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                        SOLUTION                       

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 Let's write a shell script to add a user to the file /etc/group

 This process consists of the following steps:

        1. Prompt for the group name

        2. Read the response

        3. Check to see whether the specified group name exists;

           if it does, obtain the group_id;

           otherwise, set the group_id to 150

        4. Prompt for the user's id

        4. Read the response

        5. Append new entry to the file /etc/group

 Please note, this exercise is for a user who has a good 

 understanding of Shell variables and commands.  Those with 

 limited understanding should skip this example by typing a q.

 A typical /etc/group file is shown above.

   root:X:0:root,admin,oper

   other:X:1:

   cron:X:2:cron

   bin:X:3:bin,who,dxu,dxu2

   uucp::4:uucp

   user:X:100:john:paul:jean:anna

   demo::200:demo,vdemo,cdemo

   Each line represents a group and is composed of: 

          group name, 

          group password,

          group id, and 

          group members, 

   Note that a password may or may not be present !

 Let's begin constructing the script.

 First we need to prompt for the group name and read it.  

 Note, the function read reads one word into its argument.

     echo "Please enter group name"

     read gname

 Check to see if the specified group name exists, and if so,

 obtain the corresponding group id.

     if grep "^$gname:" /etc/group > /dev/null

     then

         gid=`grep "^$gname:" /etc/group | cut -f3 -d:|tail -1`

  Note, grep "^$gname:" /etc/group > /dev/null"  says that grep will 

  look for the occurrence of the string representing the group name at

  the beginning of the line in each entry of the file /etc/group.  

  If one is found, the result is sent to the null device.  

 cut -f3 -d: will look for the third field, with field delimiter :.

  tail -1 filters out all but last line.

MOREC

 If the specified group id does not exist, then set the group id to 150.

     else

         gid=150

     fi

 Now get the user's id (userid)

    echo "Please enter the user's id"

    read userid

 Finally append an entry to the /etc/group file.

    echo $gname::$gid:$userid >> /etc/group

 If the group name was cti and the group id was 150 

 and the user id nicole, this would have the following effect on /etc/group:

   root:X:0:root,admin,oper

   other:X:1:

   cron:X:2:cron

   bin:X:3:bin,who,dxu,dxu2

   uucp::4:uucp

   user:X:100:john:paul:jean:anna

   demo::200:demo,vdemo,cdemo

   cti::150:nicole