Explanation
The export command makes a shell variable available to child processes of the current shell. It can also be used to list all the exported variables. The -v option makes the export command print the variable name and value for each exported variable. The declare command can be used to set or display the attributes and values of shell variables. The -x option makes a variable exported to subsequent commands. The +x option removes the export attribute from a variable. The visible command is not a valid shell command. References:
* LPI 101-500 Exam Objectives, Topic 103.1, Weight 4
* LPI Learning Materials, Chapter 3.1, Shell Variables and Environment
* Web Search Results, 1