启动包含管道命令的子进程时，找不到文件错误 [英] File not found error when launching a subprocess containing piped commands

问题描述

我需要在本地主机上使用Python运行命令date | grep -o -w '"+tz+"'' | wc -w.我正在使用subprocess模块，并使用check_output方法，因为我需要捕获同一模块的输出.

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.

但是它给我抛出了一个错误:

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

推荐答案

您必须添加shell=True才能执行Shell命令. check_output试图找到一个名为:date | grep -o -w '"+tz+"'' | wc -w的可执行文件，但他找不到它. (不知道为什么您从错误消息中删除了基本信息).

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).

查看两者之间的区别:

>>> 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'

并且:

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

阅读有关常用参数的文档，以了解更多信息shell自变量及其如何更改其他自变量的解释的信息.

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.

请注意，您应尽量避免使用shell=True，因为产生外壳可能会带来安全隐患(即使您不执行像Shellshock这样的不受信任的输入攻击也可以执行！).

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!).

子流程模块的文档中有一小节关于替换外壳管道. 您可以通过在python中生成两个进程并使用subprocess.PIPE:

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]

您可以编写一些简单的包装函数来轻松定义管道:

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

在此位置，您可以编写pipeline('date | grep 1')或pipeline('date', 'grep 1')或pipeline(['date'], ['grep', '1'])

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

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