What environment variable holds the username of the user who started the shell session?

The environment variable that holds the username of the user who started the shell session is $USER. This variable is set automatically when a user logs in and can be accessed by various commands and scripts to identify the current user.

In a typical Linux session, $USER reflects the login name of the user, which is often used for personalizing outputs or for controlling access to resources based on user identity. It serves as a straightforward reference point for scripts and applications to dynamically know which user is executing a command.

The other options listed serve different purposes: $HOSTNAME contains the name of the machine on the network, $HOME points to the current user's home directory, and $SHELL indicates the path of the shell that the user is currently using. None of these variables provide the username associated with the user session, which is why $USER is the correct choice for this question.