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Admin 02-18-2007 08:36 AM

Using Cron Jobs
 
What is Cron?

Cron is very simply a Linux module that allows you to run commands at predetermined times or intervals. In Windows, it’s called Scheduled Tasks. The name Cron is in fact derived from the same word from which we get the word chronology, which means order of time.

Using Cron, a developer can automate such tasks as mailing ezines that might be better sent during an off-hour, automatically updating stats, or the regeneration of static pages from dynamic sources. Systems administrators and Web hosts might want to generate quota reports on their clients, complete automatic credit card billing, or similar tasks. Cron has something for everyone!

There are two different modes you can use to add a cron job:

Standard - Provides a range of pre-set options that you can choose. This is the simplest method and is recommended.

Advanced (Unix Style) - Provides the regular Unix style cron options. Recommended for users who are used to this method of entering cron entries.

Warning: You need to have a good knowledge of Linux commands before you can use cron jobs effectively. Check your script with your hosting administrator before adding a cron job.

To add or modify a cron job:

Click on the Cron jobs button on the home page.

If you want to use the Standard mode, click on the Standard button.


Enter the e-mail address to send the results of running the cron job in the top field.

Enter the command the you want to run in the Command to run field.

Click on one option from each of the available lists.

Click on the Save Crontab button. Your cron job has now been added or updated.

Note: You can always click on the Reset Changes button to change the cron job back to whatever it was before you started entering information.


If you want to use the Advanced mode, click on the Advanced (Unix Style) button.


Enter the times for the cron job in the Minute, Hour, Day, Month, or Weekday fields. Refer to the following page to check exactly how to enter values in these fields - http://www.redhat.com/support/resour...cron/cron.html.

Enter the cron job script in the Command field.

Click on the Commit Changes button. Your cron job has now been added or updated.

Note: You can always click on the Reset Changes button to change the cron job back to whatever it was before you started entering information.

Admin 04-20-2007 11:28 AM

Cron

This file is an introduction to cron, it covers the basics of what cron does,
and how to use it.

What is cron?
Cron is the name of program that enables unix users to execute commands or scripts (groups of commands) automatically at a specified time/date. It is normally used for sys admin commands, like makewhatis, which builds a
search database for the man -k command, or for running a backup script,
but can be used for anything. A common use for it today is connecting to
the internet and downloading your email.

This file will look at Vixie Cron, a version of cron authored by Paul Vixie.

How to start Cron
Cron is a daemon, which means that it only needs to be started once, and will lay dormant until it is required. A Web server is a daemon, it stays dormant until it gets asked for a web page. The cron daemon, or crond, stays dormant until a time specified in one of the config files, or crontabs.
On most Linux distributions crond is automatically installed and entered into the start up scripts. To find out if it's running do the following:

cog@pingu $ ps aux | grep crond
root 311 0.0 0.7 1284 112 ? S Dec24 0:00 crond
cog 8606 4.0 2.6 1148 388 tty2 S 12:47 0:00 grep crond

The top line shows that crond is running, the bottom line is the search
we just run. If it's not running then either you killed it since the last time you rebooted,or it wasn't started.

To start it, just add the line crond to one of your start up scripts. The
process automatically goes into the back ground, so you don't have to force it with &. Cron will be started next time you reboot. To run it without rebooting, just type crond as root:

root@pingu # crond

With lots of daemons, (e.g. httpd and syslogd) they need to be restarted
after the config files have been changed so that the program has a chance to reload them. Vixie Cron will automatically reload the files after they have been edited with the crontab command. Some cron versions reload the files every minute, and some require restarting, but Vixie Cron just loads the files if they have changed.

Using cron
There are a few different ways to use cron (surprise, surprise).
In the /etc directory you will probably find some sub directories called
'cron.hourly', 'cron.daily', 'cron.weekly' and 'cron.monthly'. If you place
a script into one of those directories it will be run either hourly, daily,
weekly or monthly, depending on the name of the directory.

