File not found error when launching a subprocess containing piped commands

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

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'])

The most common cause of FileNotFound with subprocess, in my experience, is the use of spaces in your command. If you have just a single command (not a pipeline, and no redirection, wildcards, etc), use a list instead.

# Wrong, even with a valid command string
subprocess.run(['grep -o -w "+tz+"'])

# Fixed; notice also 
subprocess.run(["grep", "-o", "-w", '"+tz+"'])

This change results in no more FileNotFound errors, and is a nice solution if you got here searching for that exception with a simpler command.

If you need a pipeline or other shell features, the simple fix is to add shell=True:

subprocess.run(
    '''date | grep -o -w '"+tz+"'' | wc -w''',
    shell=True)

However, if you are using python 3.5 or greater, try using this approach:

import subprocess

a = subprocess.run(["date"], stdout=subprocess.PIPE)
print(a.stdout.decode('utf-8'))

b = subprocess.run(["grep", "-o", "-w", '"+tz+"'],
                   input=a.stdout, stdout=subprocess.PIPE)
print(b.stdout.decode('utf-8'))

c = subprocess.run(["wc", "-w"],
                   input=b.stdout, stdout=subprocess.PIPE)
print(c.stdout.decode('utf-8'))

You should see how one command's output becomes another's input just like using a shell pipe, but you can easily debug each step of the process in python. Using subprocess.run is recommended for python > 3.5, but not available in prior versions.

The FileNotFoundError happens because - in the absence of shell=True - Python tries to find an executable whose file name is the entire string you are passing in. You need to add shell=True to get the shell to parse and execute the string, or figure out how to rearticulate this command line to avoid requiring a shell.

As an aside, the shell programming here is decidedly weird. On any normal system, date will absolutely never output "+tz+" and so the rest of the processing is moot.

Further, using wc -w to count the number of output words from grep is unusual. The much more common use case (if you can't simply use grep -c to count the number of matching lines) would be to use wc -l to count lines of output from grep.

Anyway, if you can, you want to avoid shell=True; if the intent here is to test the date command, you should probably replace the rest of the shell script with native Python code.

Pros:

The person trying to understand the program only needs to understand Python, not shell script.
The script will have fewer external dependencies (here, date) rather than require a Unix-like platform.

Cons:

Reimplementing standard Unix tools in Python is tiresome and sometimes rather verbose.

With that out of the way, if the intent is simply to count how wany times "+tz+" occurs in the output from date, try

p = subprocess.run(['date'],
    capture_output=True, text=True,
    check=True)
result = len(p.stdout.split('"+tz+"'))-1

The keyword argument text=True requires Python 3.7; for compatibility back to earlier Python versions, try the (misnomer) legacy synonym universal_newlines=True. For really old Python versions, maybe fall back to subprocess.check_output().

If you really need the semantics of the -w option of grep, you need to check if the characters adjacent to the match are not alphabetic, and exclude those which are. I'm leaving that as an exercise, and in fact would assume that the original shell script implementation here was not actually correct. (Maybe try re.split(r'(?<=^|\W)"\+tz\+"(?=\W|$)', p.stdout).)

In more trivial cases (single command, no pipes, wildcards, redirection, shell builtins, etc) you can use Python's shlex.split() to parse a command into a correctly quoted list of arguments. For example,

>>> import shlex
>>> shlex.split(r'''one "two three" four\ five 'six seven' eight"'"nine'"'ten''')
['one', 'two three', 'four five', 'six seven', 'eight\'nine"ten']

Notice how the regular string split() is completely unsuitable here; it simply splits on every whitespace character, and doesn't support any sort of quoting or escaping. (But notice also how it boneheadedly just returns a list of tokens from the original input:

>>> shlex.split('''date | grep -o -w '"+tz+"' | wc -w''')
['date', '|', 'grep', '-o', '-w', '"+tz+"', '|', 'wc', '-w']

(Even more parenthetically, this isn't exactly the original input, which had a superfluous extra single quote after '"+tz+"').

This is in fact passing | and grep etc as arguments to date, not implementing a shell pipeline! You still have to understand what you are doing.)

File not found error when launching a subprocess containing piped commands

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