Often times one may choose to run a command line process or script at a specific time and date. Two commands that one might consider for this purpose are At and Crontab respectively, with the latter being the more complex of the two.
At
At is a relatively straightforward affair, that allows one to execute one or more commands on a single date and time. Many systems come with At installed, but some, such as Raspbian, a flavour of Debian designed to run on the Raspberry Pi, do not. To begin enter a command as thus:
This will then put you into an At prompt. Enter a command at the prompt. When you are finished typing the first command hit enter. If you wish to type another command do so now. If not type control+d to finish. It will add a
In the example a new empty file named “file.txt” will be created in the /home/admin directory at 5:00 PM on Tuesday. There is also a way to use At without entering its command prompt. You can use At as a single line command by piping the output from an echo statement through it.
Crontab
Like the At command, Crontab allows one to dictate when a process or script is executed, but rather than simply running once, it runs repeatedly at a specified interval. This is particularly useful for maintenance tasks such as running backups, or monitoring tasks such as tracking system load averages.
When Crontab is invoked for the first time, a new crontab file is created and opened, for the current user, in the systems default text editor. Each line is separate scheduled event, with timing based on 6 values in order: Minute (0-59), Hour (0-23), Day of the Month (1-31), Month of the Year (1-12), day of the Week (1-7), and Year (1900+) respectively. In the case of Day of the Week it is important to note that the week in this case starts on Monday. To invoke crontab, use the following command:
To run a backup script once every Sunday at 2am one might ad a line as follows:
Both At and Crontab can be configured in many more complicated and useful ways. To learn more check out their Man pages.
