shell中字符串比较 Error unary operator expected

if [ "$vari" == "" ]
then
    echo "OK"
fi
报错[: =: unary operator expected
原因：$vari为空，导致语句变成if [ == ]，故而报错。
解决方法：加入冗余字符。可改成如下所写法：
if [ "$vari"x == x ]
then
    echo "OK"
fi