How can you set an environment variable for the current shell session?

To set an environment variable for the current shell session, using the export command is the correct approach. When you execute export VARIABLE_NAME=value, it not only assigns the value to the specified variable but also marks it for export to any child processes that are subsequently created from the shell. This means that the variable will be available to any subshells or programs invoked from that session, ensuring that the variable's value can be utilized by those processes.

The other methods do not achieve the same effect. For example, using the set command assigns a value to a shell variable but does not export it for use by child processes. This means the variable would only exist within the current shell and not be available to any external programs.

Similarly, env is used primarily to run a command in a modified environment and does not set a variable for subsequent commands in the same shell session. The define command is not a standard shell command for creating environment variables, so it would not work in a typical shell context.

Thus, using export is essential for making an environment variable accessible both within the current session and in any subprocesses, which is a common requirement in shell operations.