Unix question

[Big Planet]

Hi geeks , How can I do substr and instr kind of operaion in unix shell script . is it possible ? like i have one paramter as "hostname" .. I want to trim quotes surrounding the hostname . One more question , is possible to read another text file line by line using a shel script and then edit that file . Actually I have this configuration file for my system which have keywords like $HOSTNAME$ , $SCHEMA$ which I want to replace with actual values at the time of installation by asking questions to the user.   TIA --Big planet