Use /usr/bin/env python ... why?

[Steve Holden]

Tim Cargile" <tecargile at hotmail.com> wrote ...
> There was a previous thread on this that referenced a python.org FAQ
> that I thought was incomplete.  As a casual observer I would
> like to submit the following - in the order of most to least efficient
> and least to most flexible (perhaps):
>
> Use '#!/usr/bin/python' when:
>
>       The location of 'python' can be or is guaranteed to
>       be '/usr/bin' AND one one wants the least execution
>       overhead AND does not wish to alter the 'python' runtine
>       environment. Or when PATH searching is not possible
>       for some reason (restricted environment - cron job).
>
> Use '#!/python' when:
>
Don't think you want a leading slash in there, old chap. Very few people
actually put the interpreter in the root directory (though of course we all
know that's where it belongs :-)

>       The location of 'python' cannot be guaranteed to be 'usr/bin'
>       (or any other locaton) AND one one is willing to incur the
>       overhead of a PATH search AND a PATH search is possible
>       AND does not wish to alter the 'python' runtine environment.
>       I have tested this under Cygin with both 'ksh' and 'python'
>       and it works as one would expect.
>
That's as may be, but (for example) Red Hat 8.0 certainly won't have any of
that. I get the same result whether the slash is included or not:

[sholden at munger sholden]$ cat pytest
#!python

print "Hello";

[sholden at munger sholden]$ ls -l pytest
-rwxrwxr-x    1 sholden  sholden        26 Jan  9 16:28 pytest
[sholden at munger sholden]$ ./pytest
bash: ./pytest: python: bad interpreter: No such file or directory
[sholden at munger sholden]$ python
Python 2.2.1 (#1, Aug 30 2002, 12:15:30)
[GCC 3.2 20020822 (Red Hat Linux Rawhide 3.2-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

"python pytest" works just fine, of course.

> Use '#!/usr/bin/env python' when:
>
>       The location of 'env' is guaranteed to be '/usr/bin'
>       AND when it is desired to alter the environment prior
>       to execution of 'python' AND when one is willing to
>       incur the overhead (not as minimal as a PATH search)
>       of starting 'env' and one cannot guarantee the location
>       of 'python' and one is willing to incur the (minimal)
>       overhead of a PATH search for 'python'.  Incidentally,
>       I could measure the performance hit (in 10ths of seconds)
>       of running with the startup overhead of 'env' on my
>       900 Mhz laptop under Cygwin.  On a System with 2000 users
>       (not even my laptop) it might make a difference).
>
> Use '#!env python' when:
>
>       The location of 'env' cannot be guaranteed AND the
>       location of 'python' cannot be guaranteed AND
>       it is desired to alter the environment prior
>       to execution of 'python' AND one is willing to
>       incur the overhead (admittedly minimal) of searching
>       the PATH for 'env' AND starting 'env' (not so minimal)
>       and serching the PATH for 'python'.  A lot of 'AND's in this one.
>
I think this will have the same problems outlined above for "python".

> It appears as though the PRIMARY intent of the 'env' utility
> is to alter the environment - not force a PATH search for the
> executable.  I have never seen a "#!" line that actually altered
> the environment.  Has anyone?
>
> Oh well, at least I have not seen these:
>
>  #!env /usr/bin/python
>
>    and
>
> #!/usr/bin/env /usr/bin/python
>
> These works also, but have yet to observe them in the wild.
>
> I'm wondering why I see so many (like 100%) '#!/usr/bin/env python'
> lines ... in the Cygwin release, at least.  I don't believe this
> is a religious issue.  It is simple mechanics/logic.  Computers
> are only machines ... except for the UNIVAC ... it was God.
>
Mostly you see it because it's about the most likely to succeed, I suspect.
Sure, it has problems, but then what doesn't?

regards
--
Steve Holden                                  http://www.holdenweb.com/
Python Web Programming                 http://pydish.holdenweb.com/pwp/
Bring your musical instrument to PyCon!    http://www.python.org/pycon/

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