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

Victorious Knight (Games)
Victorious Knight 1.3 Device: iOS Universal Category: Games Price: $1.99, Version: 1.3 (iTunes) Description: New challenges awaits you! Experience fresh RPG experience with a unique combat mechanic, packed with high quality 3D... | Read more »
Agent Gumball - Roguelike Spy Game (Gam...
Agent Gumball - Roguelike Spy Game 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Someone’s been spying on Gumball. What the what?! Two can play at that game! GO UNDERCOVERSneak past enemy... | Read more »
Runaway Toad (Games)
Runaway Toad 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: It ain’t easy bein’ green! Tap, hold, and swipe to help Toad hop to safety in this gorgeous new action game from the creators of... | Read more »
PsyCard (Games)
PsyCard 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: From the makers och Card City Nights, Progress To 100 and Ittle Dew PSYCARD is a minesweeper-like game set in a cozy cyberpunk... | Read more »
Sago Mini Robot Party (Education)
Sago Mini Robot Party 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: -- Children's Technology Review Editor's Choice -- | Read more »
How to get a high score in every level o...
Sky Charms is an adorable match three puzzler that provides a decent challenge thanks to its creative level design. It regularly presents something new, forcing you to think on your feet. [Read more] | Read more »
Apestorm: Full Bananas (Games)
Apestorm: Full Bananas 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: ***Launch sale – limited time only!*** Fugitive Apes have taken to the skies in search of revenge after humans have... | Read more »
How to create bigger words in Spellspire
Words have power. At least they do in Spellspire,a game about blasting out magical attacks by making words out of a jumble of letters. And it's a lot of fun. But if you want to be the best, you're going to have to think tactically when you start... | Read more »
Steel Media and DeePoon have partnered f...
Virtual reality is the next big thing, and 148Apps's publisher,Steel Media, wants to know what the hottest upcoming games are. [Read more] | Read more »
Airline Director 2 - Tycoon Game (Games...
Airline Director 2 - Tycoon Game 1.2.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.2.1 (iTunes) Description: Airline Director 2 is a management game set in the challenging field of commercial aviation. As the... | Read more »

Price Scanner via MacPrices.net

Aleratec Releases Mac Software Upgrade for 1...
California based Aleratec Inc., designer, developer and manufacturer of Portable Device Management (PDM) charge/sync products for mobile devices and professional-grade duplicators for hard disk... Read more
Sale! Amazon offers 27-inch iMac, 13-inch 2.9...
Amazon has the 27″ 3.2GHz 5K iMac and the 13″ 3.9GHz Retina MacBook Pro on sale for $300 off MSRP, each including free shipping, for a limited time: - 27″ 3.2GHz/1TB HD 5K iMac (model MK462LL/A): $... Read more
Apple refurbished 13-inch Retina MacBook Pros...
Apple has Certified Refurbished 13″ Retina MacBook Pros available for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.7GHz... Read more
13-inch 2.7GHz/128GB Retina MacBook Pro on sa...
Take $200 off MSRP on the price of a new 13″ 2.7GHz/128GB Retina MacBook Pro (model MF839LL/A) at Amazon. Shipping is free: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1099.99 $200 off MSRP Act now if... Read more
Apple refurbished clearance 15-inch Retina Ma...
Apple has Certified Refurbished 2014 15″ 2.2GHz Retina MacBook Pros available for $1609, $390 off original MSRP. Apple’s one-year warranty is included, and shipping is free. They have refurbished 15... Read more
27-inch 5K iMacs on sale for up to $150 off M...
B&H Photo has 27″ 5K iMacs on sale for up to $150 off MSRP including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2199 $100 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $1849.99 $150... Read more
What Does The Refreshed 12-Inch MacBook Tell...
A lot of commentators are complaining that Apple’s update of the 12-Inch MacBook last week is a bit of a damp squib. I don’t know what they were expecting, since it would be very unlike Apple to do a... Read more
Free Wittify Keyboard Now Available On The Ap...
A team of Harvard Business School students have announced that the Wittify Keyboard, a new app utility for iOS devices, is now available on the Apple App Store. The Wittify keyboard and application... Read more
Apple Reports First Year-Over-Year Quarterly...
Apple on TUesday announced financial results for its fiscal 2016 second quarter ending March 26, 2016. The Company posted quarterly revenue of $50.6 billion and quarterly net income of $10.5 billion... Read more
13-inch 2.7GHz Retina MacBook Pros on sale fo...
Take $130-$150 off MSRP on the price of a new 13″ 2.7GHz Retina MacBook Pro at Amazon. Shipping is free: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1169 $130 off MSRP - 13″ 2.7GHz/256GB Retina MacBook... Read more

Jobs Board

Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the largest premier retailer of Apple products in the nation. In order to support our growing customer base, we are currently looking for a driven Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
*Apple* Solutions Consultant - … (United Sta...
Job Summary As an Apple Solutions Consultant, you'll be the link between our future customers and our products. You'll showcase your entrepreneurial spirit as you Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.