Why does this valid shell command throw an error in python via subprocess?

Question

The line awk -F'[][]' '/dB/ { print $2 }' <(amixer sget Master) in bash returns my system's current volume (e.g. "97%").

I tried to incorporate this in Python 3

#!/usr/bin/env python3
import subprocess
command = "awk -F'[][]' '/dB/ { print $2 }' <(amixer sget Master)"
output = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE).stdout.read()

print(output)

However the output from the shell returns

/bin/sh: 1: Syntax error: "(" unexpected
b''

Why does this fail and how do I fix my code?

Funny that you tagged this with "bash" but failed to read where the error message came from... — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Jun 16, 2016 at 5:28
@UlrichEckhardt I'm very new to linux, can you explain what you mean by this? — DenverCoder10
– DenverCoder10, Commented Jun 16, 2016 at 5:32
Simple, it was a hint that you should read error messages. Your's for example didn't come from Bash (/bin/bash or /usr/bin/bash) but from /bin/sh, which is a different shell. If you had tried your "valid shell command" (which is a thing that doesn't exist, because there are many shells!) on the shell that reported the failure, you would have been one step closer to understanding the problem and solving it. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Jun 16, 2016 at 5:37
Thanks, for some reason I thought 'sh' and bash were synonymous and assumed it was my implementation with python that was the issue. — DenverCoder10
– DenverCoder10, Commented Jun 16, 2016 at 5:48

anishsane · Accepted Answer · 2016-06-16 05:47:43Z

3

As already pointed out, the syntax you are using is a bash syntax (a.k.a. bashism). The default shell used in subprocess.Popen is /bin/sh & it does not support process substitution.

You can specify the shell to be used via executable argument.

Try this:

output = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE, executable="/bin/bash").stdout.read()

answered Jun 16, 2016 at 5:47

anishsane

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Andreas Louv · Accepted Answer · 2016-06-16 05:17:06Z

3

Because you are using bashism in form of a process substitution, and your /bin/sh doesn't support that:

<(...)

Changing this to a pipe should solve your problem:

command = "amixer sget Master | awk -F'[][]' '/dB/ { print $2 }'"

Alternative you can start bash from within sh:

command = "bash -c 'amixer sget Master | awk -F'\\''[][]'\\'' '\\''/dB/ { print $2 }'\\'''"

But as you will soon realize, quoting and escaping will become a nightmare

answered Jun 16, 2016 at 5:17

Andreas Louv

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