Bash condition

Check ENV var

if [ -z "${VAR}" ]; then
    echo "VAR is unset or set to the empty string"
fi
if [ -z "${VAR+set}" ]; then
    echo "VAR is unset"
fi
if [ -z "${VAR-unset}" ]; then
    echo "VAR is set to the empty string"
fi
if [ -n "${VAR}" ]; then
    echo "VAR is set to a non-empty string"
fi
if [ -n "${VAR+set}" ]; then
    echo "VAR is set, possibly to the empty string"
fi
if [ -n "${VAR-unset}" ]; then
    echo "VAR is either unset or set to a non-empty string"
fi


                        +-------+-------+-----------+
                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      |
+-----------------------+-------+-------+-----------+

The ${VAR+foo} construct expands to the empty string if VAR is unset or to foo if VAR is set to anything 
(including the empty string).

The ${VAR-foo} construct expands to the value of VAR if set (including set to the empty string) and foo if unset. 
This is useful for providing user-overridable defaults 
(e.g., ${COLOR-red} says to use red unless the variable COLOR has been set to something).

The reason why [ x"${VAR}" = x ] is often recommended for testing whether a variable is either unset or set to the empty string 
is because some implementations of the [ command (also known as test) are buggy. 
If VAR is set to something like -n, then some implementations will do the wrong thing when given [ "${VAR}" = "" ] because the first argument to [ is erroneously interpreted as the -n operator, not a string.