17.5. subprocess — 子进程管理 ¶

The subprocess 模块允许卵生新进程，连接到它们的输入/输出/错误管道，并获得它们的返回代码。此模块打算替换几个较旧的模块和函数：

os.system
os.spawn*
					

信息关于如何 subprocess 模块可以用于替换这些模块和函数 (可以在以下各节中找到)。

另请参阅

PEP 324 – PEP 提出 subprocess 模块

17.5.1. 使用 subprocess 模块 ¶

The recommended approach to invoking subprocesses is to use the following convenience functions for all use cases they can handle. For more advanced use cases, the underlying Popen 接口可以直接使用。

subprocess. call ( args , * , stdin=None , stdout=None , stderr=None , shell=False , timeout=None ) ¶

运行的命令描述通过 args . Wait for command to complete, then return the returncode 属性。

The arguments shown above are merely the most common ones, described below in 经常使用的自变量 (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the Popen constructor - this function passes all supplied arguments other than timeout directly through to that interface.

The timeout 自变量会被传递给 Popen.wait() . If the timeout expires, the child process will be killed and then waited for again. The TimeoutExpired exception will be re-raised after the child process has terminated.

范例：

>>> subprocess.call(["ls", "-l"])
0
>>> subprocess.call("exit 1", shell=True)
1
						

注意

不使用 stdout=PIPE or stderr=PIPE with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.

3.3 版改变： timeout 被添加。

subprocess. check_call ( args , * , stdin=None , stdout=None , stderr=None , shell=False , timeout=None ) ¶

Run command with arguments. Wait for command to complete. If the return code was zero then return, otherwise raise CalledProcessError 。 CalledProcessError object will have the return code in the returncode 属性。

The arguments shown above are merely the most common ones, described below in 经常使用的自变量 (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the Popen constructor - this function passes all supplied arguments other than timeout directly through to that interface.

The timeout 自变量会被传递给 Popen.wait() . If the timeout expires, the child process will be killed and then waited for again. The TimeoutExpired exception will be re-raised after the child process has terminated.

范例：

>>> subprocess.check_call(["ls", "-l"])
0
>>> subprocess.check_call("exit 1", shell=True)
Traceback (most recent call last):
   ...
subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1
						

注意

不使用 stdout=PIPE or stderr=PIPE with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.

3.3 版改变： timeout 被添加。

subprocess. check_output ( args , * , input=None , stdin=None , stderr=None , shell=False , universal_newlines=False , timeout=None ) ¶

Run command with arguments and return its output.

If the return code was non-zero it raises a CalledProcessError 。 CalledProcessError object will have the return code in the returncode attribute and any output in the output 属性。

The arguments shown above are merely the most common ones, described below in 经常使用的自变量 (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the Popen constructor - this functions passes all supplied arguments other than input and timeout directly through to that interface. In addition, stdout is not permitted as an argument, as it is used internally to collect the output from the subprocess.

The timeout 自变量会被传递给 Popen.wait() . If the timeout expires, the child process will be killed and then waited for again. The TimeoutExpired exception will be re-raised after the child process has terminated.

The input 自变量会被传递给 Popen.communicate() and thus to the subprocess’s stdin. If used it must be a byte sequence, or a string if universal_newlines=True . When used, the internal Popen object is automatically created with stdin=PIPE ，和 stdin argument may not be used as well.

范例：

>>> subprocess.check_output(["echo", "Hello World!"])
b'Hello World!\n'
>>> subprocess.check_output(["echo", "Hello World!"], universal_newlines=True)
'Hello World!\n'
>>> subprocess.check_output(["sed", "-e", "s/foo/bar/"],
...                         input=b"when in the course of fooman events\n")
b'when in the course of barman events\n'
>>> subprocess.check_output("exit 1", shell=True)
Traceback (most recent call last):
   ...
subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1
						

By default, this function will return the data as encoded bytes. The actual encoding of the output data may depend on the command being invoked, so the decoding to text will often need to be handled at the application level.

This behaviour may be overridden by setting universal_newlines to True as described below in 经常使用的自变量 .

To also capture standard error in the result, use stderr=subprocess.STDOUT :

>>> subprocess.check_output(
...     "ls non_existent_file; exit 0",
...     stderr=subprocess.STDOUT,
...     shell=True)
'ls: non_existent_file: No such file or directory\n'
						

注意

不使用 stdout=PIPE or stderr=PIPE with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.

