Need Simple Way To Determine If File Is Executable

Discussion in 'Python' started by Tim Daneliuk, Dec 14, 2006.

  1. Tim Daneliuk

    Tim Daneliuk Guest

    I have a program wherein I want one behavior when a file is set as executable
    and a different behavior if it is not. Is there a simple way to determine
    whether a given named file is executable that does not resort to all the
    lowlevel ugliness of os.stat() AND that is portable across Win32 and *nix?

    Thanks,
     
    Tim Daneliuk, Dec 14, 2006
    #1
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  2. os.access(pathToFile, os.X_OK)
     
    John McMonagle, Dec 14, 2006
    #2
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  3. That won't work on Windows.

    You have to define what do you mean by "a file is set as executable"
    on Windows.
    a.exe is executable and nobody would discuss that. I can supress the
    extension and type simply: a, on the command line, and get a.exe
    executed. Same for a.com
    What about a.bat? cmd.exe is executed and runs the batch file. I can
    even omit the extension. Is a.bat executable then?
    What about a.py? Another process starts and handles the file
    (python.exe). Is a.py executable then?
    I can type a.mdb on the command prompt and launch an Access
    application. Is a.mdb executable then?
    If I type a.doc on the command prompt, Word is executed and opens
    that file. Is a.doc executable then?

    The answer may be so narrow to just consider .exe .com and a few
    more, or so broad to consider all things that os.startfile can handle
    without error.


    --
    Gabriel Genellina
    Softlab SRL

    __________________________________________________
    Correo Yahoo!
    Espacio para todos tus mensajes, antivirus y antispam ¡gratis!
    ¡Abrí tu cuenta ya! - http://correo.yahoo.com.ar
     
    Gabriel Genellina, Dec 15, 2006
    #3
  4. Tim Daneliuk

    Tim Golden Guest

    [Tim Daneliuk]
    I'm fairly certain the answer is no. What follows is a
    relatively low-level and certainly not portable discussion.

    The last couple of times this question came up on the list
    I looked into the implementation and experimented a bit
    but in short I would say that os.stat / os.access were
    near enough useless for determining executablility under
    Windows. That's not down to Python as such; it's simply
    passing back what the crt offers.

    Of course that raises the slightly wider issue of: should
    the Python libs do more than simply call the underlying
    crt especially when that's known to give, perhaps misleading
    results? But I'm in no position to answer that.

    I suggest that for Windows, you either use the PATHEXT
    env var and determine whether a given file ends with
    one of its components. Or -- and this depends on your
    definition of executable under Windows -- use the
    FindExecutable win32 API call (exposed in the win32api
    module of pywin32 and available via ctypes) which will
    return the "executable" for anything which has an
    association defined. So the "executable" for a Word
    doc is the winword.exe program. The "executable" for
    an .exe is itself.

    TJG
     
    Tim Golden, Dec 15, 2006
    #4
  5. Tim Daneliuk

    Tim Daneliuk Guest

    This seems to work, at least approximately:

    os.stat(selected)[ST_MODE] & (S_IXUSR|S_IXGRP|S_IXOTH

    It probably does not catch every single instance of something
    that could be considered "executable" because this is a sort
    of fluid thing in Windows (as you point out).
     
    Tim Daneliuk, Dec 15, 2006
    #5
  6. Tim Daneliuk

    Tim Roberts Guest

    This will tell you that "x.exe" is executable, even if "x.exe" contains
    nothing but zeros.

    On the other hand, I'm not convinced that any other solution is better.
     
    Tim Roberts, Dec 16, 2006
    #6
  7. Isn't the same with any other recipe, portable or not? Unless the OS
    actually tries to load and examine the file contents, which the OS's
    I'm aware of, don't do.
     
    Gabriel Genellina, Dec 16, 2006
    #7
  8. Tim Daneliuk

    Tim Roberts Guest

    Yes, of course, you're right. I was about to delve into a philosophical
    discussion about the difference in handling this between Linux and Windows,
    but they're both just conventions. One is based on an arbitrary flag, one
    is based on a file extension. Contents are irrelevant.
     
    Tim Roberts, Dec 17, 2006
    #8
  9. Tim Daneliuk

    Roger Upole Guest

    On windows, you can use win32file.GetBinaryType to check if a file is actually
    a binary executable.

    Roger
     
    Roger Upole, Dec 17, 2006
    #9
  10. Tim Daneliuk

    Tim Daneliuk Guest

    Yabut ... what about things like batch files? Does it return them
    as executable as well?
     
    Tim Daneliuk, Dec 17, 2006
    #10
  11. Tim Daneliuk

    Roger Upole Guest

    No, it's strictly for binary executables.

