Creating new commands
-	csh allows you to specify aliases
        alias   h       history
        alias   w       'who|sort'
        alias   j       jobs -l
        alias   ll      'ls -l'
-	Bourne shell and all the other shells can use shell scripts to create
	new commands
-	Shell scripts are simple text files containing a series of shell
	commands
$ sh nu
      7
$ chmod u+x nu
$ nu
      7
$ cat nu
who | wc -l

-	Specify parameters to the command that we have created
-	$1 corresponds to the first parameter
-	$2 corresponds to the second parameter, and so on
-	If you wanted complete list of parameters, use $*
-	$0 is the name of the program itself
-	You can specify the output from a command in your echo by enclosing
	them in backward single quote `
$ echo Today is `date`
Today is Wed Jan 24 19:43:56 EST 1996
$
-	In additional to the positional parameters such as $1, $2, $3, etc.,
	we can have other variables
-	Shell uses variables such as PATH, MAIL, etc. (covered earlier)
-	We can even define our own variables (USERNAME in assignment #1)
-	The variables set in a shell script are destroyed when the script
	finishes execution
-	The reason is that the shell script starts a subshell and supplies
	the commands from the shell scripts to this subshell

$ cat useless
hmmm=hello
echo $hmmm
$ sh useless
hello
$ echo $hmmm

$

-	. command allows you to retain the values of the variables defined
	in a subshell
$ chmod u+x useless
$ . useless
hello
$ echo $hmmm
hello

-	The variables defined in a shell are not available to its subshells

$ USERNAME='Pawan Lingras'
$ echo $USERNAME
Pawan Lingras
$ echo 'echo $USERNAME' > testy
$ sh testy

-	The export command makes the variable available to a subshell

$ export USERNAME
$ sh testy
Pawan Lingras
$

-	Iteration
	General format is
	for <var> in <list of words>
	do
		<commands>
	done
-	Example
	for i in *
	do
		cat $i
	done

#
while true
do
        for search in $*
        do
                if date | egrep $search
                then
                        while who | egrep lingras
                        do
                        hi=`tty lingras`
                        echo 'This is ' $search 'message: ' $* > $hi
                        sleep 10
                        done
                        exit

                fi
        done
        sleep 20

done

-	The above program has an outer while loop which runs forever
-	Inner for loop repeats the body for every parameter supplied
	to the script
-	Inside the for loop we check to see if the variable search appears
	in the output of the date command
-	The inner most while loop starts executing when a match is found
	-	gets the terminal on which the user lingras is working and
		stores it in a variable called hi
	-	echoes a message onto the terminal stored in $hi
-	When the innermost while is done, the program hits the exit statement
	and exits out of the script
-	Your first script is supposed to loop indefinitely
-	if someone is not working on tty11 or tty12
	-	echo a message "Can slip in now"
	-	else echo a message "Cannot slip in now"
-	if who | grep tty11
	then
		if who | grep tty12
		then
			echo "Cannot slip in now"
		else
			echo "Can slip in now"
		fi
	else
		echo "Can slip in now"
	fi
-	The previous algorithm is probably not the best logic try to
	find a better algorithm using &&, ! or ||
-	The second script is to be called del
-	It accepts parameters and for every file in the list
	of parameters
	show the top few lines
	bottom few lines
	ask the user if she wants to delete the file
	if the answer is yes delete the file

	for i in $*
	do
		head -5 $i
		echo 'DO you want to delete ' $i '(y/n)\c'
		read answer
		# if ..... look at page 141 to see how to setup the condition
	done
-	read command reads input from the user and stores in the variable
	that follows the word read
	read answer
	stores the input in the variable answer