PEP 324 — subprocess 新的進程模組（subprocess – New process module）

用 subprocess 模組替換舊函數（Replacing older functions with the subprocess module）

本節中的「a ==> b」表示可以將 b 換用 a。

注意：如果找不到被執行的程式，本節中的所有函數（或多或少）都會靜默地失敗；本模組將觸發 OSError 異常。

以下示例假設 subprocess 模組是用 from subprocess import * 語句導入的。

替換反引號包裹的 /bin/sh shell 命令

    output=`mycmd myarg`
    ==>
    output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]

替換 shell 管道

    output=`dmesg | grep hda`
    ==>
    p1 = Popen(["dmesg"], stdout=PIPE)
    p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
    output = p2.communicate()[0]

替換 os.system()

    sts = os.system("mycmd" + " myarg")
    ==>
    p = Popen("mycmd" + " myarg", shell=True)
    sts = os.waitpid(p.pid, 0)

注意：

• 通常沒有必要通過 shell 調用程式。
• 查看 returncode 屬性要比查看退出狀態更為容易。

現實中的程式碼實例可能會如下所示：

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

替換os.spawn*

P_NOWAIT 示例：

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

P_WAIT 示例：

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

Vector 示例：

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

環境變數示例：

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

替換 os.popen*

    pipe = os.popen(cmd, mode='r', bufsize)
    ==>
    pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout

    pipe = os.popen(cmd, mode='w', bufsize)
    ==>
    pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin


    (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)

替換popen2.*

注意：如果 popen2 函數的 cmd 參數是個字元串，則命令將通過 /bin/sh 執行。如果是個列表，則直接執行命令。

    (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和popen3.Popen4的工作方式基本和subprocess.Popen一樣，除了：

• 如果執行失敗，subprocess.Popen將觸發異常。
• capturestderr參數將用 stderr 替換。
• stdin=PIPE和stdout=PIPE必須指定。
• popen2默認會關閉所有文件描述符，而 subprocess.Popen 則必須指定close_fds=True才行。