How do I pass a variable to a python script run from crontab? [closed]

Question

I am running a python script using the following command:

0,30 * * * * python /home/pi/pythonscripts/script.py &

and I would like to pass a variable 'test' to the script in the crontab, something like so:

0,30 * * * * python /home/pi/pythonscripts/script.py & test='hello'

Does anybody in the raspberry pi community have any idea how to do this?

Answer 1

The ampersand (for sending a process in the background) is not necessary for commands issued to cron via the crontab as processes are already forked. The given examples work fine without the &. Actually the first version shows working parameters as it invokes the python interpreter with the filename of the script to be executed as parameter.

0,30 * * * * python /home/pi/pythonscripts/script.py foobar

or

0,30 * * * * /home/pi/pythonscripts/script.py foobar

where script.py contains the first line #!/bin/python3 and has executable flags.

An example how to use and print the passed parameter (per question of the OP):

import sys 
for arg in sys.argv:
    print(arg)

Parsing through the list argv that contains the parameters passed to the script. argv[0] contains the name (and path) of the script. argv[1] is the first parameter, and so on. For more serious handling of arguments it is advisable to use argparse (tutorial) instead of reinventing the wheel.

Answer 2

First, you need to remove the & sign as mentioned above. Cron forks processes already.

There are several ways to pass parameter to your script. Here are some examples (but not limited to):

1. Using sys.argv list. Pass it as an argument script.py parameter1
2. Using Python getopt module to parse parameters. Similar to above, but with more possibilities to do things like script.py --param1=yes --param2=data
3. Using os.environ dictionary. Pass it using environment variables PARAM1=yes script.py

Methods #1 and #2 are usually preferred.