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Featured Articles: Crontab
## An Introduction
## Eric De La Cruz Lugo <>

What??? It�s 5:00 PM! You have a date with someone very important at 5:15 PM! Before you go, you have several things to do; send e-mail to 20 people (telling them the same thing (Your cat had kittens!), transfer files from 6 (six) FTP sites, check when and where a user is connecting to your system, make a back up and compile that new device support in your machine's kernel and if that wasn't enough, your provider sent you an e-mail telling you that the connection will end in 45 minutes and will be back on line at 8:30 PM for routine maintenance of their servers. What you can do?

If you where a Windows user, then you would have to give up and wait until the next day, because after your date you'd probably want some sleep before connecting to the net again (after all you are only human.. aren't you?).

But you are not a Windows user, you are a FreeBSD user and that single fact make you capable to do things that a mere mortal (read Windows/DOS User) can�t do.

In FreeBSD (and other Unix-like OS as well) a cool command exists that lets you run instructions (like a shell script) at a given time. The name of this command is crontab.

What is it and how does it work?

The Cron daemon

As the FreeBSD System Manager's Manual says, cron is "a daemon that executes scheduled commands".

	"Cron searches /var/cron/tabs for crontab files which are 
	named after accounts in /etc/passwd; crontabs found are loaded
	into memory.  Cron also searches for /etc/crontab which is in 
	a different format.  Cron then wakes up every minute, 
	examining all stored crontabs, checking each command to see if
	it should be run in the current minute.  When executing 
	commands, any output is mailed to the owner of the crontab (or
	to the user named in the MAILTO environment variable in the
	crontab, if such exists)".

And here is where all begins, the key to the solution is the crontab file. This file has a special format, as we will see later, and it can be named after the user name running it.

A crontab file contains instructions to the cron daemon in the general form of "run this command at this time on this date". Each user, as we saw before, has their own crontab, and commands in any given crontab will be executed as the user who owns the crontab. There are also some restrictions using the crontab command.

If the "/var/cron/allow" file exists on the system, then you must be listed there in order to be allowed to use this command. If the allow file does not exist but the "/var/cron/deny" file does, then you must not be listed in that file in order to use this command. If neither of these files exists, then depending on site-dependent configuration parameters, only the super user will be allowed to use this command, or all users will be able to use it.

In this case, we assume that all users in your system are able to use the crontab command. The format of this file is shown below.

Each line has five time and date fields, the time and date fields are:

	field			allowed values
	-----			--------------
	minute			0-59
	hour			0-23
	day of month		1-31
	month			1-12 (or names, see below)
	day of week		0-7 (0 or 7 is Sun (Sunday), or use names)

A field may be an asterisk (*), which always stands for "first-last".

A crontab file example is given below, the # character is taken as a comment:

	# run a shell script named "daily.job" one minute after midnight,
	# every day -- output mailed to eric
	1 0 * * *       $HOME/bin/daily.job 
	# run another shell script named monthly at 1:25pm on the first of
	# every month -- output mailed to eric
	25 13 1 * *     $HOME/bin/monthly
	# run at 10 pm on Monday, send e-mail to my friends
	0 22 * * 1      $home/bin/mail.to.friends
	# Check at 9:05 AM every Monday what users are logged in the 
	# "itesocci.gdl.iteso.mx" machine, output mailed to eric.
	5 9 * * mon     finger @itesocci.gdl.iteso.mx 
	# Make a FTP session at 11:30 PM following the commands stored in the
	# "mp3" file keep in sync every day.
	30 23 * * * ftp -n -v < /home/maestros/ar/eric/bin/mp3

In the FTP session as shown above the file "mp3" contains ftp commands:

	iteso.mx:/home/maestros/ar/eric/bin% cat mp3  
	user ptagh share
	lcd /home/maestros/ar/eric/mp3
	cd unsorted  
	mget "*.mp3"

As you can see the username and password are readable, that�s why this "mp3" file most be readable only by the owner (use the apropiate parameters with the chmod command (check the manual: "man chmod").

	iteso.mx:/home/maestros/ar/eric/bin%chmod 600 mp3

The command above, makes the file "mp3" readable and writeable only for the owner.

In the e-mail, section as shown above, we have a shell script named "mail.to.friends", this shell script is shown below:

	mail -s "My cat have kitties!!" [email protected], 
	[email protected], [email protected], [email protected] <

In order to run this script from our crontab file we need to make it executable, we do this typing something like:

	iteso.mx:/home/maestros/ar/eric/bin%chmod a+x mail.to.friends

Now that we know how the cron daemond and the crontab command works, we can create a crontab file say "eric" (in your case put your account name or anyting you want) with your favorite editor (vi, pico, joe, emacs...etc). In this file we can put the example file shown above (of course we assume that you can make your own changes on this file in order to do what you really want), and after that, only type:

	iteso.mx:/home/maestros/ar/eric/bin%crontab eric

And that�s it! Now the cron daemond will check every minute for the time it will execute the commands, and the output will be mailed to (in this case) the user "eric".

If you want to stop a crontab just type:

	iteso.mx:/home/maestros/ar/eric/bin%crontab -r

I hope this article helps those who want to obtain the maximum of an OS like FreeBSD and other Unix like systems.

After all, automation is suposed to be one of the best features for a serious OS, isn�t it?

More references

I strongly suggest that you read the manuals for cron and crontab. You can also check:

Have a good time "croning" ;)


Eric De La Cruz Lugo, works for the "CONSTRUCTORA KEPLER S.A. DE C.V." a Construction company in Mexico. Right know he is working on a project in Merida, Yucatan, building a Power Plant. He is a loyal FreeBSD user and admin since 1993 at ITESO University (Guadalajara, Jalisco, Mexico). He can be reached at <> or <>.

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Last modified: $Date: 1999/06/26 05:42:50 $
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