Here you are, you've written a nice filter that reads data from cmd^.in and you want to give it some data. In the easy case, the data is in one file and you can use the CommandTool's input redirection (``<'') facility. What if your data is spread across several files? What you want to do is concatentate those files together and feed them into a pipeline to your filter. The ``cat'' command does just that. It takes a list of file names and concatentates their contents onto cmd^.out, so you can pipe it to whatever you want.

Here you are, you've written a nice filter that writes data onto cmd^.out and you want to check on what it's doing. In the easy case you can just break the pipeline after your filter and use the CommandTool's output redirection (``>'') facility to dump the pipeline into a file. What if you want to examine your data as it trickles through the pipeline? What you want is something that sits in a pipeline and copies its input to its output, but also copies its input to a file. The ``tee'' command does just that. It takes a list of file names and writes whatever it receives on its cmd^.in onto each of those files and also onto its cmd^.out.