3.1 版新增。

3.3 版改变： timeout 被添加。

3.4 版改变： input 被添加。

subprocess. DEVNULL ¶

Special value that can be used as the stdin , stdout or stderr 自变量对于 Popen and indicates that the special file os.devnull 会被使用。

3.3 版新增。

subprocess. PIPE ¶

Special value that can be used as the stdin , stdout or stderr 自变量对于 Popen and indicates that a pipe to the standard stream should be opened. Most useful with Popen.communicate() .

subprocess. STDOUT ¶

Special value that can be used as the stderr 自变量对于 Popen and indicates that standard error should go into the same handle as standard output.

exception subprocess. SubprocessError ¶

来自此模块的所有其它异常的基类。

3.3 版新增。

exception subprocess. TimeoutExpired ¶

子类化的 SubprocessError , raised when a timeout expires while waiting for a child process.

cmd ¶

用于卵生子级进程的命令。

timeout ¶

超时 (以秒为单位)。

output ¶

Output of the child process if this exception is raised by check_output() 。否则， None .

3.3 版新增。

exception subprocess. CalledProcessError ¶

子类化的 SubprocessError ，被引发当进程运行通过 check_call() or check_output() 返回非零退出状态。

returncode ¶

Exit status of the child process.

cmd ¶

用于卵生子级进程的命令。

output ¶

Output of the child process if this exception is raised by check_output() 。否则， None .

17.5.1.1. 频繁使用的自变量 ¶

为支持各种使用案例， Popen constructor (and the convenience functions) accept a large number of optional arguments. For most typical use cases, many of these arguments can be safely left at their default values. The arguments that are most commonly needed are:

args is required for all calls and should be a string, or a sequence of program arguments. Providing a sequence of arguments is generally preferred, as it allows the module to take care of any required escaping and quoting of arguments (e.g. to permit spaces in file names). If passing a single string, either shell 必须为 True (see below) or else the string must simply name the program to be executed without specifying any arguments.

stdin , stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively. Valid values are PIPE , DEVNULL , an existing file descriptor (a positive integer), an existing file object, and None . PIPE indicates that a new pipe to the child should be created. DEVNULL indicates that the special file os.devnull will be used. With the default settings of None , no redirection will occur; the child’s file handles will be inherited from the parent. Additionally, stderr 可以是 STDOUT , which indicates that the stderr data from the child process should be captured into the same file handle as for stdout .

若 universal_newlines is False the file objects stdin , stdout and stderr will be opened as binary streams, and no line ending conversion is done.

若 universal_newlines is True , these file objects will be opened as text streams in 通用换行符 mode using the encoding returned by locale.getpreferredencoding(False) 。对于 stdin , line ending characters '\n' in the input will be converted to the default line separator os.linesep 。对于 stdout and stderr , all line endings in the output will be converted to '\n' . For more information see the documentation of the io.TextIOWrapper class when the newline argument to its constructor is None .

注意

The newlines attribute of the file objects Popen.stdin , Popen.stdout and Popen.stderr are not updated by the Popen.communicate() 方法。

若 shell is True , the specified command will be executed through the shell. This can be useful if you are using Python primarily for the enhanced control flow it offers over most system shells and still want convenient access to other shell features such as shell pipes, filename wildcards, environment variable expansion, and expansion of ~ to a user’s home directory. However, note that Python itself offers implementations of many shell-like features (in particular, glob , fnmatch , os.walk() , os.path.expandvars() , os.path.expanduser() ，和 shutil ).

3.3 版改变： 当 universal_newlines is True , the class uses the encoding locale.getpreferredencoding(False) 而不是 locale.getpreferredencoding() 。见 io.TextIOWrapper 类，了解有关此变化的更多信息。

注意

阅读 安全注意事项 章节先于使用 shell=True .

这些选项及所有其它选项的更详细描述在 Popen 构造函数文档编制。

17.5.1.2. Popen 构造函数 ¶

此模块中底层进程的创建和管理的处理是通过 Popen 类。它提供了很大的灵活性，以便开发者能够处理方便函数未涵盖的不常见情况。

class subprocess. Popen ( args , bufsize=-1 , executable=None , stdin=None , stdout=None , stderr=None , preexec_fn=None , close_fds=True , shell=False , cwd=None , env=None , universal_newlines=False , startupinfo=None , creationflags=0 , restore_signals=True , start_new_session=False , pass_fds=() ) ¶

在新进程中执行子级程序。在 POSIX (便携式操作系统接口)，类使用 os.execvp() 类似行为来执行子级程序。在 Windows，类使用 Windows CreateProcess() 函数。自变量到 Popen 如下所示。

args should be a sequence of program arguments or else a single string. By default, the program to execute is the first item in args if args is a sequence. If args is a string, the interpretation is platform-dependent and described below. See the shell and executable arguments for additional differences from the default behavior. Unless otherwise stated, it is recommended to pass args as a sequence.

