Using cron
When you're on a Unix/Linux server and you want server software to launch at certain times and dates, you use cron. Programs can be launched as often as every minute, as seldom as once every year, or any recurring period between those two extremes.
cron is a computer daemon, which is a program that sits idly in the background until it is invoked or some condition occurs that causes it to perform its task. The cron program is a Unix/Linux clock daemon that executes commands at specified dates and times according to instructions in a crontab file.
I will show you how to make a crontab file.
A crontab file is a list of commands, one per line, that are to be executed at a specified time. The date and time come first in the crontab line, then the command to be executed.
Whether you set up cron via a server web page interface, cPanel for example, or set up cron via telnet/SSH with your server's command line, you'll need to know how to create the date/time and the command.
How To Create the Date/Time
The date/time part of the crontab line has six segments, a space between each segment. Every segment contains a number and/or one or more symbols.
The first segment represents the minute, the second represents the hour, then the day of the month, the month itself, and the day of the week.
At the exact moment all segments are true, that's when the command runs (the command that follows the date/time).
A number is literal. An asterisk character is a wildcard. To cause a command to execute every hour, every day, every month, any day of the week, always at 3 minutes after the hour, use this date/time:
3 * * * *
To have it execute only on Mondays at 3 minutes after the hour, use this date/time:
3 * * * 1
Days of the week are represented by the numbers 0 through 6, with 0 being Sunday and 6 being Saturday.
Months are represented by the numbers 1 through 12, with 1 being January.
Minutes are represented by numbers 0 through 59, hours by numbers 0 through 23, and days of the month by numbers 1 through 31.
Another symbol that can be used is the slash character "/" to represent a division.
To cause a command to execute whenever the hour is evenly divisible by 4, every day, every month, any day of the week, always at 3 minutes after the hour, use this date/time:
3 */4 * * *
The above date/time will cause a command to execute at 0:03, 4:03, 8:03, 12:03, 16:03, and 20:03, every day, month, and day of the week.
To cause a command to execute every minute whenever the day is Friday, the 13th, use this date/time:
* * 13 * 5
Another symbol that can be used is the hyphen character "-" to represent a range.
To cause a command to execute every minute whenever the day is Friday, the 13th, and when the months is anywhere from February through July, inclusive, use this date/time:
* * 13 2-7 5
Another symbol that can be used is the comma character "," to separate a list of numbers.
To cause a command to execute every minute whenever the day is Friday, the 13th, and when the month is February, October, or November, use this date/time:
* * 13 2,10,11 5
Remember that the only spaces in the date/time are the ones that separate the segments. The segments themselves may contain no spaces.
With the above, you can make pretty much any schedule you wish.
Here is an example that I'll let you puzzle out:
13,18 * 5,8,10-15,25,31 * 0
(The answer is near the end of this article.)
How To Create the Command
On the line in the crontab file, following the five date/time segments, is a space followed by the command to execute. Unlike the date/time segments, the command may contain spaces within itself.
The command is something the Unix/Linux operating system would understand if it was typed directly via the operating system command line. (That's why the command on the crontab file line is called a "command," because it's what would otherwise be typed on the command line.)
To invoke a system command, the command following the date/time would be the command name.
To invoke a script, the command would be the location of the interpreter, then a space, and then the location of the script.
If the perl interpreter is at /usr/bin/perl, and the perl script is located at /home/public_html/cgi-bin/script.cgi, the command would be constructed like this:
/usr/bin/perl /home/public_html/cgi-bin/script.cgi
To run script.cgi every 15 minutes, the entire crontab line would be:
*/15 * * * * /usr/bin/perl /home/public_html/cgi-bin/script.cgi
(The above most be all one line in the crontab file. This is mentioned in case it wraps in the email you're reading.)
Creating a Crontab File with cPanel
With cPanel, and with some other server web page interface control panels, setting up cron is a matter of clicking some links until the right page is found.
On that page is a form with six fields for each schedule, the first five fields being for the date/time segments and the last field being for the command.
Fill in the date/time and command according to what you learned above.
Some server web page interface control panels provide an example of running a CGI file with a command that starts with the word "GET". This should be okay in many instances.
For commands that launch scripts for sending an email to a list, or other scripts that require some time to run, a GET command might trigger a timeout if the script takes too long, just like a browser might time out.
It is okay to use
GET http://example.com/cgi-bin/script.cgi
in most instances when the script will run for less than a minute or two. Otherwise, use the "interpreter location followed by script location" method of specifying the command.
With cPanel, clicking the submit button creates the crontab file.
Creating a Crontab File On the Server Command Line
Put a list of commands and their schedules in a plain text file. Upload the file to your server.
The file can be named anything that makes sense to you. For illustration purposes, however, let's assume the file is named "mycron.txt"
Once mycron.txt has been uploaded, telnet or SSH to your server.
At your server's command line, type
crontab mycron.txt
This will create the crontab file, a specially formatted file generated from mycron.txt
If you want to verify that the crontab file indeed contains your cron schedules, type
crontab -l
(the letter el, not the number 1). That will display a list of the current cron schedules in the crontab file.
Now you know more about how to set up cron than most people do.
Oh, yes, here is the answer to the puzzle presented earlier in the article:
Whenever the minute matches 13 or 18,
and
any hour,
and
whenever the day of the month is
and
any month,
and
whenever the day of the week is a Sunday.
In other words, whenever Sunday falls on any of the days listed, the program will run at 13 and at 18 minutes after every hour.
Will Bontrager
