crontab, an abbreviation of chronic table (periodical time table), is a task scheduler which schedules tasks in period of time on UNIX systems. Each user in UNIX systems have its own "crontab".

By executing crontab -e, you can open the crontab of the user you logged in. The following figure shows the contents of my crontab.

As you can see, I'm using crontab to automate the updates of home brew, npm and arcanist (Arcanist is a CLI code review tool as a part of the code review, project planning and hosting suite: Phabricator).

The left side sequences of numbers like 0 0 * * * means "00:00 AM each day", and sets the execution time of the right side command. You can have a user-friendly tuning interface for this sequence of numbers at crontab.guru.

The right side commands only supports several environment variables. You can imagine a shell environment without any *rc file loaded, that is what a crontab command gets.

Since a command executed by crontab gets such kind of shell environment, even the PATH environment variable is not set yet, and you have to write down the full path of a command.

A command executed by crontab can be a CLI command comes with the UNXI system or a user script programmed by yourself. If you are executing a system command which requires root privilege, you have to ensure the command you are executing is listed in your sudoers file. If you are executing a shell script in a crontab, you have to use chmod +x your_script_name to make your script executable.

Saving your crontab file is different on different editors. In VIM, you can save with :w⏎。 If you are using macOS, you may get a following dialog after saving your crontab file and you have to click "OK" to allow the system accept your editing.

Each time a command executed by crontab, you would get a mail. You can check this mail in command line. You can use mail command to read these mails.

Then each day you come to your desktop, just open your command line terminal, and type mail, then you can know which CLI softwares were updated. As you can see, the npm on my computer was updated today.

By default, each command executed by crontab sends a mail to the user who sets the command up. But you can change the destination address by adding the following line on the top of the crontab file (open crontab file by typing crontab -e).

You can also disable the mail notifications by setting MAILTO to "", which like the following code shows:

By executing man crontab, you can see details of the usages of crontab.

My crontab scripts for macOS: WeZZard/com.wezzard.crontab.macos

My crontab scripts for developing: WeZZard/com.wezzard.crontab.dev