Scheduling tasks using Cron

Overview

• In this lesson we are going to learn how to automate routine tasks on the Raspberry Pi.
• This is helpful is ever you need to back up files, upload content to a web server or sample data from a sensor.
• Cron is a program that was designed to schedule routine tasks (commands, scripts or programs) on Linux systems, such as the Raspberry Pi.
• The command crontab is used to edit the list of scheduled tasks.
• Each user on the computer has their own cron table, so you can think of cron as your own personal assistant.
• In this lesson we will automate the running of the atm_sensor_get.py python program using cron.

Learning Objectives

• Running python programs from the Terminal
• Learn how to find the directory path to applications and programs on your computer
• Learn how to use the text editing software nano
• Learn how to create scheduled tasks in cron

Running python programs from the Terminal

• In this example we are going to run the atm_sensor_get.py program using the Terminal.
• Up to now, we have only run python programs from within the Python3 IDLE3 environment.
• To run our atm_sensor_get.py program we will use the command python3.

Open the Terminal and navigate to your project’s directory

• From the Raspberry Pi top bar menu click on the Terminal icon.
• Once in your default home directory (/home/pi) enter ls to list all contents.
• Find your project’s directory (e.g. botanica-park-lake) and use the change directory (cd) command to enter the directory (e.g. cd botanica-park-lake)
• Enter the command ls to list all directory contents.

• You should see the python file atm_sensor_get.py
• To run this file, enter the command python3 atm_sensor_get.py
  • python3 – name of application
  • atm_sensor_get.py – script or program to run
• The printouts we can see in the Terminal are the outputs from print() statements within our python code.

Finding the directory path to applications and python programs

• In this example we will find the path of the python3 application and scratch on the Raspberry Pi.
• A path shows the location where a software application can be found within the directory structure on your computer.
• To find the path for an application we use the Terminal command which followed by the name of the software application.

Open the Terminal and navigate to your project’s directory

• From the Raspberry Pi top bar menu click on the Terminal icon.
• Once in your default home directory (/home/pi) enter ls to list all contents.
• Find your project’s directory (e.g. botanica-park-lake) and use the change directory (cd) command to enter the directory (e.g. cd botanica-park-lake)
• Enter the command ls to list all directory contents.

• To find path to a current directory enter the command pwd (present working directory).
• This is helpful when we need to know the path to a program we have written.
• In this case, the path to the botanical-park-lake directory is /home/pi/botanica-park-lake
• The full path to the atm_sensor_get.py program is /home/pi/botanica-park-lake/atm_sensor_get.py

• We can find the path to other commonly used Linux commands.
• For example, the command ls is also an application. To find its path enter which ls
• The directory path to the ls application is found in the /bin directory (short for binary files)

• Try this with other commands such as cat, cd, touch and pwd.
• Do you see a pattern?
• You can find other commands here: Linux commands - ["Linux Commands"]
• You can also find the path to other programming applications, such as Scratch
• Enter the command which scratch

Finding an application or file within a directory

• Sometimes when we use the Terminal command which to get the path for an application, we also want to navigate to the directory to see the application within the directory.
• To navigate to the /usr/bin directory enter the command cd /usr/bin

• To list all the files and directories enter the command ls
• Unfortunately, in this directory there are more than 1000 files (1335 files to be exact).
• You can count the number of files with the command ls -1 | wc
  • ls -1 - this command lists all files and directories in one column (number one -1, not the letter L)
  • | - this is the pipe symbol (above the Enter key – is a vertical line). It takes the output from one command and makes it the input for another command.
  • wc - this command is short for Word Count

Narrowing down file search results using the Terminal command grep

• We can narrow our search down a little using another command called grep
• In the same directory enter the command ls -1 | grep python3
• This command will highlight only those entries that have the file name python3
  • ls -1 - list all files as a single column (-1 option)
  • | - pipe symbol
  • grep python3 - search for file names containing the text “python3”

• Using the | pipe symbol we can also do a word count at the very end to see how many python3 files there were with the command ls -1 | grep python3 | wc
• As you can see, the | pipe symbol is very handy.

Using the terminal text editor nano

• Nano is a text file editor that is used for creating and modifying text-based files.
• It is a simple editor and is widely used on Linux platforms.
• We need to learn how to use nano so that we can write scheduling tasks.
• In this example we will use nano to create a new file, add some text to the file and then save the file.

Create a new file using nano

• Navigate to your project directory using the Terminal command 'cd
• To create an empty file named test1.txt that can be edited in nano enter the command nano test1.txt

• This will open the nano test editor to an empty document.

• Enter text using the keyboard. You won’t be able to use the mouse.
• To navigate simply use the keyboard arrow keys.

Saving files in nano

• When you have finished editing your document enter CTRL+X keys to Exit.
• You will be prompted to Save your file.
• Enter y for yes and press Enter. Lowercase y is fine.

• You will be asked if the file is to be saved as test1.txt
• Accept the name test1.txt and press Enter.

• When the file is saved you will be returned to the main Terminal window.

Learn how to create scheduled tasks in cron

• Cron is a tool to configure scheduled tasks in Linux.
• In this example we will use the command crontab -e to create one scheduled task.
• We will configure cron to run our python program atm_sensor_get.py once every hour so that we can start to collect atmospheric data.

Running cron for the first time

• To run Cron enter the Terminal command crontab -e
• The first time you run crontab you will need to select an editor.
• Choose the relevant number to select nano and press Enter.
• Your selection options may different to those shown below.

• When the Cron table opens you should see the following comments (#) in the first few lines of the file.
• Comments will not be executed. If required, they can be deleted to make the cron table file less cluttered.

Setting when to run cron task

• The makeup for a cron entry is made up of 6 components:
  • minute (0-59)
  • hour (0-23)
  • day of month (1-31)
  • month of year (1-12)
  • day of week (0-6 – Sunday to Saturday)
  • command to be executed

• If a number is replaced with an * (asterix), then that rule applies for every unit
• Here are some examples related to the set time for a task to run:
  • 10 2 * * * – 2:10am every day
  • 00 15 2 * * - 3pm on the 2nd of every month
  • 05 10 * * * - 10:05am every day
  • 03 05 * * * - 5:03am every day
  • 00 15 * * 5 – Every Thursday at 3:00pm
  • 05 * * * * - 5 minutes past the hour, every hour

• We will run our program at 5 minutes past the hour, every hour so the time rule is 05 * * * *

Entering the command for a cron task

• We would like to run our atm_sensor_get.py file using Python3.
• The full command would therefore be python3 /home/pi/botanica-park-lake/atm_sensor_get.py
• The full path needs to be given for our python program atm_sensor_get.py
• We could also give the full path to the python3 application.
• In this case the full command line would be /usr/bin/python3 /home/pi/botanica-park-lake/atm_sensor_get.py

11. Creating a complete cron entry

Now that we have all the details we need for our cron entry we can enter it in one line Scroll using the arrow keys to the last entry in the Cron table Enter the following line: 5 * * * * python3 /home/pi/botanica-park-lake/atm_sensor_get.py