling a pyramid shape through a variety of different levels. [Read more] | Read more »
Clash Royale: The Road to Legendary Aren...
Supercell recently celebrated its 10th anniversary and their best title, Clash Royale, is as good as it's ever been. Even for lapsed players, returning to the game is as easy as can be. If you want to join us in picking the game back up, we've put... | Read more »
The Magic Gladiator class arrives in MU...
The Magic Gladiator class is now available in MU Origin 2 following the most recent patch. It also marks the start of Abyss Season 11 and the introduction of Couple Skills and Couple Dungeons. [Read more] | Read more »
The 5 Best Racing Games
With KartRider Rush+ making a splash this past week, we figured it was high time we updated our list of the best mobile racing games out there. From realistic racing sims to futuristic arcade racers (and even racing management games!), check out... | Read more »
KartRider Rush+ Guide - Tips for new rac...
KartRider Rush+ continues to be a surprisingly refreshing and fun kart racer that's entirely free-to-play. The main reason for this is just how high its skill ceiling is. Check out the video above if you're curious to know what top level play looks... | Read more »
KartRider Rush+ might be good, actually?
It's hard to find good racing games on mobile. Most of them are free-to-play, and free-to-play racers generally suck. Even Nintendo couldn't put together a competent Mario Kart game, opting instead for a weird score chaser that resembles--but feels... | Read more »
LifeAfter, NetEase's popular surviv...
A new map will be making its way into NetEase's popular survival game LifeAfter. The map is set to arrive on May 28th and will introduce a volcano that's teetering on the verge of eruption, bringing a host of added challenges to the game. [Read... | Read more »

Price Scanner via MacPrices.net

Memorial Day Weekend Sale: Take $300 off thes...
Apple resellers are offering $300 discounts on select 16″ MacBook Pros as part of their Memorial Day Weekend 2020 sales. Prices start at $2099: – 16″ 2.6GHz 6-Core Space Gray MacBook Pro: $2099 at... Read more
Best Memorial Day Weekend 2020 Apple AirPods...
Apple resellers are offering discounts ranging up to $50 off MSRP on AirPods as part of their Memorial Day Weekend 2020 sales. These are the best deals today on various AirPods models. See our... Read more
Memorial Day Weekend Sale: 10″ Apple iPads fo...
Amazon is offering new 10.2″ iPads for $80-$100 off Apple’s MSRP as part of their Memorial Day Weekend 2020 sale, with prices starting at only $249. These are the same iPads sold by Apple in their... Read more
Memorial Day Weekend Sale: 2020 Apple iPhone...
Sprint is offering Apple’s new 2020 64GB iPhone SE for $0 per month for 18 months as part of their Memorial Day Weekend 2020 sale. New line of service and trade-in required. Offer is valid from 5/22/... Read more
Amazon’s popular $100 Apple Watch Series 5 di...
Amazon has Apple Watch Series 5 GPS + Cellular models on sale for up to $100 off Apple’s MSRP today. Shipping is free. These are the same Apple Watch models sold by Apple in their retail and online... Read more
2020 13″ 4-Core MacBook Air on sale for $949,...
Apple reseller Adorama has the new 2020 13″ 1.1GHz 4-Core Space Gray MacBook Air on sale today for $949 shipped. Their price is $50 off Apple’s MSRP, and it’s the lowest price currently available for... Read more
Apple Retail Chief Announces Staggered Reopen...
NEWS: 05.20.20 – In the midst of a global pandemic, after its retail outlets were shuttered temporarily in mid-March as a mitigation measure enacted by Apple during the peak of the spread of COVID-19... Read more
Apple’s Pro Display XDR models in stock today...
Abt Electronics has Apple’s new 32″ Pro Display XDR models in stock and on sale today for up to $305 off MSRP. Shipping is free. Their prices are currently the lowest available for these new models... Read more
Apple restocks refurbished iPhone 8 for $339
Apple has restocked Apple Certified Refurbished 64GB iPhone 8 models for only $339. Apple dropped their price on this phone last month by $160, from $499 to $339. Each refurbished iPhone comes with a... Read more
New at AT&T: 50% off iPhone 11 for new cu...
AT&T is offering a 50% off the 64GB iPhone 11 for new customers who switch to AT&T and open a new line of service. Discount applied over a 30 month period The fine print: “iPhone 11 64GB for... Read more

Jobs Board

Cub Foods - *Apple* Valley - Now Hiring Par...
Cub Foods - Apple Valley - Now Hiring Part Time! United States of America, Minnesota, Apple Valley New Retail Operations Post Date 5 days ago Requisition # Read more
Senior Data Engineer - *Apple* - Theorem, L...
Job Summary Apple is seeking an experienced, detail-minded data engineeringconsultant to join our worldwide business development and strategy team. If you are Read more
Cub Foods - *Apple* Valley - Now Hiring Par...
Cub Foods - Apple Valley - Now Hiring Part Time! United States of America, Minnesota, Apple Valley New Retail Operations Post Date 4 days ago Requisition # Read more
Medical Screener - *Apple* Hill - Evenings...
Medical Screener - Apple Hill - Evenings Tracking Code D-MS-AH-E Job Description Medical Screener - Temporary We Are Hiring: WellSpan Health has a temporary Medical Read more
*Apple* Systems Administrator - Solidus Tech...
Solidus is searching for an Apple Systems Engineer. The engineer will be responsible for desktop and server infrastructure. This includes support for operating Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.