how to find variable is empty or "" in shell script

Question

how to find variable is empty in shell script

Answer 1

if [[ "$variable" == "" ]] ...

Answer 2

Presuming bash:

var=""
if [ -n "$var" ]; then
    echo "not empty"
else
    echo "empty"
fi

Answer 3

In bash at least: if [[ -z "$var" ]]

the command line "man test" is your friend.

Answer 4

[ "$variable" ] || echo empty
: ${variable="value_to_set_if_unset"}