You can feed a text file to a shell script in various ways - typical syntax is:

cat somefile.txt | somescript.sh
or
somescript.sh < somefile.txt
or
cat somefile.txt | somescript.sh -
or...

http://www.google.com/search?q=shell+scripting+tutorial

Shameless plug:

http://steve-parker.org/sh/sh.shtml

A bourne Shell Programming/Scripting Tutorial for learning about using the Unix shell.
Centres on the Bourne shell, but mentions when bash...