⚙️ Introduction to Cron

What is Cron? Cron is a command line program/utility that is used to schedule jobs. Along with this Cron is also a job scheduler/task scheduler. It helps to automate the process of executing tasks repeatedly. The jobs are the tasks which are other programs or shell commands.

For example, if a user wants to automate the process of updating his/her laptop with apt update && apt upgrade, they can do it via cron. Cron is very flexible the tasks could be scheduled in terms of minutes, hours, days, weeks, months and years. For example, update the laptop every day on 12:00.

Cron is most suitable for scheduling repetitive tasks.

Cron’s name originates from Chronos, the Greek word for time.

🤔 How to use it?

Cron works via a configuration file called as Crontab (Cron Table). This configuration file specifies the shell commands that needs to be scheduled.

Each line of a Crontab file represents a job. Here is an example of config file from Wikipedia.

# * * * * * <command to execute>
# | | | | |
# | | | | day of the week (0–6) (Sunday to Saturday;
# | | | month (1–12)             7 is also Sunday on some systems)
# | | day of the month (1–31)
# | hour (0–23)
# minute (0–59)

• * - Represents minute (0 - 59)

• * - Represents hour (0 - 23)

• * - Represents day of the month (1 - 31)

• * - Represents month (1 - 12)

• * - Represents the day of the week (0 - 6), Sunday to Saturday

The syntax for cron-expression excepts five fields, which describe the time and date and followed by the shell command.

For example, assuming that default shell for the user is Bash, this below cron-expression will execute the command echo at 12:10 every day.

10 12 * * * echo "Terminal"

A much complex example would be something like this,

10 12 2 2 * cal

The above cron-expression will execute the command cal, after every second month, second day of the month, at *12:10*.

The below command is used to edit the user configuration file regardless of where the file is stored.

crontab -e

🚀 Conclusion

This is an introductory blog for Cron. Please visit Wikipedia or run man crontab on command-line for more info.