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

FINAL FANTASY Ⅸ (Games)
FINAL FANTASY Ⅸ 1.0.4 Device: iOS Universal Category: Games Price: $16.99, Version: 1.0.4 (iTunes) Description: ==========●Special sale price for the FINAL FANTASY IX release! ●20% off from February 10 to February 21, 2016... | Read more »
Tennis Club Story (Games)
Tennis Club Story 1.03 Device: iOS Universal Category: Games Price: $4.99, Version: 1.03 (iTunes) Description: Aim for the ace position of tennis club prestige in this simulation! Your leadership decides if players make it to the big... | Read more »
Check out the new Pirate Attack update i...
Love pirates and board games? Well, you'll love the new Pirate Attack themed update that just launched in Game of Dice. It adds a bunch of new content themed around pirates, like an all new event map based on a pirate ship which revamps the toll... | Read more »
Splash Cars guide - How to paint the tow...
Splash Cars is an arcade driving game that feels like a hybrid between Dawn of the Plow and Splatoon. In it, you'll need to drive a car around to repaint areas of a town that have lost all of their color. Check out these tips to help you perform... | Read more »
The best video player on mobile
We all know the stock video player on iOS is not particularly convenient, primarily because it asks us to hook a device up to iTunes to sync video in a world that has things like Netflix. [Read more] | Read more »
Four apps to help improve your Super Bow...
Super Bowl Sunday is upon us, and whether you’re a Panthers or a Broncos fan you’re no doubt gearing up for it. [Read more] | Read more »
LooperSonic (Music)
LooperSonic 1.0 Device: iOS Universal Category: Music Price: $4.99, Version: 1.0 (iTunes) Description: LooperSonic is a multi-track audio looper and recorder that will take your loops to the next level. Use it like a loop pedal to... | Read more »
Space Grunts guide - How to survive
Space Grunts is a fast-paced roguelike from popular iOS developer, Orange Pixel. While it taps into many of the typical roguelike sensibilities, you might still find yourself caught out by a few things. We delved further to find you some helpful... | Read more »
Dreii guide - How to play well with othe...
Dreii is a rather stylish and wonderful puzzle game that’s reminiscent of cooperative games like Journey. If that sounds immensely appealing, then you should immediately get cracking and give it a whirl. We can offer you some tips and tricks on... | Read more »
Kill the Plumber World guide - How to ou...
You already know how to hop around like Mario, but do you know how to defeat him? Those are your marching orders in Kill the Plumber, and it's not always as easy as it looks. Here are some tips to get you started. This is not a seasoned platform... | Read more »

Price Scanner via MacPrices.net

Apple refurbished 15-inch Retina MacBook Pros...
Apple has Certified Refurbished 2015 15″ Retina MacBook Pros available for up to $380 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2... Read more
BookBook For iPad Pro Coming Soon
The iPad Pro is a device unlike any other, and with Apple Pencil, it’s the ideal portable sketchpad: all that’s missing is the modern easel and portfolio to go. TwelveSouth’s BookBook for iPad Pro... Read more
12-inch 1.2GHz Silver Retina MacBook on sale...
B&H Photo has the 12″ 1.2GHz Silver Retina MacBook on sale for $1399 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price for this model from... Read more
iPads on sale at Target: $100 off iPad Air 2,...
Target has WiFi iPad Air 2s and iPad mini 4s on sale for up to $100 off MSRP on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for... Read more
Target offers Apple Watch for $100 off MSRP
Target has Apple Watches on sale for $100 for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary: - Apple... Read more
Apple refurbished 2014 13-inch Retina MacBook...
Apple has Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and shipping is free... Read more
Macs available for up to $300 off MSRP, $20 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more
Watch Super Bowl 50 Live On Your iPad For Fre...
Watch Super Bowl 50 LIVE on the CBS Sports app for iPad and Apple TV. Get the app and then tune in Sunday, February 7, 2016 at 6:30 PM ET to catch every moment of the big game. The CBS Sports app is... Read more
Two-thirds Of All Smart Watches Shipped In 20...
Apple dominated the smart watch market in 2015, accounting for over 12 million units and two-thirds of all shipments according to Canalys market research analysts’ estimates. Samsung returned to... Read more
12-inch 1.2GHz Retina MacBooks on sale for up...
B&H Photo has 12″ 1.2GHz Retina MacBooks on sale for $180 off MSRP. Shipping is free, and B&H charges NY tax only: - 12″ 1.2GHz Gray Retina MacBook: $1499 $100 off MSRP - 12″ 1.2GHz Silver... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Subject Matter Expert - Experis (Uni...
This position is for an Apple Subject Matter Expert to assist in developing the architecture, support and services for integration of Apple devices into the domain. Read more
*Apple* Macintosh OSX - Net2Source Inc. (Uni...
…: * Work Authorization : * Contact Number(Best time to reach you) : Skills : Apple Macintosh OSX Location : New York, New York. Duartion : 6+ Months The associate would Read more
Computer Operations Technician ll - *Apple*...
# Web Announcement** Apple Technical Liaison**The George Mason University, Information Technology Services (ITS), Technology Support Services, Desktop Support Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.