LESSOPEN

LESSOPEN is a Windows environment variable that specifies a list of programs that the operating system should use to open files with specific extensions. When a file is double-clicked in Windows File Explorer, the system checks the LESSOPEN variable to see if any of the registered programs can handle the file's extension. If a match is found, the specified program is launched to open the file.

The value of the LESSOPEN variable is a semicolon-separated list of program paths and their corresponding file extensions. For example, a typical LESSOPEN entry might look like this: "C:\Program Files\MyViewer\viewer.exe *.txt;*.log". This entry tells Windows that if a file with a .txt or .log extension is encountered, it should be opened with viewer.exe.

LESSOPEN is often used in conjunction with text editors or viewers that support custom file associations. It allows users to configure their system to open certain file types with their preferred applications, rather than relying solely on the default Windows associations. This can be particularly useful for developers who work with various log files or specialized text formats. The variable can be set and modified through the System Properties dialog in Windows.