在 POSIX (便携式操作系统接口)，若 args is a string, the string is interpreted as the name or path of the program to execute. However, this can only be done if not passing arguments to the program.

注意

shlex.split() can be useful when determining the correct tokenization for args , especially in complex cases:

>>> import shlex, subprocess
>>> command_line = input()
/bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'"
>>> args = shlex.split(command_line)
>>> print(args)
['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"]
>>> p = subprocess.Popen(args) # Success!
						

Note in particular that options (such as -input ) and arguments (such as eggs.txt ) that are separated by whitespace in the shell go in separate list elements, while arguments that need quoting or backslash escaping when used in the shell (such as filenames containing spaces or the echo command shown above) are single list elements.

在 Windows，若 args is a sequence, it will be converted to a string in a manner described in 在 Windows 将自变量序列转换成字符串 . This is because the underlying CreateProcess() operates on strings.

The shell 自变量 (默认为 False ) specifies whether to use the shell as the program to execute. If shell is True , it is recommended to pass args as a string rather than as a sequence.

在 POSIX (便携式操作系统接口) 采用 shell=True , the shell defaults to /bin/sh 。若 args is a string, the string specifies the command to execute through the shell. This means that the string must be formatted exactly as it would be when typed at the shell prompt. This includes, for example, quoting or backslash escaping filenames with spaces in them. If args is a sequence, the first item specifies the command string, and any additional items will be treated as additional arguments to the shell itself. That is to say, Popen does the equivalent of:

Popen(['/bin/sh', '-c', args[0], args[1], ...])
						

在 Windows 采用 shell=True ， COMSPEC environment variable specifies the default shell. The only time you need to specify shell=True on Windows is when the command you wish to execute is built into the shell (e.g. dir or copy ). You do not need shell=True to run a batch file or console-based executable.

注意

阅读 安全注意事项 章节先于使用 shell=True .

bufsize will be supplied as the corresponding argument to the open() function when creating the stdin/stdout/stderr pipe file objects:

• 0 means unbuffered (read and write are one system call and can return short)
• 1 means line buffered (only usable if universal_newlines=True i.e., in a text mode)
• any other positive value means use a buffer of approximately that size
• negative bufsize (the default) means the system default of io.DEFAULT_BUFFER_SIZE will be used.

Changed in version 3.3.1: bufsize now defaults to -1 to enable buffering by default to match the behavior that most code expects. In versions prior to Python 3.2.4 and 3.3.1 it incorrectly defaulted to 0 which was unbuffered and allowed short reads. This was unintentional and did not match the behavior of Python 2 as most code expected.

The executable argument specifies a replacement program to execute. It is very seldom needed. When shell=False , executable replaces the program to execute specified by args . However, the original args is still passed to the program. Most programs treat the program specified by args as the command name, which can then be different from the program actually executed. On POSIX, the args name becomes the display name for the executable in utilities such as ps 。若 shell=True , on POSIX the executable argument specifies a replacement shell for the default /bin/sh .

stdin , stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively. Valid values are PIPE , DEVNULL , an existing file descriptor (a positive integer), an existing 文件对象 ，和 None . PIPE indicates that a new pipe to the child should be created. DEVNULL indicates that the special file os.devnull will be used. With the default settings of None , no redirection will occur; the child’s file handles will be inherited from the parent. Additionally, stderr 可以是 STDOUT , which indicates that the stderr data from the applications should be captured into the same file handle as for stdout.

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

警告

The preexec_fn parameter is not safe to use in the presence of threads in your application. The child process could deadlock before exec is called. If you must use it, keep it trivial! Minimize the number of libraries you call into.

注意

若需要为子级修改环境使用 env 参数而不是处理它在 preexec_fn 。 start_new_session 参数可以代替以前常用的 preexec_fn 以在子级中调用 os.setsid()。

若 close_fds is true, all file descriptors except 0 , 1 and 2 will be closed before the child process is executed. (POSIX only). The default varies by platform: Always true on POSIX. On Windows it is true when stdin / stdout / stderr are None , false otherwise. 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 .

