使用双括号给出-bash：期望的条件二元运算符

Question

为什么这不起作用

if [[ screen -ls | grep -q "Hello" ]];
 then 
    echo "Hello"; 
fi

它给出错误

-bash: unexpected token `|', conditional binary operator expected
-bash: syntax error near `-ls'

但这有效

if screen -ls | grep -q "Hello";
 then 
    echo "Hello"; 
fi

不应该是相反的方式吗？我该如何修复双括号。