What is thunar?

Thunar

Thunar is a file manager for Linux and other Unix-like systems, written using the GTK+ 2 toolkit, and shipped with Xfce version 4.4 RC1 and later. Thunar is developed by Benedikt Meurer, and was originally intended to replace XFFM, Xfce's previous file manager. Thunar was initially called Filer but was changed to Thunar due to a name clash.

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Some articles on thunar:

Thunar - API
... Thunar provides an API for third party developers The "thunar-vfs" provides a rich cross platform API for high level file system operations, This is deprecated for the 1.2.0 release in favor of the native gvfs ... Thunar can also be extended by writing scripts to be placed in the context menu for various file types ...
Xfce - Applications - Thunar
... Thunar is the default file manager for Xfce, replacing Xffm ... It resembles GNOME's Nautilus and is designed for speed and a low memory footprint as well as being highly customizable through plugins ...