How to access python return value from bash script

Question

I'm trying to understand how to access from a bash script the return value of a python script.

Clarifying through an example:

foo.py

def main():
    print ("exec main..")
    return "execution ok"

if __name__ == '__main__':
    main()

start.sh

script_output=$(python foo.py 2>&1)
echo $script_output

If I run the bash script, this prints the message "exec main..".

How can I store in script_output the return value (execution ok)? If I direct execution ok to stdout, the script_output will capture all the stdout (so the 2 print statement).

Is there any way to implement this?

Thanks! Alessio

Answer 1

Add a proper exit code from your script using the sys.exit() module. Usually commands return 0 on successful completion of a script.

import sys

def main():
    print ("exec main..")
    sys.exit(0)

and capture it in shell script with a simple conditional. Though the exit code is 0 by default and need not be passed explicitly, using sys.exit() gives control to return non-zero codes on error cases wherever applicable to understand some inconsistencies with the script.

if python foo.py 2>&1 >/dev/null; then
    echo 'script ran fine'
fi

Answer 2

You can get the previous command's output status through $?. If the python script ran successfully without any stderr, it should return 0 as exit code else it would return 1 or any number other than 0.

#!/bin/bash
python foo.py 2>&1 /dev/null
script_output=$?
echo $script_output

Answer 3

Bash contains only return code in $?, so you can't use it to print the text from python's return. My solution is write in to the stderr in python script, next print only stderr in bash:

import sys
    
    
def main():
    print ("exec main..")
    sys.stderr.write('execution ok\n')
    return "execution ok"
    
if __name__ == '__main__':
    main()

Bash:

#!/bin/bash
    
script_output=$(python foo.py 1>/dev/null)
echo $script_output

Output:

execution ok