Check if variable is empty

Reference

https://serverfault.com/questions/7503/how-to-determine-if-a-bash-variable-is-empty

This will return true if a variable is unset or set to the empty string (“”).

if [ -z "${VAR}" ];

Here is the same thing but in handy table form:

                        +-------+-------+-----------+
                VAR is: | unset | empty | non-empty |
+-----------------------+-------+-------+-----------+
| [ -z "${VAR}" ]       | true  | true  | false     |
| [ -z "${VAR+set}" ]   | true  | false | false     |
| [ -z "${VAR-unset}" ] | false | true  | false     |
| [ -n "${VAR}" ]       | false | false | true      |
| [ -n "${VAR+set}" ]   | false | true  | true      |
| [ -n "${VAR-unset}" ] | true  | false | true      |
+-----------------------+-------+-------+-----------+