________  SWAT MAGAZINE ISSUE TWENTY-FOUR DECEMBER 1999 ________
  /        \______________________________________________/        \
 /              Guide to the UNIX command line - part 2             \
/                    By qwaszx <qwaszx@europe.com>                   \
-----------------------------------------------------------------------

Last month I started this tutorial, describing some of the commands you
can use in UNIX, and introducing the concept of files along with
copying, moving and deleting. This article will continue with the idea
of file permissions...

If you log in to your UNIX account and type in the following command:

ls -l

then you should get a directory listing of your home directory, which
should look something like

drwx------   3 qwaszx   users        1024 Nov 17 12:23 bin
-rw-------   1 qwaszx   users         747 Nov  9 17:23 dead.letter
drwx------   2 qwaszx   users        1024 Nov 17 13:50 incoming
drwx------   2 qwaszx   users        1024 Nov 14 01:52 mail
-rw-------   1 qwaszx   users        1558 Nov 20 19:52 mbox
drwx--x--x   5 qwaszx   users        1024 Aug 27 12:59 public_html

Now the important thing in this case is all that gobbledegook on the
left. That is showing the permissions of all your files, ie. who
can access them.

First off - an explanation of how permissions work...

There are three types of user that you can set permissions for. These
are:
	User  - that's you
	Group - that's everyone who is in your 'group'. This could just
                be you, or it could be everyone on the server. it just
                depends on how your administrator has set things up.
	Other - Everyone else basically

And there are three different actions that you can prevent or allow a
user to do to that file:

	Read    - User can read the content of the file
	Write   - User can change and delete the file
	Execute - User can run the file as a program. For directories,
                  the execute flag allows you to set that directory as
	          your current directory.

The column on the left shows what permissions are set for each group of
users. ie.

-rwxrwxrwx 2 qwaszx   users        1024 Nov 14 01:52 mail
 \_/\_/\_/      |       |           |                 |
  |  |  |     Owner    Group     Filesize            Filename
  |  |  Other
  |  Group
  User

To change these permissions, you use the chmod command. The format is
as follows:

chmod (a|ugo)(+|-)rwx filename
or
chmod 777 filename

the numbers is just another way of writing the permission information.
For now, we will use the first form of the command as it is easier to
use.
Say for example you wanted to give everyone read access to your file,
you would use the following command:

chmod a+r filename

The a means all users, the plus means turn ON the required permission,
and the r means set the read permissions.

These letter can be put together in any combination eg. 
   chmod go-rwx private_file
would remove all permissions for anyone except yourself.

The uses of permissions are obvious, and it is a good idea to do the
following commands on your shell account:

chmod go-rwx ~/*          # Protects all files in your home directory
chmod go-rw ~             # Protects your home directory itself
chmod go+rx ~/public_html # Allows read access to your website

In the next article I will cover networking commands in UNIX (Telnet,
commmand line FTP, etc.)

---
Written by qwaszx for SWAT Magazine
Send comments, suggestions, praise to <qwaszx@europe.com>
Send Flames to <gillbates@microsoft.com>
Standard disclaimer applies - I am not reponsible for what you do with
this information, and for any inaccuracies. blah.. blah.. blah......