Cron для raspberry pi

Cron для raspberry pi

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Scheduling tasks with Cron

Cron is a tool for configuring scheduled tasks on Unix systems. It is used to schedule commands or scripts to run periodically and at fixed intervals. Tasks range from backing up the user’s home folders every day at midnight, to logging CPU information every hour.

The command crontab (cron table) is used to edit the list of scheduled tasks in operation, and is done on a per-user basis; each user (including root ) has their own crontab .

A graphical application for Cron is available by installing the gnome-schedule package:

You can then launch the program Scheduled Tasks from the main menu.

Run crontab with the -e flag to edit the cron table:

Select an editor

The first time you run crontab you’ll be prompted to select an editor; if you are not sure which one to use, choose nano by pressing Enter .

Add a scheduled task

The layout for a cron entry is made up of six components: minute, hour, day of month, month of year, day of week, and the command to be executed.

This cron entry would run the backup.sh script every day at midnight.

View scheduled tasks

View your currently saved scheduled tasks with:

Run a task on reboot

To run a command every time the Raspberry Pi starts up, write @reboot instead of the time and date. For example:

This will run your Python script every time the Raspberry Pi reboots. If you want your command to be run in the background while the Raspberry Pi continues starting up, add a space and & at the end of the line, like this:

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Setting Up A Cron Job On The Raspberry Pi

Cron jobs are an easy way to schedule a script or program to run at specific dates and times in regular recurring cycles. These can be used for server maintenance tasks and scheduled backups, right out to real world applications for things like turning on or off lights and other automation. In this tutorial we will go through the steps of setting up a cron job on a Raspberry Pi along with some of the basic configuration options.

There are a lot of different operating systems out there for the Raspberry Pi, so we are going to focus on the most popular: Raspbian. We are using the version dated: 2019-7-10 (Pi 4 Compatible) available from the Raspberry Pi Foundations’ Download Page. You don’t need to use the Raspberry Pi 4, any Raspberry Pi will do. However, deviating from the recommended operating system version may result in different / additional steps so if you are very new to this, we recommend following the tutorial exactly.

Requirements:

This tutorial requires several items:

  • A Raspberry Pi
  • 1 x microSD card with Raspbian Buster installed (2019-7-10)
  • 1 x Power Supply for Pi4 or Pi 3B+ & Earlier
  • A USB Keyboard & Mouse
  • A HDMI compatible monitor
  • Internet access

Step 1 — Get Everything Prepared

In this short tutorial, we are going to get started with the assumption that you have already set up your Raspberry Pi, installed Raspbian, and all of the basic configuration on first boot has been done.

Step 2 — How It Works

Cron is a time based scheduler found in “Unix-like” operating systems (such as Raspbian). Just like setting up a notification or recurring notification in a calendar or scheduling app for day to day appointments, Cron allows you to schedule scripts and programs. This can be as simple as running something every 5 minutes, every hour of every day, to precise things like specific times on specific days only in specific months. Overall, a very useful tool to have!

Step 3 — How To Create And Edit Cron Jobs

Cron is a part of the Raspbian operating system and isn’t terribly difficult to work with. In older versions of Raspbian we had gnome-scheduler, a graphical editor for Cron Jobs. However, this hasn’t been pulled into Raspbian Buster at the time of writing this tutorial. To keep this compatible with the Raspberry Pi 4, we will go through how to set it up the old fashioned way! Start by firing up terminal and run the crontab command with the -e flag to edit the table of jobs:

You will be prompted to select an editor – we recommend using “nano”. Select it by entering the corresponding number, and hit enter to continue.

Step 4 – Creating A Scheduled Task

The editor should have now opened – so we can create our first scheduled task. Tasks are scheduled using a specifically formatted line containing six components. From left to right these are:

minute / hour / day of month / month / day of week / command to execute

Example:

This group of different parameters allows a lot of control as to when your code is executed. Each task is added to a new line in the cron table with these 6 components.

In the next steps we will go through each of these components before assembling what we have learned to create a few different example schedules.

Step 5 — Minutes

Since minutes are the first component of a schedule we will go into depth on this one. Minutes can be entered as an asterisk (representing any/all), a number (between 0-59), a comma separated list of numbers, a range of numbers, step values, or a combination of these commands.

  • Inputting an “ * “ means “any value” and will run the code any minute – meaning your script would run every minute of the hour.
  • Inputting a number between 0 and 59 will run the script at a specific minute. An entry of 5 would mean your script runs 5 minutes into an hour.
  • Inputting a comma separated list of numbers like: 5,15,47 would result in your script running at minute 5, 15, and 47 of the hour.
  • A range of minutes can also be entered using a dash. 5-47 would result in your script running every minute between minute 5 and 47 of an hour.
  • A step value can also be set up. Using */5 would run your script every 5 minutes of an hour (the equivalent of entering 5,10,15,20,25,30,35,40,45,50,55)

Step 6 – Hours, Days, Months, Days of Week

Hours are the second component of the schedule. Just like minutes, hours can be entered as an asterisk, a number (between 0-23), a comma separated list, a range of numbers, a step value, or a combination of the above. We wont go into the same level of detail as above in this example as the same principles apply to hours, days, months, and day of week as they do to minutes.

The third component is Days of a Month. This component accepts numbers between 1 and 31. In months where there are less than 31 days, numbers above the number of days are ignored. Days of a month can be used to run programs on odd or even days (like our irrigation system) using a step value within a range value. Example: Odd Days would be entered as 1-31/2 (every second day of the month starting with 1) while even days is entered as 2-30/2 (every second day of the month starting with day 2)

The forth component is Months. Just like minutes and hours, months can be entered as an asterisk, a number (between 1-12), a comma separated list, a range of numbers, a step value, or a combination of the above.

The fifth component is Day of the Week. This is a bit different as it allows you to run a daily script on a specific day of the week rather than a the day of a month. If you wanted to run something on Mondays or Sundays regardless of what number the day is, this would be where you enter it. Just like every previous component, days of the week can be entered as an asterisk, a number (between 0-6), a comma separated list, a range of numbers, a step value, or a combination of the above. The week starts on Sunday (0) and goes to Saturday (6). Entering 7 will work, however it is non-standard. It is the same as entering 0 and represents Sunday.

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Cron для raspberry pi

Copy raw contents

Copy raw contents

Scheduling tasks with Cron

Cron is a tool for configuring scheduled tasks on Unix systems. It is used to schedule commands or scripts to run periodically and at fixed intervals. Tasks range from backing up the user’s home folders every day at midnight, to logging CPU information every hour.

The command crontab (cron table) is used to edit the list of scheduled tasks in operation, and is done on a per-user basis; each user (including root ) has their own crontab .

A graphical application for Cron is available by installing the gnome-schedule package:

You can then launch the program Scheduled Tasks from the main menu.

Run crontab with the -e flag to edit the cron table:

Select an editor

The first time you run crontab you’ll be prompted to select an editor; if you are not sure which one to use, choose nano by pressing Enter .

Add a scheduled task

The layout for a cron entry is made up of six components: minute, hour, day of month, month of year, day of week, and the command to be executed.

This cron entry would run the backup.sh script every day at midnight.

View scheduled tasks

View your currently saved scheduled tasks with:

Run a task on reboot

To run a command every time the Raspberry Pi starts up, write @reboot instead of the time and date. For example:

This will run your Python script every time the Raspberry Pi reboots. If you want your command to be run in the background while the Raspberry Pi continues starting up, add a space and & at the end of the line, like this:

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