I'm looking for any open source command line tool or tools which will allow me to index and search a large number of plain text files. Approximate search would be a plus. The tool only needs to print the files that match, although some match context would be useful. A GUI tool isn't useful for my application, nor is anything that searches files one by one (grep for example). I'm basically targeting unix platforms (osx, linux, bsd).
EDIT: I'm not interested in any sort of tool that is system-wide, or needs to run in the background. Basically, I want to build an index for a directory tree full of text files and then later be able to search against it. Preferably the index is one or a few files that I can specify the location of.