3.2 版改变： 默认为 close_fds was changed from False to what is described above.

pass_fds is an optional sequence of file descriptors to keep open between the parent and child. Providing any pass_fds forces close_fds 到 True . (POSIX only)

3.2 版新增： The pass_fds 参数被添加。

若 cwd 不是 None , the function changes the working directory to cwd before executing the child. In particular, the function looks for executable (or for the first item in args ) relative to cwd if the executable path is a relative path.

若 restore_signals is true (the default) all signals that Python has set to SIG_IGN are restored to SIG_DFL in the child process before the exec. Currently this includes the SIGPIPE, SIGXFZ and SIGXFSZ signals. (POSIX only)

3.2 版改变： restore_signals 被添加。

若 start_new_session is true the setsid() system call will be made in the child process prior to the execution of the subprocess. (POSIX only)

3.2 版改变： start_new_session 被添加。

若 env 不是 None ，它必须是为新进程定义环境变量的映射；使用这些代替继承当前进程环境的默认行为。

注意

若指定， env 必须提供要执行程序要求的任何变量。在 Windows，为运行 并行汇编 指定 env must 包括有效 SystemRoot .

若 universal_newlines is True , the file objects stdin , stdout and stderr are opened as text streams in universal newlines mode, as described above in 经常使用的自变量 , otherwise they are opened as binary streams.

若给定， startupinfo 将是 STARTUPINFO 对象，会被传递给底层 CreateProcess 函数。 creationflags , if given, can be CREATE_NEW_CONSOLE or CREATE_NEW_PROCESS_GROUP . (Windows only)

Popen objects are supported as context managers via the with statement: on exit, standard file descriptors are closed, and the process is waited for.

with Popen(["ifconfig"], stdout=PIPE) as proc:
    log.write(proc.stdout.read())
						

3.2 版改变： 添加上下文管理器支持。

17.5.1.3. 异常 ¶

Exceptions raised in the child process, before the new program has started to execute, will be re-raised in the parent. Additionally, the exception object will have one extra attribute called child_traceback , which is a string containing traceback information from the child’s point of view.

最常引发的异常是 OSError . This occurs, for example, when trying to execute a non-existent file. Applications should prepare for OSError 异常。

A ValueError 会被引发若 Popen is called with invalid arguments.

check_call() and check_output() 会引发 CalledProcessError if the called process returns a non-zero return code.

All of the functions and methods that accept a timeout parameter, such as call() and Popen.communicate() 会引发 TimeoutExpired if the timeout expires before the process exits.

此模块中定义的异常都继承自 SubprocessError .

3.3 版新增： The SubprocessError 基类被添加。

17.5.2. 安全注意事项 ¶

Unlike some other popen functions, this implementation will never implicitly call a system shell. This means that all characters, including shell metacharacters, can safely be passed to child processes. If the shell is invoked explicitly, via shell=True , it is the application’s responsibility to ensure that all whitespace and metacharacters are quoted appropriately to avoid shell injection vulnerabilities.

当使用 shell=True ， shlex.quote() function can be used to properly escape whitespace and shell metacharacters in strings that are going to be used to construct shell commands.

17.5.3. Popen 对象 ¶

实例化的 Popen 类具有下列方法：

Popen. poll ( ) ¶

校验子级进程是否已终止。设置并返回 returncode 属性。

Popen. wait ( timeout=None ) ¶

等待子级进程终止。设置并返回 returncode 属性。

若进程未终止后于 timeout 秒，引发 TimeoutExpired 异常。捕获此异常并试着等待是安全的。

注意

这会死锁，当使用 stdout=PIPE or stderr=PIPE 子级进程生成足够输出到管道，这会阻塞等待的 OS 管道缓冲以接受更多数据。使用 Popen.communicate() 当使用管道时能避免这种情况。

注意

函数的实现是使用忙循环 (不阻塞调用且短休眠)。使用 asyncio 模块对于异步等待：见 asyncio.create_subprocess_exec .

