Section F.

                           INTRODUCTION TO UNIX

                      File Permissions and Security

       Every file and directory has permissions associated with it
       which determine who may or may not access that file or directory.
       Just as the system administrator assigns you a user name and user
       ID (UID), you are also assigned a default group name and ID (GID),
       and special groups may be set up by the system administrator to
       allow users to share files while working on projects, yet not
       allow all users to access them.

       There are three types of permissions:

       1) r - gives permission to read or copy
       2) w - gives permission to write to, delete, or save
       3) x - gives permission to run an executable file (program) or
              in the case of a directory, allows you to cd to that

       You can set permissions for three classes of users:

       1) you, the owner of the file or directory
       2) your group
       3) all others.

       Display permissions of files/directories using the long format of
       the ls command:

       % ls -l
       drwx------  2 user_name     512 Jan  1 00:01 directory_name
       -rw-------  1 user_name      50 Dec 25 12:00 ordinary_file_name

       The first character indicates the type of file:

       d indicates a directory
       - indicates an ordinary file

       Unix considers the next nine characters as three sets of three.
       The first set indicates the owner's permissions, the  second
       indicates the groups' permissions, and the last three refer to all
       other users.

       The chmod command is used to change permissions and requires three
       numbers between 0 and 7 (which correspond to binary digits and are
       defined below), the first for owner, the second for group, and the
       third for all others.

       Number   Binary       Permission   Permissions
                Equivalent   Bits

       0        000          ---          no permission
       1        001          --x          execute
       2        010          -w-          write only
       3        011          -wx          write and execute
       4        100          r--          read only
       5        101          r-x          read and execute
       6        110          rw-          read and write only
       7        111          rwx          read, write, execute

       Example: To reset the permissions of the file ordinary_file_name
                displayed above

       % chmod 700 ordinary_file_name
       % ls -l ordinary_file_name
       -rwx------  1 user_name      50 Dec 25 12:00 ordinary_file_name

       Only you can access the file and you can read, write, or execute

       Note: "Symbolic" arguments may be used instead with the chmod

             % chmod +x doit.csh

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