I’ve been using cron to execute a Python script used by my Raspberry Pi automatic cat feeder for some time now. After coming up with another script that needs to run on a regular schedule I added a second cronjob entry into the crontab.

That’s easy enough: first, in a terminal open the crontab to reveal existing jobs:

sudo nano crontab -e

Add the cronjob to the crontab by creating a new entry:

* * * * * /home/pi/Documents/Development/CatFeeder.py
* * * * * /home/pi/Documents/Development/Maintenance/Monitoring.py

The CatFeeder.py script has been running without a hitch, while the Monitoring.py script is the one being added.

After saving the file (this is nano, so CTRL+X, Y, Enter to save and exit) I expected the job to run every 60 seconds. However, nothing seemed to be happening. The new script never successfully executed. Meanwhile, the CatFeeder.py script was running flawlessly.

The script is an executable, I made sure of that by running chmod +x Monitoring.py, so that’s not the issue. I double-checked that the script ran without errors by invoking it manually: sudo python Monitoring.py. To no surprise the script ran as expected.

If the script runs fine, and a cronjob I already have in the crontab runs fine, then what gives?

SheBang #!: Specifying the Language Interpreter

There’s a very interesting line of code, or a comment rather, that I’ve noticed floating around in most of the Python scripts I’ve seen:

#!/usr/bin/env python

My tiny brain paid this line little attention at first, “how important could it be”, I thought. Enter the shebang #! character sequence. This sequence is treated as an interpreter directive by program loaders such as cron. That is, it specifies which interpreter to use when executing the script. 

Without this line of code the cronjob will not automatically use Python as the interpreter and will fail to execute the script. Add this line to the beginning of your script and watch your cronjob flourish!

What Was That You Said About the Script Running Without Issue?

It was previously mentioned that the Monitoring.py script executed without issue when invoked manually, like so:

sudo python Monitoring.py

This does indeed run without issue, even without the shebang #! line added to the file. However, the script name is preceded with the interpreter to use, python. If we execute the script a different way we’ll more accurately see what’s happening when the cronjob runs it.

In the terminal use the ./ command to execute the script:

./Monitoring.py

Without the shebang #!, Python is not used as the interpreter, and errors are shown in the terminal:

pi@raspberrypi:~/Documents/Development/Maintenance $ ./Monitoring.py
./Monitoring.py: line 2: import: command not found
./Monitoring.py: line 3: syntax error near unexpected token `1,'
./Monitoring.py: line 3: `sys.path.insert(1, os.path.join(sys.path[0], '..'))'

Now add the shebang #!/usr/bin/env python at the beginning of the script and execute it again. Unless your script has other syntactical problems it should run successfully.

Sam Storino

Hi, I’m Sam.
I’m a programmer and a DIYer. When I’m not finding things to build I enjoy cooking, hiking, camping and traveling the world with my best friend. Say Hello!

www.storiknow.com