public folder file permissions

Gavin McCullagh gmccullagh at
Wed Apr 18 09:41:43 BST 2007


On Tue, 17 Apr 2007, Ian Moore wrote:

> File access permissions. Basically we have about 5 machines which users
> log onto with using a location specific user account.

It's up to you but I try to avoid location-specific accounts like the
plague.  It keeps people in that tortured "file X is on computer Y only"
mentality.  If they are given their own accounts they can have their own
files and settings whichever computer they sit down at.  I'm sure you have
your reasons of course, but if at all possible, I'd give people individual
accounts.  However, that doesn't solve this problem:

> As people often work on the same documents and files but at different
> times and from different workstations we saved most things to a shared
> directory called 'public'.

First an executive summary:

 - make sure all involved users are in some group 
 - use chgrp to set the group of the shared directory
 - use chmod to set the setgid bit and give write access to the group on
   your shared directory.
 - set everyone's default umask to 0002 in /etc/profile

Now an explanation:

There's probably two things you need to look at.  One is which group files
are created with by default (setgid on directory) and the other is the
permissions files are created with (umask).  What I presume you want is a
situation like this:

gavinmc at boing:~$ ls -la /shared/
total 8
drwxrwsr-x  2 root    admin 4096 2007-04-18 09:09 .
drwxr-xr-x 21 root    root  4096 2007-04-18 09:03 ..
-rw-rw-r--  1 gavinmc admin    0 2007-04-18 09:03 somefile

So /shared is owned by "root", with the group "admin" and somefile has been
created by gavinmc allowing members of the group "admin" write access.  The
group "admin" has write access so all of those users can create files in
/shared.  Also, the setgid bit is set on /shared which means that new files
created in that folder will automatically have the admin group associated.  

This is done with
	sudo mkdir /shared
	sudo chgrp admin /shared
	sudo chmod g+w /shared
	sudo chmod g+s /shared

Now, the next thing is, when a user "sarah" creates a file, the group must
have write access to the file.  As Denis says, umask is what you want here.
Every user session has a "umask" which dictates what permissions are given
to files they create.  The norm is 022, meaning those in the file's group
and others get read access but not write access.  Below I create a file
somefile3, then change my umask and create another.  Note in the second
case that the group "admin" gets write access.

gavinmc at boing:~$ umask 
gavinmc at boing:~$ touch /shared/somefile3
gavinmc at boing:~$ ls -la /shared/somefile3 
-rw-r--r-- 1 gavinmc admin 0 2007-04-18 09:17 /shared/somefile3
gavinmc at boing:~$ umask 0002
gavinmc at boing:~$ touch /shared/somefile4
gavinmc at boing:~$ ls -la /shared/somefile4
-rw-rw-r-- 1 gavinmc admin 0 2007-04-18 09:18 /shared/somefile4

If you look at the last lines of /etc/profile, the umask for all users gets
set there.  Edit the file and change the umask command from 0022 to 0002.
Then logout and log back in again.  At a shell, type umask to make sure you
get 0002.  Then create a file and you should see group write permissions on
the new files you create.

As /etc/profile sets everyones umask, the same should now be true for
everyone else.

> The partial solution we found was to connect to this public directory
> using a samba share, shortcut link located on the desktop. This was
> already setup anyway for our Window machine on the network. 

This is due to the way samba maps unix to windows file permissions.  There
is a "create mask" setting in smb.conf which you can modify similarly to
linux umask which sets the permissions created when files are created over
a samba share.

One final warning.  You should explain to people that they must be careful
two people do not edit files at the same time.  Linux itself (unlike
windows) doesn't usually enforce file locking.  This can be good and bad,
depending on the situation.


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