如果变量未设置或设置为空字符串("")，则返回true。

if [ -z "$MyVar" ]
then
   echo "The variable MyVar has nothing in it."
elif ! [ -z "$MyVar" ]
then
   echo "The variable MyVar has something in it."
fi

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

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

检查变量v是否未设置

if [ "$v" == "" ]; then
   echo "v not set"
fi

如果变量未设置或设置为空字符串("")，则返回true。

if [ -z "$MyVar" ]
then
   echo "The variable MyVar has nothing in it."
elif ! [ -z "$MyVar" ]
then
   echo "The variable MyVar has something in it."
fi

假设Bash:

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