Entry

How can I get a file's permissions?

Oct 8th, 2006 04:14
James Stroud, unknown unknown, Fredrik Lundh, James Stroud

Problem:

I have been able to get a file's owner and group IDs using the os.stat 
function but it doesn't seem to give me the file's permissions.

Solution:

the first member of the stat tuple (ST_MODE) contains the permissions, 
as a bitmask.

you can use the functions and macros in the "stat" module to decipher 
them.

    st = os.stat(myfile)
    mode = st[stat.ST_MODE]
    if mode & stat.S_IREAD:   # same as stat.S_IRUSR
        print "readable"
    if mode & stat.S_IWRITE:  # same as stat.S_IWUSR
        print "writable"
    if mode & stat.S_IEXEC:   # same as stat.S_IXUSR
        print "executable"


Note that (within stat):

   (S_IREAD, S_IWRITE, S_IEXEC) == (S_IRUSR, S_IWUSR, S_IXUSR)


Also note the following identities:

   (S_IRUSR >> 2) == (S_IWUSR >> 1) == S_IXUSR
   (S_IRGRP >> 2) == (S_IWGRP >> 1) == S_IXGRP
   (S_IROTH >> 2) == (S_IWOTH >> 1) == S_IXOTH

And, for symmetry:

   (S_IRUSR >> 6) == (S_IRGRP >> 3) == S_IROTH
   (S_IWUSR >> 6) == (S_IWGRP >> 3) == S_IWOTH
   (S_IXUSR >> 6) == (S_IXGRP >> 3) == S_IXOTH


Note as well these identities (in *NIX):

   S_IRWXU == (S_IRUSR | S_IWUSR | S_IXUSR) == 0700
   S_IRWXG == (S_IRGRP | S_IWGRP | S_IXGRP) == 0070
   S_IRWXO == (S_IROTH | S_IWOTH | S_IXOTH) == 0007

      0777 == (S_IRWXU | S_IRWXG | S_IRWXO) == 0777

Its should be obvious how the latter is equivalent to "a+rwx".


After studying the above, the following illustrative function for adding
permissions to a file might begin to make sense:

def addmodes(thepath, modes):
  """
  thepath is a path to a file or directory (a str type)
  modes is a tuple of modes from the stat module
  """
  import operator
  import stat
  st = os.stat(thepath)[stat.ST_MODE]
  os.chmod(thepath, reduce(operator.or_, (st,) + modes))


Problem cont.:

Thanks for your help but I only seem to be able to check whether or not 
the user running the script has "rwx" permissions.  I am looking to find 
the permissions for owner/group/other, preferably in numeric mode.

Solution:

did you try printing the "mode" value?  I'm pretty sure it does contain 
the "numeric mode" you're looking for.
try this:

    st = os.stat(myfile)
    mode = st[stat.ST_MODE]
    print "mode is", oct(mode & 0777)


(looking in the stat module source file may also help; it's in Python's 
Lib directory)