3.3 版改变： timeout 被添加。

从 3.4 版起弃用： 不使用 endtime parameter. It is was unintentionally exposed in 3.3 but was left undocumented as it was intended to be private for internal use. Use timeout 代替。

Popen. communicate ( input=None , timeout=None ) ¶

Interact with process: Send data to stdin. Read data from stdout and stderr, until end-of-file is reached. Wait for process to terminate. The optional input 参数应该是要发送给子级进程的数据，或为 None , if no data should be sent to the child. The type of input must be bytes or, if universal_newlines was True , a string.

communicate() 返回元组 (stdout_data, stderr_data) .

注意，若想要把数据发送给进程的 stdin，需要创建 Popen 对象采用 stdin=PIPE 。同样，要获取任何东西除了 None 在结果元组，需要给出 stdout=PIPE and/or stderr=PIPE 也。

若进程未终止后于 timeout 秒， TimeoutExpired 异常会被引发。捕获此异常并试着通信不会丢失任何输出。

不杀除子级进程若超时到期，所以为正确清理行为良好的应用程序，应杀除子级进程并完成通信：

proc = subprocess.Popen(...)
try:
    outs, errs = proc.communicate(timeout=15)
except TimeoutExpired:
    proc.kill()
    outs, errs = proc.communicate()
						

注意

读取数据缓冲在内存中，所以不要使用此方法若数据尺寸很大 (或不受限制)。

3.3 版改变： timeout 被添加。

Popen. send_signal ( signal ) ¶

发送信号 signal 到子级。

注意

在 Windows，SIGTERM 是别名化的 terminate() 。可以将 CTRL_C_EVENT 和 CTRL_BREAK_EVENT 发送给进程，启动时采用 creationflags 参数包括 CREATE_NEW_PROCESS_GROUP .

Popen. terminate ( ) ¶

Stop the child. On Posix OSs the method sends SIGTERM to the child. On Windows the Win32 API function TerminateProcess() 被调用以停止子级。

Popen. kill ( ) ¶

Kills the child. On Posix OSs the function sends SIGKILL to the child. On Windows kill() 是别名化的 terminate() .

下列属性也可用：

Popen. args ¶

The args 自变量是被传递给 Popen – 程序自变量 (或单个字符串) 的序列。

3.3 版新增。

Popen. stdin ¶

若 stdin 自变量为 PIPE ，此属性是可写流对象返回通过 open() 。若 universal_newlines 自变量为 True ，流是文本流，否则流是字节流。若 stdin 自变量不是 PIPE ，此属性为 None .

Popen. stdout ¶

若 stdout 自变量为 PIPE ，此属性是可读流对象返回通过 open() 。从流读取提供来自子级进程的输出。若 universal_newlines 自变量为 True ，流是文本流，否则流是字节流。若 stdout 自变量不是 PIPE ，此属性为 None .

Popen. stderr ¶

若 stderr 自变量为 PIPE ，此属性是可读流对象返回通过 open() 。从流读取提供来自子级进程的错误输出。若 universal_newlines 自变量为 True ，流是文本流，否则流是字节流。若 stderr 自变量不是 PIPE ，此属性为 None .

警告

使用 communicate() 而不是 .stdin.write , .stdout.read or .stderr.read 能避免由于任何其它 OS 管道缓冲填满和阻塞子级进程而导致死锁。

Popen. pid ¶

子级进程的进程 ID。

注意：若设置 shell 自变量对于 True ，这是卵生 Shell 的进程 ID。

Popen. returncode ¶

子级返回代码，设置通过 poll() and wait() (和间接通过 communicate() )。 None 值指示进程仍未终止。

负值 -N 指示子级被终止，通过信号 N (仅 POSIX)。

17.5.4. Windows Popen 帮手 ¶

The STARTUPINFO 类和以下常量只可用于 Windows。

class subprocess. STARTUPINFO ¶

部分支持 Windows STARTUPINFO 结构用于 Popen creation.

dwFlags ¶

A bit field that determines whether certain STARTUPINFO attributes are used when the process creates a window.

si = subprocess.STARTUPINFO()
si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW
							

hStdInput ¶

若 dwFlags 指定 STARTF_USESTDHANDLES , this attribute is the standard input handle for the process. If STARTF_USESTDHANDLES is not specified, the default for standard input is the keyboard buffer.

hStdOutput ¶

若 dwFlags 指定 STARTF_USESTDHANDLES , this attribute is the standard output handle for the process. Otherwise, this attribute is ignored and the default for standard output is the console window’s buffer.

hStdError ¶

若 dwFlags 指定 STARTF_USESTDHANDLES , this attribute is the standard error handle for the process. Otherwise, this attribute is ignored and the default for standard error is the console window’s buffer.

wShowWindow ¶

若 dwFlags 指定 STARTF_USESHOWWINDOW , this attribute can be any of the values that can be specified in the nCmdShow parameter for the ShowWindow function, except for SW_SHOWDEFAULT . Otherwise, this attribute is ignored.