    Roger
     
    Roger Upole, Dec 18, 2006
    #11
  12. A similar function exists on Linux too. But even if a file has the
    right file format, if it does not have the execute bit set, won't run.
    And you could set that bit on a JPG image too - and nothing good would
    happen, I presume. So one must determine first what means "the file is
    executable".
     
    Gabriel Genellina, Dec 18, 2006
    #12
  13. Tim Daneliuk

    Tim Daneliuk Guest

    Well... sure, but that isn't the point. Here is the problem I was
    trying to solve:

    I wrote and maintain the 'twander' cross-platform file browser:

    http://www.tundraware.com/Software/twander/

    I was working on a new release and wanted to add file associations
    to it. That is, if the user selected a file and double clicked or
    pressed Enter, I wanted the following behavior (in the following
    steps, "type" means nothing more than "a file whose name ends with
    a particular string"):

    1) If an association for that file type exists, run the associated program.

    2) If an association for that file type does not exist:

    a) If the file is not "executable", see if there is a "default"
    association defined and run that program if there is.

    b) If the file *is* "executable", run it.


    So ... all I really needed to know is whether or not the OS thinks the
    file is executable. Obvious - and this is true on most any system -
    you can create the situation where the file appear executable from
    the OS's point of view, but it is not actually. But this is a pathology
    that no application should really be expected to cope with...
     
    Tim Daneliuk, Dec 18, 2006
    #13
  14. This is what os.startfile does. The underlying Win32 functions would
    be ShellExecute, FindExecutable & their variants.
    Will you maintain your own registry for associations?


    --
    Gabriel Genellina
    Softlab SRL

    __________________________________________________
    Correo Yahoo!
    Espacio para todos tus mensajes, antivirus y antispam ¡gratis!
    ¡Abrí tu cuenta ya! - http://correo.yahoo.com.ar
     
    Gabriel Genellina, Dec 18, 2006
    #14
  15. Tim Daneliuk

    Tim Daneliuk Guest

    And on Windows, that's exactly what I use.
    Yes, because I want common configuration syntax and runtime
    semantics across FreeBSD, Linux, Windows, et al. The only
    semantic difference is that, on Windows, if my own association
    is not found, then the Windows association will apply. This
    cannot be done in the *nix environment - at least not easily -
    because there is no common association repository across the
    various window managers, nor is there a startfile() type
    call in the POSIX world.


    This is implemented already and largely works as planned.
    There are a few subtleties I want to work into the next
    release, but things work as expected today...
     
    Tim Daneliuk, Dec 18, 2006
    #15
  16. Really? I don't think so. Afaik on Linux executable binary files need an
    ELF header.

    [[email protected]]-[10:15:43] >> ~/Bilder
    --> chmod a+x VM-Background.png

    [[email protected]]-[10:15:46] >> ~/Bilder
    --> ./VM-Background.png
    bash: ./VM-Background.png: cannot execute binary file

    As you can see, binary files without such a header are not executed...

    Bye
    lunar
     
    Sebastian 'lunar' Wiesner, Dec 20, 2006
    #16
  17. No, they aren't! Try this:

    [[email protected]]-[10:24:44] >> ~/test
    --> dd if=/dev/zero of=test.sh count=1
    1+0 records in
    1+0 records out
    512 bytes (512 B) copied, 6.5e-05 seconds, 7.9 MB/s

    [[email protected]]-[10:24:46] >> ~/test
    --> chmod a+x test.sh

    [[email protected]]-[10:24:55] >> ~/test
    --> ./test.sh
    bash: ./test.sh: cannot execute binary file

    A file containing only zeros isn't executed...
     
    Sebastian 'lunar' Wiesner, Dec 20, 2006
    #17
  18. you're confusing the shell's "is this file executable" check with the
    loader's "can I execute this file" check:

    $ export PATH=.:$PATH
    $ dd if=/dev/zero of=ls count=1
    1+0 records in
    1+0 records out
    $ ls -l ls
    -rw-rw-r-- 1 slab slab 512 Dec 20 03:33 ls
    $ chmod a+x ls
    $ ls
    -bash: ./ls: cannot execute binary file

    </F>
     
    Fredrik Lundh, Dec 20, 2006
    #18
  19. ???
    Am I blind or is there really no difference between you shell example an
    mine?
    As far as I can see, you are doing exactly the same thing as I did...
    So what are trying to proof?

    Sebastian
     
    Sebastian 'lunar' Wiesner, Dec 20, 2006
    #19
  20. There are other executable loaders for `a.out` and `COFF` in the kernel,
    and with the `binfmt_misc` module you can make anything with a "magic"
    header executable, including Python scripts/bytecode and even JPEG images.

    http://www.tat.physik.uni-tuebingen.de/~rguenth/linux/binfmt_misc.html

    Ciao,
    Marc 'BlackJack' Rintsch
     
    Marc 'BlackJack' Rintsch, Dec 20, 2006
    #20
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