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Crontab
CRON is a very powerful tool in the *nix world. It is used to schedule jobs, tasks, or scrips. Any action performed on the command line can be executed via CRON.
Each user has their own crontab file which they can use to run their own scheduled tasks. The permissions for these tasks are identical to the users permissions at the command line. The system also has a series of cron related files and folders. These are /etc/crontab, /etc/cron.hourly, /etc/cron.daily /etc/cron.daily, /etc/cron.monthly. The items above with the '.' in the name are directories where scrips can be stored or linked to. The are run in the order that they appear in the directory. The directory contents are actionable by cron through the commands below which are found in /etc/crontab
# 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
/etc/crontab
The start of the file is shown blow and is there to pass parameters to the cron process before it starts to action on any of the line items. Each item is pretty self explanatory.
SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root HOME=/Issuing Commands
Each command line argument that cron encounters has to include when cron is to run the command line. Below is the format and syntax of the cron timetable.
.---------------- minute (0 - 59) | .------------- hour (0 - 23) | | .---------- day of month (1 - 31) | | | .------- month (1 - 12) OR jan,feb,mar,apr ... | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat | | | | | * * * * * command to be executedMultiple date/time values
There are several ways of specifying multiple date/time values in a field:
- The comma (',') operator specifies a list of values, for example: "1,3,4,7,8" (space inside the list must not be used)
- The dash ('-') operator specifies a range of values, for example: "1-6", which is equivalent to "1,2,3,4,5,6"
- The asterisk ('*') operator specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to 'every hour' (subject to matching other specified fields).
Example
Here is a simple example for rotating log files for Squint.
# squint squid reports # Daily at 3am 00 03 * * * root /usr/local/bin/squint.cron.sh daily # Weekly, on Mondays 00 01 * * 1 root /usr/local/bin/squint.cron.sh weekly # Monthly, on the first day of the month 00 02 1 * * root /usr/local/bin/squint.cron.sh monthlyNotice that root appears in the fifth field. This is a requirement of some implementations of cron.
More info
Issue the command man cron or search in Google, for more detailed information.
The most common version of cron was written by Paul Vixie. The most recent implementations should conform to IEEE Std1003.2-1992
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