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cron🔗

allows users to schedule the execution.

Installation🔗

sudo apt update && sudo apt install cron
sudo systemctl enable cron

Cron is mostly installed in all the systems and consists of two things

  • cron or crond : daemon
  • crontab : command interface for interacting with daemon

Cron Syntax🔗

*    *    *    *    *   /home/user/bin/somecommand.sh
|    |    |    |    |            |
|    |    |    |    |    Command or Script to execute
|    |    |    |    |
|    |    |    | Day of week(0-6 | Sun-Sat)
|    |    |    |
|    |    |  Month(1-12)
|    |    |
|    |  Day of Month(1-31)
|    |
|   Hour(0-23)
|
Min(0-59)

*: matches all values, so above script runs every minute

Usage🔗

  • add crontab entry
crontab -e

NOTE: crontab entries do not source ~/.bashrc so always use full script path and environment variable. Either hardcode or source in the executing script

NOTE: Make sure your script is executible

  • cron logs

    tail -f /var/log/cron

Example crons entries🔗

  • Use https://crontab.guru to validate your cron

If you want to run four times a day between Monday and Friday, you can use the step operator ( / ) and the range operator ( - ).

0 */6 * * Mon-Fri /home/user/somejob.sh

Run Every Hour at Minute 15🔗

15 * * * * /path/to/script.sh

Run Every Day at 2 AM🔗

0 2 * * * /path/to/script.sh

Run Every Sunday at 3 PM🔗

0 15 * * 0 /path/to/script.sh

Run Every Month on the 1st at 4 AM🔗

0 4 1 * * /path/to/script.sh

Advanced Features🔗

Environment Variable🔗

SHELL=/bin/bash  
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
* * * * * /path/to/script.sh
* * * * * /path/to/script.sh >> /var/log/script.log 2>&1

Special Strings🔗

Use predefined schedules:

  • @reboot: Run at startup.
  • @daily: Run once a day.
  • @weekly: Run once a week.
  • @monthly: Run once a month.
  • @yearly: Run once a year.
@daily /path/to/script.sh

Pro Tips🔗

Use cron.d for System Jobs: Place system-wide cron jobs in /etc/cron.d/