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

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

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

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

我也看到过

if [ "x$variable" = "x" ]; then ...

这显然是非常健壮和外壳无关的。

此外，“empty”和“unset”之间也有区别。请参阅如何判断字符串是否未在Bash shell脚本中定义。

检查变量v是否未设置

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