Python标准库 - subprocess

编写Python脚本时, 经常要执行Linux操作系统命令, 如mkdir zzzz. 目前比较推荐的方法是使用subprocess模块.

通过该模块的帮助文档, 可看到其主要提供了4个API, 和相应的使用说明.

Main API

========

call(...): Runs a command, waits for it to complete, then returns

    the return code.

check_call(...): Same as call() but raises CalledProcessError()

    if return code is not 0

check_output(...): Same as check_call() but returns the contents of

    stdout instead of a return code

Popen(...): A class for flexibly executing a command in a new process

结合API的说明来看, 若执行简单的命令, 像前面所说mkdir zzzz, 前三个API都可以, 只是处理细节稍有不同.

In [54]: subprocess.call("mkdir /tmp/zzzz", shell=True)

Out[54]: 0

In [55]: subprocess.call("mkdir /tmp/zzzz", shell=True)

mkdir: cannot create directory `/tmp/zzzz': File exists

Out[55]: 1

In [56]: subprocess.check_call("mkdir /tmp/zzzz", shell=True)

Out[56]: 0

In [57]: subprocess.check_call("mkdir /tmp/zzzz", shell=True)

mkdir: cannot create directory `/tmp/zzzz': File exists

---------------------------------------------------------------------------

CalledProcessError                        Traceback (most recent call last)

<ipython-input-57-2a531671f16e> in <module>()

----> 1 subprocess.check_call("mkdir /tmp/zzzz", shell=True)

/usr/local/python27/lib/python2.7/subprocess.pyc in check_call(*popenargs, **kwargs)

    184         if cmd is None:

    185             cmd = popenargs[0]

--> 186         raise CalledProcessError(retcode, cmd)

    187     return 0

    188

CalledProcessError: Command 'mkdir /tmp/zzzz' returned non-zero exit status 1

In [59]: subprocess.check_output("mkdir /tmp/zzzz", shell=True)

Out[59]: ''

In [60]: subprocess.check_output("mkdir /tmp/zzzz", shell=True)

mkdir: cannot create directory `/tmp/zzzz': File exists

---------------------------------------------------------------------------

CalledProcessError                        Traceback (most recent call last)

<ipython-input-60-4911eea4ecd3> in <module>()

----> 1 subprocess.check_output("mkdir /tmp/zzzz", shell=True)

/usr/local/python27/lib/python2.7/subprocess.pyc in check_output(*popenargs, **kwargs)

    217         if cmd is None:

    218             cmd = popenargs[0]

--> 219         raise CalledProcessError(retcode, cmd, output=output)

    220     return output

    221

CalledProcessError: Command 'mkdir /tmp/zzzz' returned non-zero exit status 1

又若执行复杂的命令或脚本, 需要获取其标准输出, 和标准错误输出, 就要用到Popen接口了. 前三个API, 其实是第四个的简化版, 是将参数直接传给了Popen的构造函数, 不过大部分参数都是保留了默认值, 来看下该构造函数的样子.

def __init__(self, args, bufsize=0, executable=None,

             stdin=None, stdout=None, stderr=None,

             preexec_fn=None, close_fds=False, shell=False,

             cwd=None, env=None, universal_newlines=False,

             startupinfo=None, creationflags=0):

注意参数shell=False, 其含义为, 是否将args(要执行的命令)置于操作系统的shell环境中运行.

下面给出一个代码段, 用到了Popen, 算是对subprocess模块的小结.

#!/usr/bin/env python

# -*- coding: utf-8 -*-

import subprocess

def exec_cmd(cmd):

    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE,

                            stderr=subprocess.PIPE, shell=True)

    stdout, stderr = proc.communicate()

    if proc.returncode != 0:

        return proc.returncode, stderr

    return proc.returncode, stdout

def main():

    cmd = "ls -l"

    # cmd = "ls -lz"

    returncode, stdoutdata = exec_cmd(cmd)

    print str(returncode) + " - " + stdoutdata

if __name__ == '__main__':

    main()