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On Unix, with *shell=False* (default): In this case, the Popen class uses
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:meth:`os.execvp` to execute the child program. *args* should normally be a
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sequence. A string will be treated as a sequence with the string as the only
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item (the program to execute).
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On Unix, with *shell=True*: If args is a string, it specifies the command string
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to execute through the shell. If *args* is a sequence, the first item specifies
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the command string, and any additional items will be treated as additional shell
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sequence. If a string is specified for *args*, it will be used as the name
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or path of the program to execute; this will only work if the program is
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being given no arguments.
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:meth:`shlex.split` can be useful when determining the correct
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tokenization for *args*, especially in complex cases::
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>>> import shlex, subprocess
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>>> command_line = raw_input()
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/bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'"
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>>> args = shlex.split(command_line)
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['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"]
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>>> p = subprocess.Popen(args) # Success!
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Note in particular that options (such as *-input*) and arguments (such
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as *eggs.txt*) that are separated by whitespace in the shell go in separate
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list elements, while arguments that need quoting or backslash escaping when
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used in the shell (such as filenames containing spaces or the *echo* command
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shown above) are single list elements.
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On Unix, with *shell=True*: If args is a string, it specifies the command
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string to execute through the shell. This means that the string must be
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formatted exactly as it would be when typed at the shell prompt. This
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includes, for example, quoting or backslash escaping filenames with spaces in
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them. If *args* is a sequence, the first item specifies the command string, and
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any additional items will be treated as additional arguments to the shell
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itself. That is to say, *Popen* does the equivalent of::
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Popen(['/bin/sh', '-c', args[0], args[1], ...])
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On Windows: the :class:`Popen` class uses CreateProcess() to execute the child
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program, which operates on strings. If *args* is a sequence, it will be
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needed: Usually, the program to execute is defined by the *args* argument. If
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``shell=True``, the *executable* argument specifies which shell to use. On Unix,
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the default shell is :file:`/bin/sh`. On Windows, the default shell is
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specified by the :envvar:`COMSPEC` environment variable.
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specified by the :envvar:`COMSPEC` environment variable. The only reason you
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would need to specify ``shell=True`` on Windows is where the command you
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wish to execute is actually built in to the shell, eg ``dir``, ``copy``.
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You don't need ``shell=True`` to run a batch file, nor to run a console-based
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*stdin*, *stdout* and *stderr* specify the executed programs' standard input,
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standard output and standard error file handles, respectively. Valid values
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Doc/library/subprocess.rst
