How do I pass a Python Variable to Bash?

Question

Hello,

How would I pass a Python variable to the Bash shell? It should work like this: foo="./RetVar.py 42"

Replace the double-quotes with `s

I have tried printing and sys.exiting the result, but to no avail. How would I accomplish my goal?

Answer 1

foo="$(scriptthatprintssomething)"

That's it. print. Or sys.stdout.write(). Or the like. If the script isn't executable then you'll need to specify the interpreter explicitly.

foo="$(python scriptthatprintssomething.py)"

Answer 2

Your desired form works just fine:

$ cat >Retvar.py
#!/usr/bin/python
import sys
print sys.argv[1]
$ chmod +x RetVar.py 
$ foo=`./RetVar.py 42`
$ echo $foo
42
$ 

so presumably the ways in which you had tried printing were incorrect. (This is quite independent from using the older-style backquotes, or newer-style constructs such as $()). If you still have this problem, can you show us the minimal example of Python code that reproduces it, to help us help you?

Answer 3

In bash both `cmd` and $(cmd) will be replaced by the output of the command. This allows you to assign the output of a program to a variable like

foo=`some command`

or

foo=$(some command)

Normally you wrap this in double quotes so you can have spaces in your output. It must be double quotes as stuff inside single quotes will not be executed.