On Linux and macOS, the system property `user.home` is set to the home directory provided by the operating system.
If the directory name provided is empty or only a single character, the value of the environment variable `HOME` is used instead.

The directory name and the value of $HOME are usually the same and valid. The fallback to `$HOME` is unusual and unlikely to occur except in environments such as `systemd` on Linux or when running in a container such as Docker.