cron - daemon to execute scheduled commands
Description:
Cron should be started from /etc/rc or /etc/rc.local.
Cron searches /var/spool/cron for crontab files which are named after accounts in /etc/passwd; crontabs found are loaded into memory. Cron also searches for /etc/crontab and the files in the /etc/cron.d/ directory, which are 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).
Additionally, cron checks each minute to see if its spool directory's modtime (or the modtime on /etc/crontab) has changed, and if it has, cron will then examine the modtime on all crontabs and reload those which have changed. Thus cron need not be restarted whenever a crontab file is modified.
Cron (a Linux process that performs background work, often at night) is set up by default on your RedHat system. So you don't have to do anything about it unless you would like to add some tasks to be performed on your system on a regular basis or change the time at which cron performs its duties.
Please note that some of the cron work might be essential for your system functioning properly over a long period of time. Among other things cron may:
- rebuild the database of files which is used when you search for files with the locate command,
- clean the /tmp directory,
- rebuild the manual pages,
- "rotate" the log files, i.e. discard the oldest log files, rename the intermediate logs, and create new logs,
- perform some other checkups, e.g. adding fonts that you recently copied to your system.
Therefore, it may not be the best idea to always switch your Linux machine off for the night--in such a case cron will never have a chance to do its job. If you do like switching off your computer for the night, you may want to adjust cron so it performs its duties at some other time.
To find out when cron wakes up to perform its duties, have a look at the file /etc/crontab, for example:
cat /etc/crontab
It may contain something like this:
# 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
You can see that there are four categories of cron jobs: performed hourly, daily, weekly and monthly. You can modify those or add your own category. Here is how it works.
The columns in the entries show: minute (0-59), hour (0-23), day of month (1-31), month of year (1-12), day of week (0-6--Sunday to Saturday). The "*" means "any valid value".
Thus, in the example quoted, the hourly jobs are performed every time the computer clock shows "and one minute", which happens every hour, at one minute past the hour. The daily jobs are performed every time the clock shows 2 minutes past 4 o'clock, which happens once a day. The weekly jobs are performed at 22 minutes past four o'clock in the morning on Sundays. The monthly jobs are performed 42 minutes past four o'clock on the first day of every month. The directory with the script file that contain the command(s) to be executed is shown as the last entry on each line.
If you wanted your jobs to be performed at noon instead of 4 in the morning, just change the 4s to 12s. Cron wakes up every minute and examines if the /etc/crontab has changed so there is no need to re-start anything after you make your changes.
If you wanted to add a job to your cron, place a script which runs your job (or a link to your script) in the directory /etc/cron.hourly or cron.daily or /etc/cron.weekly, or /etc/cron.monthly .
Here is an example of an entry in /etc/crontab which causes a job to be performed three times a week (Mon, Wed, Fri):
02 4 * * 1,3,5 root run-parts/etc/cron.weekly
