An interactive grep tool in your terminal. Percol does the following:

1. receives input lines from stdin or a file, 2. lists up the input lines, 3. waits for your input that filter/select the line(s), 4. and finally outputs the selected line(s) to stdout.

Since percol just filters the input and output the result to stdout, it can be used in command-chains with | in your shell (UNIX philosophy!).