SW_HIDE is provided for this attribute. It is used when Popen is called with shell=True .

17.5.4.1. 常量 ¶

The subprocess 模块暴露以下常量。

subprocess. STD_INPUT_HANDLE ¶

The standard input device. Initially, this is the console input buffer, CONIN$ .

subprocess. STD_OUTPUT_HANDLE ¶

The standard output device. Initially, this is the active console screen buffer, CONOUT$ .

subprocess. STD_ERROR_HANDLE ¶

The standard error device. Initially, this is the active console screen buffer, CONOUT$ .

subprocess. SW_HIDE ¶

隐藏窗口。将激活另一窗口。

subprocess. STARTF_USESTDHANDLES ¶

Specifies that the STARTUPINFO.hStdInput , STARTUPINFO.hStdOutput ，和 STARTUPINFO.hStdError attributes contain additional information.

subprocess. STARTF_USESHOWWINDOW ¶

Specifies that the STARTUPINFO.wShowWindow attribute contains additional information.

subprocess. CREATE_NEW_CONSOLE ¶

The new process has a new console, instead of inheriting its parent’s console (the default).

subprocess. CREATE_NEW_PROCESS_GROUP ¶

A Popen creationflags parameter to specify that a new process group will be created. This flag is necessary for using os.kill() on the subprocess.

此标志会被忽略，若 CREATE_NEW_CONSOLE 被指定。

17.5.5. Replacing Older Functions with the subprocess 模块 ¶

In this section, “a becomes b” means that b can be used as a replacement for a.

注意

All “a” functions in this section fail (more or less) silently if the executed program cannot be found; the “b” replacements raise OSError 代替。

In addition, the replacements using check_output() will fail with a CalledProcessError if the requested operation produces a non-zero return code. The output is still available as the output attribute of the raised exception.

In the following examples, we assume that the relevant functions have already been imported from the subprocess 模块。

17.5.5.1. Replacing /bin/sh shell backquote ¶

output=`mycmd myarg`
# becomes
output = check_output(["mycmd", "myarg"])
			

17.5.5.2. Replacing shell pipeline ¶

output=`dmesg | grep hda`
# becomes
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
p1.stdout.close()  # Allow p1 to receive a SIGPIPE if p2 exits.
output = p2.communicate()[0]
			

The p1.stdout.close() call after starting the p2 is important in order for p1 to receive a SIGPIPE if p2 exits before p1.

Alternatively, for trusted input, the shell’s own pipeline support may still be used directly:

output=`dmesg | grep hda`
# becomes
output=check_output("dmesg | grep hda", shell=True)
			

17.5.5.3. Replacing os.system() ¶

sts = os.system("mycmd" + " myarg")
# becomes
sts = call("mycmd" + " myarg", shell=True)
			

注意事项：

• Calling the program through the shell is usually not required.

更现实范例看起来像这样：

try:
    retcode = call("mycmd" + " myarg", shell=True)
    if retcode < 0:
        print("Child was terminated by signal", -retcode, file=sys.stderr)
    else:
        print("Child returned", retcode, file=sys.stderr)
except OSError as e:
    print("Execution failed:", e, file=sys.stderr)
			

17.5.5.4. Replacing the os.spawn 系列 ¶

P_NOWAIT 范例：

pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
==>
pid = Popen(["/bin/mycmd", "myarg"]).pid
			

P_WAIT 范例：

retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
==>
retcode = call(["/bin/mycmd", "myarg"])
			

向量范例：

os.spawnvp(os.P_NOWAIT, path, args)
==>
Popen([path] + args[1:])
			

环境范例：

os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
==>
Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})
			

17.5.5.5. Replacing os.popen() , os.popen2() , os.popen3() ¶

(child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdin, child_stdout) = (p.stdin, p.stdout)
			

(child_stdin,
 child_stdout,
 child_stderr) = os.popen3(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
(child_stdin,
 child_stdout,
 child_stderr) = (p.stdin, p.stdout, p.stderr)
			

(child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
(child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)
			

返回代码处理翻译如下：

pipe = os.popen(cmd, 'w')
...
rc = pipe.close()
if rc is not None and rc >> 8:
    print("There were some errors")
==>
process = Popen(cmd, stdin=PIPE)
...
process.stdin.close()
if process.wait() != 0:
    print("There were some errors")
			

17.5.5.6. Replacing functions from the popen2 模块 ¶

注意

If the cmd argument to popen2 functions is a string, the command is executed through /bin/sh. If it is a list, the command is directly executed.

