在Bash中，至少下面的命令会测试$var是否为空:

if [[ -z "$var" ]]; then
   # $var is empty, do what you want
fi

指挥人测试是你的朋友。

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

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

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 [ ${foo:+1} ]
then
    echo "yes"
fi

如果设置了变量，则打印yes。${foo:+1}在设置变量时将返回1，否则将返回空字符串。

检查变量v是否未设置

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