cron is a time-based job scheduler in Unix-like computer operating systems, cron is used to schedule jobs at fixed times, dates, or intervals; cron runs every minute and detects if there is a new command or task (reading the files /etc/crontab, /etc/cron.d/* and/var/spool/cron/crontabs/*) to run.
crontab is the program that maintains: installs, uninstalls or lists the scheduled tasks, each user has his own crontab file in /var/spool/cron/crontabs/username which should not be edited directly else using the crontab utility. If the /etc/cron.allow exists, your user must appear in it to be able to schedule tasks, if it does not exist and /etc/cron.deny exists your user must not appear in it; if neither of them exist then it depends on the default OS configuration.
crontab is also the file that contains instructions for executing scheduled tasks. A line in the crontab file can set an environment variable (var-name=value) or schedule the execution of a command; blank lines and lines beginning with # (comments) are ignored. The format for scheduling a task is:
# (0-59) (0-23) (1-31) (1-12) (0 - 7)
min hour day-of-the-month month day-of-the-week command
- * = all posibles values
- value1-value2 defines a range of values
- value1,value3,value3 defines a list
- value1-value2/step defines a step inside the range
- 0 or 7 = Sunday in the day of the week
Ejemplos
Make a backup of my home DIR
Send my home DIR last changes to a remote server every 5 minutes between 9:00 a.m – 6:00 p.m
*/5 9-18 * * * rsync -avz $HOME remote-server:/mnt/backup
Read 16 Practical examples of the rsync command for sync behaviour.
Backup my DB
Make a backup to my databases every day and every hour.
0 */1 * * * mysqldump -u root -pclave --all-databases > $HOME/bds.sql
Send a reminder
Email a remainder on the 1st and 15th of each month at 2:15 p.m
15 14 1,15 * * mail -s 'Recordatorio' myemail@myemailserver.com < /dev/null
Backup my projects
Send my projects last changes to a remote server every 2 hours between 8:30 a.m – 5:30 p.m
30 8-17/2 * * 1-5 rsync -avz $HOME/prt servidor:/mnt/backup/prt
Read 16 Practical examples of the rsync command for sync behaviour.
All together
# These are enviroment variables # Use for command execution SHELL=/bin/bash # Cron will send notification to this address. MAILTO="Put your email here" */5 9-18 * * * rsync -avz $HOME remote-server:/mnt/backup 0 */1 * * * mysqldump -u root -pclave --all-databases > $HOME/bds.sql 15 14 1,15 * * mail -s 'Recordatorio' myemail@myemailserver.com < /dev/null 30 8-17/2 * * 1-5 rsync -avz $HOME/prt servidor:/mnt/backup/prt
To edit your crontab file execute this command
crontab -e