(child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
==>
p = Popen("somestring", shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)
			

(child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
==>
p = Popen(["mycmd", "myarg"], bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)
			

popen2.Popen3 and popen2.Popen4 basically work as subprocess.Popen ，除了：

• Popen 引发异常若执行失败。
• the capturestderr 自变量被替换采用 stderr 自变量。
• stdin=PIPE and stdout=PIPE 必须指定。
• popen2 closes all file descriptors by default, but you have to specify close_fds=True with Popen to guarantee this behavior on all platforms or past Python versions.

17.5.6. Legacy Shell Invocation Functions ¶

This module also provides the following legacy functions from the 2.x commands module. These operations implicitly invoke the system shell and none of the guarantees described above regarding security and exception handling consistency are valid for these functions.

subprocess. getstatusoutput ( cmd ) ¶

返回 (status, output) 对于执行 cmd 在 Shell。

执行字符串 cmd 在 Shell 采用 Popen.check_output() 并返回 2 元素元组 (status, output) . Universal newlines mode is used; see the notes on 经常使用的自变量 了解更多细节。

A trailing newline is stripped from the output. The exit status for the command can be interpreted according to the rules for the C function wait() 。范例：

>>> subprocess.getstatusoutput('ls /bin/ls')
(0, '/bin/ls')
>>> subprocess.getstatusoutput('cat /bin/junk')
(256, 'cat: /bin/junk: No such file or directory')
>>> subprocess.getstatusoutput('/bin/junk')
(256, 'sh: /bin/junk: not found')
					

可用性：POSIX & Windows

3.3.4 版改变： 添加 Windows 支持

subprocess. getoutput ( cmd ) ¶

返回输出 (stdout 和 stderr) 对于执行 cmd 在 Shell。

像 getstatusoutput() , except the exit status is ignored and the return value is a string containing the command’s output. Example:

>>> subprocess.getoutput('ls /bin/ls')
'/bin/ls'
					

可用性：POSIX & Windows

3.3.4 版改变： 添加 Windows 支持

17.5.7. Notes ¶

17.5.7.1. Converting an argument sequence to a string on Windows ¶

在 Windows， args sequence is converted to a string that can be parsed using the following rules (which correspond to the rules used by the MS C runtime):

1. Arguments are delimited by white space, which is either a space or a tab.
2. A string surrounded by double quotation marks is interpreted as a single argument, regardless of white space contained within. A quoted string can be embedded in an 自变量。
3. A double quotation mark preceded by a backslash is interpreted as a literal double quotation mark.
4. Backslashes are interpreted literally, unless they immediately precede a double quotation mark.
5. If backslashes immediately precede a double quotation mark, every pair of backslashes is interpreted as a literal backslash. If the number of backslashes is odd, the last backslash escapes the next double quotation mark as described in rule 3.

另请参阅

shlex

提供剖析和转义命令行函数的模块。

内容表

• 17.5. subprocess — 子进程管理
  • 17.5.1. 使用 subprocess 模块
    • 17.5.1.1. 频繁使用的自变量
    • 17.5.1.2. Popen 构造函数
    • 17.5.1.3. 异常
  • 17.5.2. 安全注意事项
  • 17.5.3. Popen 对象
  • 17.5.4. Windows Popen 帮手
    • 17.5.4.1. 常量
  • 17.5.5. Replacing Older Functions with the subprocess 模块
    • 17.5.5.1. Replacing /bin/sh shell backquote
    • 17.5.5.2. Replacing shell pipeline
    • 17.5.5.3. Replacing os.system()
    • 17.5.5.4. Replacing the os.spawn 系列
    • 17.5.5.5. Replacing os.popen() , os.popen2() , os.popen3()
    • 17.5.5.6. Replacing functions from the popen2 模块
  • 17.5.6. Legacy Shell Invocation Functions
  • 17.5.7. Notes
    • 17.5.7.1. Converting an argument sequence to a string on Windows

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