File not found error when launching a subprocess containing piped commands

Question

I need to run the command date | grep -o -w '"+tz+"'' | wc -w using Python on my localhost. I am using subprocess module for the same and using the check_output method as I need to capture the output for the same.

However it is throwing me an error :

Traceback (most recent call last):
  File "test.py", line 47, in <module>
    check_timezone()
  File "test.py", line 40, in check_timezone
    count = subprocess.check_output(command)
  File "/usr/lib/python2.7/subprocess.py", line 537, in check_output
    process = Popen(stdout=PIPE, *popenargs, **kwargs)
  File "/usr/lib/python2.7/subprocess.py", line 679, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1249, in _execute_child
    raise child_exception-
OSError: [Errno 2] No such file or directory

Answer 1

You have to add shell=True to execute a shell command. check_output is trying to find an executable called: date | grep -o -w '"+tz+"'' | wc -w and he cannot find it. (no idea why you removed the essential information from the error message).

See the difference between:

>>> subprocess.check_output('date | grep 1')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.4/subprocess.py", line 603, in check_output
    with Popen(*popenargs, stdout=PIPE, **kwargs) as process:
  File "/usr/lib/python3.4/subprocess.py", line 848, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.4/subprocess.py", line 1446, in _execute_child
    raise child_exception_type(errno_num, err_msg)
FileNotFoundError: [Errno 2] No such file or directory: 'date | grep 1'

And:

>>> subprocess.check_output('date | grep 1', shell=True)
b'gio 19 giu 2014, 14.15.35, CEST\n'

Read the documentation about the Frequently Used Arguments for more information about the shell argument and how it changes the interpretation of the other arguments.

---

Note that you should try to avoid using shell=True since spawning a shell can be a security hazard (even if you do not execute untrusted input attacks like Shellshock can still be performed!).

The documentation for the subprocess module has a little section about replacing the shell pipeline. You can do so by spawning the two processes in python and use subprocess.PIPE:

date_proc = subprocess.Popen(['date'], stdout=subprocess.PIPE)
grep_proc = subprocess.check_output(['grep', '1'], stdin=date_proc.stdout, stdout=subprocess.PIPE)
date_proc.stdout.close()
output = grep_proc.communicate()[0]

You can write some simple wrapper function to easily define pipelines:

import subprocess
from shlex import split
from collections import namedtuple
from functools import reduce

proc_output = namedtuple('proc_output', 'stdout stderr')


def pipeline(starter_command, *commands):
    if not commands:
        try:
            starter_command, *commands = starter_command.split('|')
        except AttributeError:
            pass
    starter_command = _parse(starter_command)
    starter = subprocess.Popen(starter_command, stdout=subprocess.PIPE)
    last_proc = reduce(_create_pipe, map(_parse, commands), starter)
    return proc_output(*last_proc.communicate())

def _create_pipe(previous, command):
    proc = subprocess.Popen(command, stdin=previous.stdout, stdout=subprocess.PIPE)
    previous.stdout.close()
    return proc

def _parse(cmd):
    try:
        return split(cmd)
    except Exception:
        return cmd

With this in place you can write pipeline('date | grep 1') or pipeline('date', 'grep 1') or pipeline(['date'], ['grep', '1'])