How to do a blocking system call in python (and get its result as a string)

Question

How do I make a blocking operating system call in python?

For example get the output of the linux "date" command.

And put its string output result into a string variable.

Dear Stackoverflow.

What is unclear about "operating system call"

What is unclear about "blocking"

I suspect the people who "close" do not even read the question.

Is there a procedure for unblocking questions?

what is not clear about the question? please remove the downvotes. — SHernandez, Jul 12 '15 at 18:56
http://stackoverflow.com/questions/325463/launch-a-shell-command-with-in-a-python-script-wait-for-the-termination-and-ret third result for *python how to run external command and wait for return* — Padraic Cunningham, Jul 12 '15 at 19:25

score 3 · Answer 1 · answered Jul 12 '15 at 19:01

3

You can do something like this:

import subprocess
proc = subprocess.Popen(['date'],
                        stdout=subprocess.PIPE,
                        stderr=subprocess.PIPE)
stdout, stderr = proc.communicate()

The first argument to Popen is the list of command line arguments you'd like executed. The PIPE arguments which follow instruct python to capture the output of the command. communicate() waits for the command to complete and returns the stdout and stderr of the process as strings. After execution, the stdout variable will contain the text from the subprocess. Note that there are a few caveats to this pattern (particularly with large amounts of output). You can learn more here:

https://docs.python.org/2/library/subprocess.html

answered Jul 12 '15 at 19:01

matthewatabet

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This is a very good answer but F.X.'s way is more convenient and also handles errors and exceptions nice. Will probably have to accept his – SHernandez Jul 12 '15 at 19:30
@SHernandez -- That's true, but it is not available in all versions of python. The code I work with generally requires the lowest common denominator. So, I generally use the less-convenient approach and error catch where needed. The very best solution will depend upon your use case. – matthewatabet Jul 12 '15 at 19:33
It should be noted that stdout and stderr in the last line are bytestrings with a newline at the end. But the same name parameters to the function calls are operating system pipe objects, just to prevent confusion – SHernandez Jul 12 '15 at 19:34

score 2 · Accepted Answer · answered Jul 12 '15 at 19:07

2

To get the output of a command, use check_output, which has the advantage over Popen that it raises an exception if the command fails :

from subprocess import check_output
output = check_output(["date"])

(I assumed that you're not talking about real system calls.)

answered Jul 12 '15 at 19:07

F.X.

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if I want a real system call I call my mother lol – SHernandez Jul 12 '15 at 19:12
very interesting, check_output seems like a convenience wrapper isn't it? – SHernandez Jul 12 '15 at 19:12
Yes it is, see the [subprocess](https://github.com/python/cpython/blob/master/Lib/subprocess.py#L588) module. It's convenient though ;) – F.X. Jul 12 '15 at 19:17

score 1 · Answer 3 · answered Jul 12 '15 at 18:56

1

You can call:

import os

os.system('date')

answered Jul 12 '15 at 18:56

Ming

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this is nice but misses the "string result" part. result is integer. – SHernandez Jul 12 '15 at 19:02
The simple and age-old way of doing it. Still works like a charm. – Mohit Arvind khakharia Feb 17 '21 at 02:31

How to do a blocking system call in python (and get its result as a string)

3 Answers3