The asterisks ("*") in our entry tell cron that for that unit of time, the job should be run "every".
We need to add a line to the bottom of the file which looks like this: In vi or vim, you can add this line by typing G to go to the end of the file, and o to add a new line and enter insert mode.If MAILTO is defined but empty ('MAILTO=""'), no mail will be sent.Otherwise mail is sent to the owner of the crontab.Another option is using PAM (pluggable authentication module) authentication to set up users who may or may not use crontab and system cron jobs, as configured in /etc/cron.d/.The temporary directory for cron jobs can be set in environment variables (see below); if not, /tmp is used as the temporary directory.cron jobs can be allowed or disallowed for individual users, as specified in the files cron.allow and cron.deny, located in the directory /etc.
If the cron.allow file exists, a user must be listed there in order to be allowed to use a given command.
#· # Each task to run has to be defined through a single line # indicating with different fields when the task will be run # and what command to run for the task #· # To define the time you can provide concrete values for # minute (m), hour (h), day of month (dom), month (mon), # and day of week (dow) or use '*' in these fields (for 'any').#· # Notice that tasks will be started based on the cron's system # daemon's notion of time and timezones. Above, you can see that the last comment line is there to remind you how to format your entry.
#· # Output of the crontab jobs (including errors) is sent through # email to the user the crontab file belongs to (unless redirected). The format is very simple: six pieces of information, each separated by a space; the first five pieces of information tell cron when to run the job, and the last piece of information tells cron what the job is.
cron is the system process which will automatically perform tasks for you according to a set schedule.
The schedule is called the crontab, which is also the name of the program used to edit that schedule.
Let's say you have a script which backs up important files, or creates a report about system statistics, for example.