Sed is a very useful program that often is used as a filter in shell scripts.
From the sed man page:
Sed is a stream editor. A stream editor is used to perform basic text transformations on an input stream (a file or input from a pipeline).
== Simple find-and-replace in text file ==
To replace all occurrences of the string ''findme'' and replace it with the string ''replcaceme'' in a file named test.file using ''sed'' just use the follwing command.
sed -e 's/findme/replaceme/g' test.fil > newtest.fil