The 4th bit is used in cases such as ftp where you want to temporarily 
connected user to have the same writes as the owner for the duration of 
the connection.  You may notice sometimes that permissions are listed 
as:  - r - x r - s r - -

That is where the "s" comes in.  The logged in user will temporarily 
have the read and execute group rights.  Now, the 4000 value would put 
the "S" permission in the user place.  The 2000 value puts it in the 
group place, while the 1000 value is something completely different -- 
the sticky bit "T" which means that this file cannot be deleted without 
some effort.  That feature is arcane.

Garrett
 
Todd Young wrote:

> OK, I've been playing with Linux for a year, I was baptized on 
> Irix(for 4 years).....why the extra digit in the permissions below, 
> specifically the 2? I thought it was "user-group-everyone", what's the 
> fourth place for? Thanks.
>
> David Dyer-Bennet wrote:
>
>> Mine is chmod 2775 owner root.wheel, mostly (except a lot of the
>> directories in it are owned by different users, and most are in group
>> wwwadmin).  Works fine, has for years.  
>
>




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