UNIX File Protections

To allow access to the contents of a file or a directory, the
permission fields are used.  There are three types of permissions
and three types of people for the purposes of access. 

By default, all your files are created 'private'; no one else on the
system can read any of your files unless you allow them to.

     * the permission types are read (r), write (w), and execute (x)
     * the people types are user (u), group (g), and others (o)

To see the current settings, use the command "ls -lg".  That is the
command ls for listing files with options, (l) for long format and (g)
for group names.  Here's a sample:

     -rw-r--r--  1 mer      staff        1551 Nov  4 14:32 notes
     -rw-rw----  1 mer      staff          95 Oct 15 17:33 agenda
     drwxr-xr-x  2 mer      staff         512 Nov  5 19:54 shipping

The permission field appears at the very left and is the portion we
are discussing.  The first bit indicates the file type.  In the above
example, there are two regular files indicated by a '-' in the first
space and one directory indicated by 'd'.  The next nine bits are
three sets of the three permission types, one set for each of the
three types of people.

          read, write, execute for user
          |
          |  read, write, execute for group
          |  |  
          |  |  read, write, execute for others
          |  |  |
        -rwxrwxrwx


The command 'chmod' allows you to add (+) or remove (-) individual
permissions for the each of the people types; the user [yourself] (u),
members of your group (g), or others on the system (o).  The shorthand
notation might seem a bit cryptic at first but let's give it a try:

Adding read access to group and others:      chmod go+r filename	
	(group and other, plus read)
Removing read access to group and others:    chmod go-r filename
	(group and other, minus read)
Adding read and execute permission 
  to group and others:                       chmod go+rx filename
	(group and other, plus read and execute)
Removing read and execute permission
  to group and others:                       chmod go-rx filename
	(group and other, minus read and execute)

Modifying directories:

All directories must have the execute bit set in order to use them
properly.  When modifying a directory's permissions be certain to
allow execute permission.

Adding read access to group and others:      chmod go+rx directory
Removing read access to group and others:    chmod go-rx directory

For futher details, check the manual page for chmod.  The command chmod
works in two modes, absolute and symbolic.  The brief overview we
did here describes the symbolic mode.

The manual page for ls has additional information about the file
types shown by that first character in the long listing (usually a -
as most files are text files).

The UNIX command 'umask' will allow you to modify the default file
protections which your account uses.  To understand this command you
must be familiar with the absolute modes which are fully described in
the man page on chmod