Today's state of work-brain:
mkdir /tmp/fred
umask 077 /tmp/fred
Immediately after these two commands, I hit cursor-up to change the '
umask
' to '
chmod
', so that I then ran '
chmod 077 /tmp/fred
'. Fortunately I was doing this as a regular user, so my next action exposed my error.
This whole sequence of commands is a set of mistakes jumbled together in a very Unix way. My goal was to create a new
/tmp/fred
directory that was only accessible to me. My second command is not just wrong because I wanted
chmod
instead of
umask
(I should have run
umask
before the
mkdir
, not after), but because I had the wrong set of permissions for
chmod
. It was as if my brain wanted Unix to apply a '
umask 077
' to the creation of
/tmp/fred
after the fact. Since the numeric permissions you give to
umask
are the inverse of the permissions you give to
chmod
(you tell
umask
what you don't want instead of what you do), my change of
umask
to
chmod
then left
/tmp/fred
with completely wrong permissions; instead of being only accessible to me, it was fully accessible to everyone except me.
(Had I been doing this as root, I would then have been able to
cd
into the directory, put files in it, access files in it, and so on, and might not have noticed that the permissions were reversed from what I actually wanted.)
The traditional Unix
umask
itself is a very Unix command (well, shell built-in), in that it more or less directly calls
umask()
. This allows a very simple implementation, which was a priority in early Unixes like V7. A more sensible implementation would be that you specify effectively the maximum permissions that you want (for example, that things can be '755') and then
umask
would invert this to get the value it uses for
umask()
. But early Unixes took the direct approach, counting on people to remember the inversion and perform it in their heads.
In the process of writing this entry I learned that POSIX
umask
supports symbolic modes, and that they work this way. You get and set umask modes like '
u=rwx,g=rx,o=rx
' (aka '022', the traditional friendly Unix umask), and they're the same permissions as you would use with
chmod
. I believe that this symbolic mode is supported by any modern Bourne compatible shell (including
zsh
), but it isn't necessarily supported by non-Bourne shells such as
tcsh
or
rc
( which is my shell ).