Again, the inconsistency of UNIX shows up. Suppose that you have a file that you wish to encrypt by shifting all the letters down the alphabet by 1, and suppose that there is only upper case in the file. Filters are common in UNIX and are often used with redirection. In all cases, filters do not actually change their input files, but rather create a new data stream that can be either viewed directly on the screen or redirected into a new file. Another common use of filters is to hook several of them together in order to create a pipeline.
Data flows through multiple filters, each one doing one particular thing to the data stream.
These pipelines are also discussed in the shell lesson. The specific editing command follows the sed command, and it often uses special characters so it is must be enclosed in quotes, as shown above. The slashes delimit or mark off these strings from each other. If you want to use the slash as a data character, there are several ways to do it, the simplest being to change the delimiter to something else, like a colon. The name, file1.changed, is totally arbitrary, but it is often useful to give it a meaningful name so that you do not lose track of what is in each file.