A LIN file is a data file created by VBSP, a game level map compiling tool included with Valve Hammer Editor (version 4 or later). It contains information about one or more leak(s) in a Valve Map .VMF file detected by VBSP when compiling the VMF file. LIN files are generated in the same directory as the corresponding VMF file.

You can use Valve Hammer Editor to create custom level maps for various video games, such as Half-Life and Counter-Strike. The maps are saved as VMF files, which are working files that can be closed, re-opened, and edited. The files must be compiled to the Binary Space Partition (BSP) format before they can be loaded into BSP-based game engines. The VBSP tool included with Valve Hammer Editor is used to compile VMF files. If the tool detects any points (leaks) where the black infinite area around the map makes contact with an entity in the map, it generates a LIN file in the same directory as the VMF file. You can then review the leak(s) in the file, fix them, then re-compile the map so it will work correctly in the game. If the leaks are fixed, the LIN file is deleted the next time VBSP runs.

Painted line file used by X-Plane, a professional flight simulator; contains data that defines taxiway markings on the ground.

The LIN file consists of a header and commands that define different aspects of the lines, such as the texture, size, scale, and direction (rounded or sharp corners).