If you want more flexibility than this, you can edit a crontab (the name
for cron's config files). The main config file is normally /etc/crontab.
On a default RedHat install, the crontab will look something like this:

root@pingu # cat /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/
# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

The first part is almost self explanatory; it sets the variables for cron.
SHELL is the 'shell' cron runs under. If unspecified, it will default to
the entry in the /etc/passwd file.

PATH contains the directories which will be in the search path for cron
e.g if you've got a program 'foo' in the directory /usr/cog/bin, it might
be worth adding /usr/cog/bin to the path, as it will stop you having to use
the full path to 'foo' every time you want to call it.

MAILTO is who gets mailed the output of each command. If a command cron is running has output (e.g. status reports, or errors), cron will email the output to whoever is specified in this variable. If no one if specified, then the output will be mailed to the owner of the process that produced the output.

HOME is the home directory that is used for cron. If unspecified, it will
default to the entry in the /etc/passwd file.

Now for the more complicated second part of a crontab file.
An entry in cron is made up of a series of fields, much like the /etc/passwd file is, but in the crontab they are separated by a space. There are normally seven fields in one entry. The fields are:

minute hour dom month dow user cmd minute

This controls what minute of the hour the command will run on,
and is between '0' and '59' hour This controls what hour the command will run on, and is specified in the 24 hour clock, values must be between 0 and 23 (0 is midnight) dom

This is the Day of Month, that you want the command run on, e.g. to
run a command on the 19th of each month, the dom would be 19.
month This is the month a specified command will run on, it may be specified numerically (0-12), or as the name of the month (e.g. May)
dow This is the Day of Week that you want a command to be run on, it can also be numeric (0-7) or as the name of the day (e.g. sun).
user. This is the user who runs the command. cmd This is the command that you want run. This field may contain multiple words or spaces.
If you don't wish to specify a value for a field, just place a * in the
field.

e.g.
01 * * * * root echo "This command is run at one min past every hour"
17 8 * * * root echo "This command is run daily at 8:17 am"
17 20 * * * root echo "This command is run daily at 8:17 pm"
00 4 * * 0 root echo "This command is run at 4 am every Sunday"
* 4 * * Sun root echo "So is this"
42 4 1 * * root echo "This command is run 4:42 am every 1st of the month"
01 * 19 07 * root echo "This command is run hourly on the 19th of July"

Notes:

Under dow 0 and 7 are both Sunday.
If both the dom and dow are specified, the command will be executed when either of the events happen.
e.g.

* 12 16 * Mon root cmd

Will run cmd at midday every Monday and every 16th, and will produce the same result as both of these entries put together would:

* 12 16 * * root cmd
* 12 * * Mon root cmd

Vixie Cron also accepts lists in the fields. Lists can be in the form, 1,2,3
(meaning 1 and 2 and 3) or 1-3 (also meaning 1 and 2 and 3).
e.g.

59 11 * * 1,2,3,4,5 root backup.sh

Will run backup.sh at 11:59 Monday, Tuesday, Wednesday, Thursday and Friday,as will:

59 11 * * 1-5 root backup.sh

Cron also supports 'step' values.
A value of */2 in the dom field would mean the command runs every two days and likewise, */5 in the hours field would mean the command runs every 5 hours.

e.g.
* 12 10-16/2 * * root backup.sh
is the same as:
* 12 10,12,14,16 * * root backup.sh
*/15 9-17 * * * root connection.test

Will run connection.test every 15 mins between the hours or 9am and 5pm
Lists can also be combined with each other, or with steps:

* 12 1-15,17,20-25 * * root cmd

Will run cmd every midday between the 1st and the 15th as well as the 20th and 25th (inclusive) and also on the 17th of every month.

* 12 10-16/2 * * root backup.sh

is the same as:

* 12 10,12,14,16 * * root backup.sh

When using the names of weekdays or months, it isn't case sensitive, but only the first three letters should be used, e.g. Mon, sun or Mar, jul.
Comments are allowed in crontabs, but they must be preceded with a '#', and must be on a line by them self.

