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.