CODEWITHSUNDEEP
pythonbashsubprocess
LearnOPhile
LearnOPhile

Reputation: 1551

Running bash command with redirection in python

I want to use redirection from bash function return values to e.g. grep, and call it from python.

Even though I am using shell=True in subprocess, this does not seem to work.

Example that works in bash:

grep foo <(echo "fooline")

Example that gives the error /bin/sh: 1: Syntax error: "(" unexpected in python 3.7.3, Ubuntu 19.04:

#!/usr/bin/env python3
import subprocess
subprocess.call('grep foo <(echo "fooline")', shell=True)

According to answers like these, redirection should work with shell=True (and it does for redirecting actual files, but not return values).

EDIT: Added shebang and python version.

Upvotes: 1

Views: 631

Answers (2)

Viktor Qvarfordt
Viktor Qvarfordt

Reputation: 36

The inconsistency that you observe is due to the fact that shell=True gives you a sh shell, not bash.

The following is valid in bash but not in sh.

grep foo <(echo "fooline")

Example output:

sh-3.2$ grep foo <(echo "fooline")
sh: syntax error near unexpected token `('

If you use a valid sh expression your approach will work. Alternatively you can specify which shell to use with executable='/bin/bash'. You can also use something like:

subprocess.Popen(['/bin/bash', '-c', cmd])

Upvotes: 2

Jean-Fran&#231;ois Fabre
Jean-Fran&#231;ois Fabre

Reputation: 140216

The error is a shell error, nothing to do with python. sh chokes on the parentheses of python syntax.

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

Let's add a shebang (reference: Should I put #! (shebang) in Python scripts, and what form should it take?)

And also stop using shell=True and all. Use real pipes and command lines with arguments (note: this has been tested & works on windows using a native grep command, so now this is portable)

#!/usr/bin/env python3
import subprocess

p = subprocess.Popen(['grep','foo'],stdin = subprocess.PIPE)
p.stdin.write(b"fooline\n")
p.stdin.close()
p.wait()

Upvotes: 2

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