Charles Daffern added the comment:
>To be sure that it is existing program, you can use shutil.which()
I'd like to clear this up a little because this is worded as if
shutil.which()'s success implies that the shell will not fail.
Here is the setup to demonstrate:
>>> import os, shlex, shutil, subprocess
>>> open("do", "w").write("#!/bin/sh\necho Something is being done...")
__main__:1: ResourceWarning: unclosed file <_io.TextIOWrapper name='do'
mode='w' encoding='UTF-8'>
41
>>> os.chmod("do", 0o700)
Here is the behaviour using shlex.quote:
>>> subprocess.call(shlex.quote("do"), shell=True, env={'PATH': '.'})
/bin/sh: 1: Syntax error: "do" unexpected
2
Here is the behaviour when quoting properly:
>>> subprocess.call("'do'", shell=True, env={'PATH': '.'})
Something is being done...
0
Here is the output of shutil.which:
>>> shutil.which("do", path=".")
'./do'
So checking shutil.which()'s success or failure will not guard against this
case (though using its output would work around the problem).
>It's not at all obvious that the intention is to ensure such an argument
>should be treated only as a command external to the shell.
>
>If an application really wants to ensure the command is not handled as a shell
>built-in, it should use shell=False.
The shell will still search builtins if the argument is quoted, it just won't
search for keywords. So, a quoted "bind", "shopt" or "jobs" will still work,
but a quoted "case", "fi" or "done" will cause the shell to search for a
command of that name rather than treating it as syntax.
Looking at the source, shlex.quote's refusal to quote certain arguments appears
to be intentional. I would rather it quote slightly more carefully than
necessary, than quote something incorrectly.
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<http://bugs.python.org/issue26124>
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