Cron is the name of program that enables unix users to execute commands or scripts (groups of commands) automatically at a specified time/date. It is normally used for sys admin commands, like makewhatis, which builds a search database for the man -k command, or for running a backup script, but can be used for anything. A common use for it today is connecting to the internet and downloading your email.
Cron is a daemon, which means that it only needs to be started once, and will lay dormant until it is required. A Web server is a daemon, it stays dormant until it gets asked for a web page. The cron daemon, or crond, stays dormant until a time specified in one of the config files, or crontabs.
crontab -e |
Edit your crontab file, or create one if it doesn't already exist. |
crontab -l |
Display your crontab file. |
crontab -r |
Remove your crontab file. |
minute(s) hour(s) day(s) month(s) weekday(s) command(s)
| Category | Value | Description |
|---|---|---|
| minute | 0-59 | This controls what minute of the hour the command will run on. |
| hour | 0-23 | This controls what hour the command will run on. |
| day | 1-31 | This is the Day of Month, that you want the command run on. |
| month | 1-12 | This is the month a specified command will run on. |
| weekday | 0-6 | This is the Day of Week that you want a command to be run on. |
| user | root | This is the user who runs the command. |
| command | /cmd | A valid shell syntax code, executables or combinations of both |
cron every minute, hour, day, month, weekday then please include [*] in all value field. know more
Some of most used crontab syntax mentioned below,