Need help with basic Shell programming

Hi
I am trying to write a shell program which takes in three parameters: the file name of text document, the original word and the new word e.g.
$ replace.sh blah.txt Dev Shed
So this should convert all the "Dev" strings in the blah.txt to "Shed"
Following is my script
#! /bin/csh
set file1=$argv[1]
set file2=$argv[2]
set file3=$argv[3]
sed 's/file2/file3/ file1

when I run it, it says cannot open file1. Can somebody help me with this. I am a beginner and its really frustrating me.
Thanx

Moving this thread to the unix forum as it has nothing to do with C programming. As for your specific problem, you need to refer to your variables as $file1, $file2 etc.

Note the $ signs in the sed line.

Thank You. I am totally new to this forum thats why I kind of don't know where to post what. Thanx again

Now it seems as if the sed command does not work because it is basically giving me the same output when I run replace.sh blah.txt Dev Shed

sed 's/$file2/$file3/' $file1 <---- will never expand $file[23]
sed "s/$file2/$file3/" $file1 <--- does it
see the difference ! girl? boy ? 1 chromosome
are you beginning... so do it the proper way
csh is a fantastic tool
on the command line (my preferred) but
the BSD-guys did not complete the job, so
NEVER, NEVER, NEVER write csh SCRIPTS
it's buggy, to be portable prefer the old BOURN,
else KSH, BASH or other.

Hi guys. Thanx for being so helpful. I just have one last thing that I can not figure out. I wanna make permanent changes to the actual file using sed command. How do I do it. It seems like it does not change the actual contents of the file . thanx