Wikipedia
Recoll is a desktop search tool that provides efficient full text search (from single-word to arbitrarily complex boolean searches) in a friendly GUI, with minimum technical sophistication and few mandatory external dependencies. It runs under many Unix-like operating systems, and is mostly independent of the desktop environment.
Recoll was designed not to require a permanent daemon but on Linux systems Recoll can make use of inotify. Recoll updates its index at designed intervals (for example through Cron tasks) but if desired, the indexing task can run as a file-system monitoring daemon for real-time index updates.
The Recoll document conversion and text extraction architecture makes it extremely easy to write new filters, and many document types are supported.