What is the correct file extension for GLSL shaders?

There's no official extension in the spec. OpenGL doesn't handle loading shaders from files; you just pass in the shader code as a string, so there's no specific file format.

However, glslang, Khronos' reference GLSL compiler/validator, uses the following extensions to determine what type of shader that the file is for:

• .vert - a vertex shader
• .tesc - a tessellation control shader
• .tese - a tessellation evaluation shader
• .geom - a geometry shader
• .frag - a fragment shader
• .comp - a compute shader

The glslang compiler created by Khronos makes assumptions about the shader stage based on the extension, but there is no standard extension outside of this (and quite a few projects make up their own). The glslang compiler keys off of .vert, .tesc (TESsellation Control shaders), .tese (TESsellation Evaluation shaders), .geom, .frag, and .comp.

But that's about it for any form of standard extension.