Which command is used to make a variable an environment variable in Bash?

In Bash, to make a variable an environment variable, the export command is used. When you define a variable in a script or command line, it is typically local to that shell session. By using export, you ensure that the variable is made available to any child processes spawned from the shell. This is particularly useful when you want to pass configuration settings or options to scripts or programs that run after the variable's definition.

For example, if you have a variable MY_VAR="Hello", running export MY_VAR will allow any subprocess spawned after this command to access the value of MY_VAR. This command effectively promotes the variable from a local variable to an environment variable, thus expanding its scope for use in different processes.

The other choices do not serve this purpose: while set can be used to set shell options and positional parameters, it does not specifically make variables environment variables. The terms define and assign are not valid commands in Bash for defining variables directly. Therefore, the export command is the correct choice for creating environment variables in Bash.