Definition: A unix clock daemon that executes commands at specified dates and times according to instructions in a “cron table” (Also referred to as the Crontab file). On a typical unix system it is located at /usr/sbin/cron. On linux systems we use the “crontab” command which is located under /usr/bin/.
The cron command starts a process that regularly executes commands at specified dates and times. Regularly scheduled commands can be specified according to instructions found on the crontab files in the directory /var/spool/cron/crontabs. In Redhat distributions this crontab file is located in the “/etc/” directory.
Users can submit their own crontab file using the crontab(1) command.
The following format is used for scheduling the commands. This is stored in a file called the cron table.
minute hour day month weekday command
Each time component above can be specified as an integer number.
Minute is specified as a number between 00 and 59.
Hour is specified as a number between 00 and 23.
Day is specified as a number between 1 and 31.
Month is specified as a number between 1 and 12.
Weekday is specified as a number between 0 and 6 (0 being sunday).*
*( On linux systems the weekday is specified as sun, mon, tue....)
If you have installed a cgi script called report.cgi in your cgi-bin directory and wanted to run this program at 4:30 pm every sunday of December then you would type:
30 16 * 12 0 /home/user/domain.com/cgi-bin/report.cgi
Where “*” is wildcard character. This means that for a time component where * is the option entered, then it would execute at every value of that component provided it satisfies all other components' requirements.
To run the script at 6 AM and 6 PM everyday, the following format would be used:
00 06,18 * * * /home/user/domain.com/cgi-bin/report.cgi
(a comma is used to specify more than one value for a given time component.)
To run the above script from monday to friday only, the following format would be used.
00 06,18 * * 1-5 /home/user/domain.com/cgi-bin/report.cgi.
(or cron job as it is popularly referred to)
Cron jobs can be set up either by using the Cron tab manager or by uploading the cron tab file.
The cron tab manager (also called the Task Scheduler in Redhat 9.0) appears as shown below. It is a GUI interface with the various time component options displayed and one can configure the cron job as per their need.
If you have telnet or secure shell access to your system, you can setup the cron jobs by uploading the crontab file directly to the system using the following steps.
- Using notepad or any text editor, you would create your cron.txt file (or append an existing one).
- Upload the file to the root or relevant directory. (Contact your System Administrator for details)
- Install the txt file as a cron tab file by running the command 'crontab cron.txt'.
- Telnet or ssh into the system and and check the cron jobs running by typing 'crontab -l'.
- Always press the Return key after writing the cron job.
- Always use absolute path for the command line.
- Turn off the 'Word Wrap' feature in your text editor.
- Upload your file in ASCII mode.
- Cron emails you every time it runs. To stop it simply add this tag at the end of your crob job. '>/dev/null 2>&1'
- Taking one of the above examples:
00 06,18 * * 1-5 /home/user/domain.com/cgi-bin/report.cgi > /dev/null 2>&1
The crontab command is used to edit, list, create or remove a crontab file for a user.
Since unix/linux is a multi user environment, it is possible to have crontab files for multiple users.
Some crontab options are:
-e for editing the file.
-l for listing the contents of a crontab file.
-r option to remove the crontab file.
-v option to view the last time you edited the cron tab file.
-u option will tweak the crontab file for a particular user. (for superuser)
(Usage: crontab -u username cron.txt)
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