subprocess(python)

subprocess 是python在2.4版本之后引入的用于subprocess管理的module。subprocess中有一个重要的类Popen和一个重要的方法call。下面对Popen和call进行相关说明。

Popen用于新建一个subprocess，下面是它的构造函数。

class subprocess.Popen(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)

其中：

args是运行进程的命令行（与shell中输入的命令行一样）；

bufsize, if given, has the same meaning as the corresponding argument to the built-in open() function: 0 means unbuffered, 1 means line buffered, any other positive value means use a buffer of (approximately) that size. A negative bufsize means to use the system default, which usually means fully buffered. The default value for bufsize is 0 (unbuffered).

The executable argument specifies the program to execute. It is very seldom needed: Usually, the program to execute is defined by the args argument. If shell=True, the executable argument specifies which shell to use. On Unix, the default shell is /bin/sh. On Windows, the default shell is specified by the COMSPEC environment variable. The only reason you would need to specify shell=True on Windows is where the command you wish to execute is actually built in to the shell, eg dir, copy. You don’t need shell=True to run a batch file, nor to run a console-based executable.

stdin, stdout and stderr 指定了subprocess的标准输入、输出和错误流。这些值可以是subprocess.PIPE，文件描叙符（file descriptor），文件对象（file object）和None。使用subprocess.PIPE，会新建一个pipe到子进程，通过这个pipe父子进程可以进行信息传递。stderr可以为subprocess.STDOUT，表示stderr和stdout使用同样的file handle。

If preexec_fn is set to a callable object, this object will be called in the child process just before the child is executed. (Unix only)

If close_fds is true, all file descriptors except 0, 1 and 2 will be closed before the child process is executed. (Unix only). Or, on Windows, if close_fds is true then no handles will be inherited by the child process. Note that on Windows, you cannot set close_fds to true and also redirect the standard handles by setting stdin, stdout or stderr.

If shell is True, the specified command will be executed through the shell.

If cwd is not None, the child’s current directory will be changed to cwd before it is executed. Note that this directory is not considered when searching the executable, so you can’t specify the program’s path relative to cwd.

If env is not None, it must be a mapping that defines the environment variables for the new process; these are used instead of inheriting the current process’ environment, which is the default behavior.

If universal_newlines is True, the file objects stdout and stderr are opened as text files, but lines may be terminated by any of '\n', the Unix end-of-line convention, '\r', the old Macintosh convention or '\r\n', the Windows convention. All of these external representations are seen as '\n' by the Python program.

The startupinfo and creationflags, if given, will be passed to the underlying CreateProcess() function. They can specify things such as appearance of the main window and priority for the new process. (Windows only)

Example：

[codesyntax lang="python" title="Example"]

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#! /usr/bin/python
import subprocess
import time

if __name__ == '__main__':
 p = subprocess.Popen('./bt_download.out', stdin=subprocess.PIPE,
                      stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
 for i in range(0, 10):
        p.stdin.write('Info\n')
       print p.stdout.readline()
     time.sleep(5)
 p.stdin.write('Quit\n')
   print p.stdout.readline()
 time.sleep(5)

[/codesyntax]

上面的example通过subprocess，实现了父子进程的交流。需要注意的是，p.stdout.readline等操作都是阻塞操作。

call方法，同Popen的构造函数，其函数的declare如下：

subprocess.call(*popenargs, **kwargs)

其中openargs就是对应Popen中的args，kwargs就对Popen后面的一系列argument。

Example如下：

retcode = subprocess.call(["ls", "-l"])
它会等待ls命令执行完毕，retcode是子进程执行完的retcode。