Use crontab to Automate Updates of CLI Softwares

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".

Editing the crontab File

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

Editor of crontab
Editor of 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 the crontab File

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.

macOS Alert
macOS Alert

Notification Mails

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.

crontab mails
crontab 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.

crontab Mails Contents
crontab Mails Contents

Changing Notification Mails Destination

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).

crontab

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

crontab
MAILTO=""

Reading the Manual of crontab

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

Manual of crontab
Manual of crontab

Some Resources

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

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



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