Passing variables to a subprocess in python

Question

I don't know if this is the correct form

num=8
num2=10
num=subprocess.check_output("num/2", shell=True)
num2=subprocess.check_output("num2+7", shell=True)

Bash doesnt have access to your python variables, try something like that subprocess.check_output("{}/2".format(num), shell=True) — ShellCode
– ShellCode, Commented Mar 16, 2021 at 10:36

tdy · Accepted Answer · 2021-03-17 05:45:51Z

1

'num/2' is just the string "num/2". If you want it to be the variable num, you need to interpolate it, e.g., with an f-string f'{num}/2'.

Also if you're trying to use bash for division, you'll have to add double parentheses for arithmetic expansion:

>>> num = 8
>>> subprocess.check_output(f'echo $(( {num}/2 ))', shell=True)
b'4\n'

Python first interpolates {num} into 8, and then sends echo $(( 8/2 )) to shell.

answered Mar 16, 2021 at 11:39

tdy

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