Why is the argument of os.umask() inverted? (umask 0o000 makes chmod 0o777)
There is no real inconsistency, as the relation between umask
and chmod
can purely be written down with equations. Apparently, umask
sets the opposite of chmod
, it was created like this back in the old days.
Example: 022
(the default usual umask
) creates 755
. It works like this:
7 - 0 = 7
becomes the first byte7 - 2 = 5
becomes the second and third bytes
Using this example, umask 777
creates a file with chmod 000
, umask 112
will be equal to chmod 664
. As far as I know, this happened because the umask
command was originally created to indicate what permission bits the file will NOT have after it's created (hence the invertion).
While it could be annoying, it's really not hard to get used to it. Just think how you would chmod
your files, and subtract the byte you want from 7
, and you will get the umask
value. Or, when you are at the IDE, writing your code, don't use umask
, but rather create the file (with the default umask
of course) and then use, in Python, os.chmod()
instead.