python 2.7.12 on Linux behaving differently than on Windows

Steve D'Aprano steve+python at pearwood.info
Mon Dec 5 12:12:01 EST 2016


On Tue, 6 Dec 2016 02:41 am, BartC wrote:

> In that case forget Windows vs. Linux, you now have a program that will
> get command parameters processed differently depending on whether it was
> invoked from a shell or not.

Er, yeah? You say that as if it were a bad thing.

Look at it this way. Suppose you were a really cunning system administrator
who spent 8 or 10 hours a day typing in commands and arguments that were
frequently file names. So you built yourself a macro system where you type
a command:

    foo bar baz

and the macro would walk through the command line, automatically expanding
environment variables and file name globs. And because you used this 95% of
the time, and it was intended as a convenience for interactive use, you
linked the macro to the Enter key, so that only a single key press was
needed to do this processing.

But of course you're not an idiot, you know full well that there are
occasions where you want to avoid the expansion. So you include commands to
disable and enable that macro, and to give you even more fine-grained
control of what is expanded where, you build it escaping mechanisms so that
you can expand part of the command line and not other parts.

And then I come along, full of righteous indignation, and start yelling

"Wait a minute, now your application gets its command parameters processed
differently depend on whether you called it from C or used your macro!"

And you would answer:

"Of course it does, that's the whole bloody point of the macro!"

(By the way, I think that you will find that when you call Popen, if you set
shell=True it will be invoked in a subshell, which means you'll get the
full shell experience including command expansion. For good and evil.
That's a security risk, if you're getting the arguments from an untrusted
source, so don't pass shell=True unless you know what you're doing.)


-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.



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