How to use sed, a special editor for modifying files automatically. If you want to write a program to make changes in a file, sed is the tool to use.
There are a few programs that are the real workhorse in the UNIX toolbox. These programs are simple to use for simple applications, yet have a rich set of commands for performing complex actions. Don't let the complex potential of a program keep you from making use of the simpler aspects. I'll start with the simple concepts and introduce the advanced topics later on.
When I first wrote this (in 1994), most versions of sed did not allow you to place comments inside the script. Lines starting with the '#' characters are comments. Newer versions of sed may support comments at the end of the line as well.
One way to think of this is that the old, "classic" version was the basis of GNU, FreeBSD and Solaris verisons of sed. And to help you understand what I had to work with, here is the sed(1) manual page from Sun/Oracle.