How to return result from python script to shell script?

Question

I have a python script a.py which is run by a shell script b.sh . I wish to return the result (which is int) of a.py to b.sh .

I know I can use sys.exit(result) in a.py . But as I wrote set -ex in b.sh , the shell script stops after running a.py . I don't wish to use print(result) eather because I wish to print other information in a.py .

Is there any other way to return result from a.py to b.sh ?

Answer 1

You can return the exit code from the python script and look it up (and reassign to another variable if necessary) in the shell script via $? :

foo.py:

import sys
sys.exit(35)

foo.sh:

python foo.py
echo $?

Run the scripts:

> sh foo.sh
35

Answer 2

You can do something like this:

{ a.py; result=$?; } || true
# the result of a.py is in $result

|| true || true temporarily disables set -e , and the $? special variable gets stored to $result before it gets overwritten by the next command.

The downside is that, if your script actually fails (eg by raising an exception), you won't notice that. A more robust solution would be to print the needed result and catch it as output, and print the other output to the standard error.

Answer 3

I agree with chepner , that the exit codes should not be used for integer passing. Why not passing output of the python-file via stdout?

value="$(python foo.py)"
echo "${value}"

Answer 4

I would recommend writing the integer value to a file. I completely agree that exit codes should not be used for passing any information, other than success or failure of the program.

For example b.sh :

#! /bin/bash
set -ex

# Create temp file, ensure it is erased on bash script exit
tempfile="$(mktemp)"
trap 'rm -f "$tempfile"' EXIT
trap 'exit 1' HUP INT QUIT PIPE TERM

# Run program and write value to temp file
python3 ./a.py -o "$tempfile"

# Read value from temp file
myvalue="$(cat "$tempfile")"

echo "Value from a.py was: $myvalue"

And a.py :

#! /usr/bin/env python3

import argparse
from pathlib import Path

parser = argparse.ArgumentParser()
parser.add_argument(
    "-o",
    "--output-file",
    help="Write integer value to text file",
    type=str,
    required=True,
)
args = parser.parse_args()

my_value = 42
Path(args.output_file).write_text(str(my_value))