Multiuser cron

As Unix is a multiuser OS, some of the apps have to be able to support
multiple users, cron is one of these. Each user can have their own crontab
file, which can be created/edited/removed by the command crontab. This
command creates an individual crontab file and although this is a text file,
as the /etc/crontab is, it shouldn't be edited directly. The crontab file is
often stored in /var/spool/cron/crontabs/<user> (Unix/Slackware/*BSD),
/var/spool/cron/<user> (RedHat) or /var/cron/tabs/<user> (SuSE),
but might be kept elsewhere depending on what Un*x flavor you're running.

To edit (or create) your crontab file, use the command crontab -e, and this will load up the editor specified in the environment variables EDITOR or VISUAL, to change the editor invoked on Bourne-compliant shells, try:

cog@pingu $ export EDITOR=vi

On C shells:

cog@pingu $ setenv EDITOR vi

You can of course substitute vi for the text editor of your choice.
Your own personal crontab follows exactly the same format as the main
/etc/crontab file does, except that you need not specify the MAILTO
variable, as this entry defaults to the process owner, so you would be mailed the output anyway, but if you so wish, this variable can be specified.

You also need not have the user field in the crontab entries. e.g.

min hr dom month dow cmd

Once you have written your crontab file, and exited the editor, then it will
check the syntax of the file, and give you a chance to fix any errors.
If you want to write your crontab without using the crontab command, you can write it in a normal text file, using your editor of choice, and then use the crontab command to replace your current crontab with the file you just wrote.

e.g. if you wrote a crontab called cogs.cron.file, you would use the cmd
cog@pingu $ crontab cogs.cron.file

to replace your existing crontab with the one in cogs.cron.file.
You can use:

cog@pingu $ crontab -l

to list your current crontab, and

cog@pingu $ crontab -r

will remove (i.e. delete) your current crontab.

Privileged users can also change other user's crontab with:
root@pingu # crontab -u and then following it with either the name of a file to replace the existing user's crontab, or one of the -e, -l or -r options.

According to the documentation the crontab command can be confused by the su command, so if you running a su'ed shell, then it is recommended you use the -u option anyway.

Controlling Access to cron
Cron has a built in feature of allowing you to specify who may, and who
may not use it. It does this by the use of /etc/cron.allow and /etc/cron.deny files. These files work the same way as the allow/deny files for other daemons do. To stop a user using cron, just put their name in cron.deny, to allow a user put their name in the cron.allow. If you wanted to prevent all users from using cron, you could add the line ALL to the cron.deny file:

root@pingu # echo ALL >>/etc/cron.deny

If you want user cog to be able to use cron, you would add the line cog
to the cron.allow file:

root@pingu # echo cog >>/etc/cron.allow

If there is neither a cron.allow nor a cron.deny file, then the use of cron
is unrestricted (i.e. every user can use it). If you were to put the name of some users into the cron.allow file, without creating a cron.deny file, it
would have the same effect as creating a cron.deny file with ALL in it.

This means that any subsequent users that require cron access should be
put in to the cron.allow file.

Output from cron
As I've said before, the output from cron gets mailed to the owner of the
process, or the person specified in the MAILTO variable, but what if you
don't want that? If you want to mail the output to someone else, you can
just pipe the output to the command mail.
e.g.

cmd | mail -s "Subject of mail" user
If you wish to mail the output to someone not located on the machine, in the above example, substitute user for the email address of the person who wishes to receive the output.

If you have a command that is run often, and you don't want to be emailed the output every time, you can redirect the output to a log file (or /dev/null, if you really don't want the output).

e,g
cmd >> log.file

Notice we're using two > signs so that the output appends the log file and doesn't clobber previous output. The above example only redirects the standard output, not the standard error, if you want all output stored in the log file, this should do the trick:

cmd >> logfile 2>&1

You can then set up a cron job that mails you the contents of the file at
specified time intervals, using the cmd:
mail -s "logfile for cmd" <log.file

Now you should be able to use cron to automate things a bit more.
A future file going into more detail, explaining the differences between
the various different crons and with more worked examples, is planned.


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