How to Setup Crontab on Linux.

The crontab is used for running specific tasks on a regular interval. Linux crontab is similar to windows task schedules.

Crontab is very useful for routine tasks like scheduling system scanning, daily backups, etc. Crontab executes jobs automatically in the back-end at a specified time and interval.

Crontab

To Add/Edit Crontab.

crontab -e

To edit other user crontab.

crontab -u username -e

To List Crontab.

crontab -l

To List crontab by using username.

crontab -u username -l

Set Time,Month & Days in Crontab.

  • Minute – A minute value can be between 0-59
  • Hour – A hour value can be between 0-23.
  • Day_of_the_month – This value can between 1-31.
  • Month_of_the_year – This can be between 1-12.We can also define month of the year like jan, feb, mar, apr etc.
  • Day_of_the_Week – This can be the value between 0-7. We can also define day of the week like, sun, mon, tue, wed, etc.

Crontab Syntax

*(minute) *(hour) *(month's day) *(year) *(week'day) /path/of/the/script.sh/

Examples

  • Create a Script for execute using crontab.
vim example.sh
  • Schedule a cron to execute at 5am daily.
0 5 * * * /example.sh
  • Schedule a cron to execute twice a day.If we needs script file will execute at 4 AM and 4 PM daily.
0 4,16 * * * /example.sh

  • Schedule a cron to execute on every minutes.

* * * * *  /example.sh
  • Schedule a cron to execute on every Monday at 9 AM.
0 9 * * mon  /example.sh
  • Schedule a cron to execute on every 20 minutes.
*/20 * * * * /example.sh
  • Schedule a cron to execute on march.
* * * mar *  /example.sh
  • Schedule a cron to execute on selected months.
* * * jan,march,sept,dec *  /example.sh
  • Schedule a cron to execute on Sunday at 8AM.
0 8 * * sun /example.sh

  • Schedule a cron to execute on selected days at 10 AM.

0 10 * * wed,fri  /example.sh
  • Schedule a cron to execute on first monday of every month at 8 AM.
0 8 * * mon  [ $(date +%d) -le 07 ] && /example.sh
  • Schedule a cron to execute on every 8 hour.
0 */8 * * * /example.sh

  • Schedule a multiple tasks in single cron.

* * * * * /example.sh; /example1.sh
  • Schedule tasks to execute on monthly.
@monthly /example.sh

 

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