Shell Programming and Scripting

Does anyone know why the following expression return an error
[[ "$(echo yop | grep o)" ]]
while the following one not
[ "$(echo yop | grep o)" ]
??
Thanks

There are several differences between the two types of tests. The double bracket demands a logical expression while the single bracket does not.

[ $string ]

is true if string contains something. And
[ ]

if always false. You are using a variation on that. This depends on $string completely disappearing before [ sees its arguments. That will happen, but it's why

[ $string = "cat" ]

doesn't work. You must put quotes around $string for that to work. [[ is immune to that effect so

[[ $string = "cat" ]]

works fine. You can use -n with both of them to achieve the same effect:

[ -n "$string" ]
[[ -n $string ]]

But beware of

[ -n $string ]

If string is empty, it will disappear and the single bracket is back to seeing a single argument which will be true if that argument is not empty. "-n" is not empty so the test will be true.