C Shell Programming Introduction
--------------------------------

Shell scripts are a powerful tool for invoking and organizing UNIX commands.
They can substitute for systems programs written in a general purpose
language such as C.  The shell scripts may be quite simple and merely
execute a sequence of commands, one after another (see the online file called
Making a Simple Shell Script).  This keeps you from having to remember 
where you are in a command sequence (do I convert to a YUV file next or
rename the file??), look up commands in a manual, or do a large amount of
repetitive typing.  The C shell allows loops and decision sequences just
like in a programming language.  Scripts may read input from the keyboard,
display output and have their output piped.  Much can be done with the C
shell.

We are assuming at this point that you already know some of the basic UNIX
commands and terminology.  

Pop quiz:
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What is a pipe?  (NOT A CYLINDRICAL TUBE!)

A pipe, represented by the character "|", is a means of taking the "output" 
from one command or program and passing it as "input" to another command or
or program.  Here is an example:

	ls -al | wc -l

This takes the output from the directory listing and INSTEAD OF DISPLAYING
IT, it delivers it (through the pipe) to the command "wc" as input.  If you
recall, the command "wc" counts words or lines (if the "-l" argument is used
it counts lines).  The number of lines is then counted in "wc" and output
to the screen.
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Preliminaries:
--------------

 	If you are logged into a window that has a prompt of "$",
	my friends, you are in the wrong shell!  You are probably in
	the Bourne Shell (or Born Again SHell).

	Cure: type the following command to get into the C Shell

		csh

	You should now see a "%" as the prompt (unless you changed
	the PROMPT environment variable) 


Scratching the Surface:
-----------------------

Lets start with a simple example.
This simple shell script example will  a list
of unix commands.

Enter the commands into a file called "ls2" like this:

	echo "----------------------"
	echo "- Enhanced Directory -"
	echo "----------------------"
	echo " "
	ls -x
	echo "----------------------"

Change the permissions on the file with 

	chmod +x ls2

and then execute the file by typing

	ls2

You will get a fancy directory listing such as this:

----------------------
- Enhanced Directory -
----------------------
 
#shellingintro# makingsimple    script2         shellingintro
ls2             script1         script3         shellingintro~
----------------------

Of course, you could expand the "-"'s such that they span the whole
width of the directory to make it look even nicer.  

Let look at another example that changes a PREDEFINED shell variable or two.
Let's make a very short shell script (even shorter than the last one) that
changes the prompt and the history size.   
Let's create a file called "script4" containing:

	set history = 20
	set prompt = '\!% '

Now change the permissions and execute the file "script4".  What the
"hay" happened??  Nothing at all!  Certain shell scripts won't execute.
Why?  Shell scripts containing system shell variable assignments need to
be "source"'d to execute.  Try the following command instead of "script4":

	source script4

Now it works!  Notice that if you type "history" now that only the most recent 20
commands show up.  Also, notice that your command line prompt prints the
current command number.  The funny stuff in single quotes seemed to do that
job ('\!').  Don't worry about how it comes about, the C Shell can do some
amazing things.

Let us try one more example.  Modify your .cshrc file by adding the 
following alias line:

	alias hi echo "get a life!"

Save the file and try to execute it by typing:

	.cshrc

DON'T CHANGE THE PERMISSIONS TO EXECUTABLE.  Notice it will come back and
tell you "permission denied".  This, of course, is due to the fact that it
doesn't have execute permission.  BUT how come when you log in it works?
Well, the system effectively calls the .cshrc file using the source command:

	source .cshrc

The source command will do the trick.  Now when you type "hi" the computer will
respond with "get a life!"--gratitude for you.  

Let's try a different example.  Let's make a very simple assignment to 
a variable in a shell script and print the value on the screen.  Create
the file called "script5" containing:

	set a = 27
	echo $a

Follow the rules of shell scripting by changing permissions and
executing it.  A blank line comes back.  Why?  When using variable
assignments in a shell script, if there is no
comment line in the beginning of the script, things go haywire.  
When making a shell
script from now on, make sure you put a comment on the first line
(this acts as an indicator for the shell script processor).  A comment 
line starts with the "#" character.  Let us change the last 
script and call it "script6":

	# script6

	set a = 27
	echo $a

Now the shell script comes back with 

	27


One Interesting Example Using What We Know Before We Get More Complex
---------------------------------------------------------------------

First of all type 

	xcalc &

This will pop-up a calculator.  Let's write a slightly more complex
shell script that will automatically kill the calculator without having
to search for the process id number for the "kill" command.  Create the
file called "script7" with the following:

	# simple search and kill routine

	ps | awk '$4 == "xcalc" {print "kill -9 " $1}' > tmp123zxy
	chmod +x tmp123zxy
	tmp123zxy
	unalias rm
	rm tmp123zxy
	alias rm rm -i

Now execute it and notice that the calculator will disappear.  What this 
shell script does is create a temporary file containing the "kill"
command and makes it into a shell script and executes it.  Then it changes
the "rm" command so that it won't prompt the user with

	 "File script7. Remove ? (yes/no)[no] :"

Then it deletes the temporary file and changes the "rm" command back
so that it will prompt the user from now on again.