replacing string within same file
 

Hi,

I want to write a script in ksh, which parses the file and replaces string within it (not writing the changed text to another file).

I am able to write to another file, but don't know how to make changes within the file which is read.

This is exactly what sed specializes in.
For the gory details,

man sed

--- rod.

or perl

perl -pi.bak -e 's/THIS/THAT/g' file ...

this creates a .bak of the original (omit when script is proven if you prefer)