Explanation: - Login as a root user
- cat >schedule.txt
30 05 * * * /bin/echo "Hello"
- crontab -u jeff schedule.txt
- service crond restart
The cron system is essentially a smart alarm clock. When the alarm sounds, Linux runs the commands of your choice automatically. You can set the alarm clock to run at all sorts of regular time intervals. Alternatively, the system allows you to run the command of your choice once, at a specified time in the future.
Red Hat configured the cron daemon, crond. By default, it checks a series of directories for jobs to run, every minute of every hour of every day. The crond checks the /var/spool/cron directory for jobs by user. It also checks for scheduled jobs for the computer under /etc/crontab and in the /etc/cron.d directory.
Here is the format of a line in crontab. Each of these columns is explained in more detail:
#minute, hour, day of month, month, day of week, command
* * * * * command
Entries in a crontab Command Line
Field Value
Minute 0-59
Hour Based on a 24-hour clock; for example, 23 = 11 p.m.
Day of month 1-31
Month 1-12, or jan, feb, mar, etc.
Day of week 0-7; where 0 and 7 are both Sunday; or sun, mon, tue, etc.
Command: The command you want to run
