This is advanced chmod.  This file is quick and dirty and doesn't go into
as much detail about certain permissions as the standard chmod help file
does so it's harder to follow for new *nix users.  If you haven't done so
already please read the chmod and chown help txt first.

In chmod we talked about Read Write and eXecute permissions.  There is also
some more that you might file handly when the need arsises.  Well work
with the same two things as the other help txts also.

~# ls -l 
total 456 
drwx------    7 root     root         4096 Feb 24 05:50 somedir
-rw-rw-rw-    4 root     root          323 Feb 18 09:10 somefile

The first extra chmod permission we have is suid and sgid which stand 
for Set User ID and Set Group ID respectivly.  When applied to a file
they make no sense if the file isn't marked as eXecutable so if I apply
one to somefile chmod u+s somefile I'd get:

-rwSrw-rw-    4 root     root          323 Feb 18 09:10 somefile

The upper case S is there to highlight that fact that somefile is not
marked +x.  If we apply a +x to somefile chmod u+x somefile it would change:

-rwsrw-rw-    4 root     root          323 Feb 18 09:10 somefile

Which is set properly now.  Setting the suid or sgid of a file
will allow that file to run as the user or as the group to which it is
applied.  So in this example somefile is a program, it will run with
root permissions so it might be able to access a device or other file
that normally only root has access to such as /etc/passwd.  This isn't
as dangerous as it sounds and it quite common, remember that only the
owner of a file or directory can change it's permissions so it's quite safe.
Just don't go applying to to every program you have.

However when they are applied to a directory chmod u+s somedir

drws------    7 root     root         4096 Feb 24 05:50 somedir

Everything that I place in this directory from here on in until I change it
will be owned by root since root is the owner of the directory.  The same
would go for the group, chmod g+s somedir

drws--S---    7 root     root         4096 Feb 24 05:50 somedir

Of course this directory is not marked +x so it's an upper case S which
doesn't matter in this example anyway.  Now every file or directory I place
in somedir will be owned by the user root and the group root.


The next one in the chain is the sticky bit.  Since just about every other
letter is taken the sticky bit uses of all things 'T'.  The sTicky bit
sounds rather useless, in reality it's quite useful in the right cases.
The sTicky but will make any file or directroy that is marked as such stay
in the swap file for quick access to it's contents.  The /tmp directory
is usually marked +t for this reason.  The sTicky bit can only be applied
by root for obvious reasons and is only used by the User flag, it shows up as
chmod +t somedir

drws--S--t    7 root     root         4096 Feb 24 05:50 somedir

Same as with the suid or sgid if the directory or file is not marked +x
the sTicky bit will appear as upper case, but since it is applied to the
user class and root has eXecute permissiom to this directory it's lower case.


Absolutes:  I suppose since in chmod we worked with absolute permissions
I should include them in here also.  suid is 4000, sgid is 2000 and sTicky
is 1000.  So to set the suid, world eXecute and user Read permissions
for somefile we would use this:  chmod 4511 somefile producing

dr-s--x--x    7 root     root         4096 Feb 24 05:50 somefile

And since I didn't cover this in chmod if you leave off the #000 and only use
three numbers for absolute chmod assumes you aren't changing them
There, quick and dirty without a lot of detail.  Be sure to reread chmod
if some of this seems confusing.