How can I use Predefined Variable With Shell Command

[blueray453]

When I run a command with predefined variable, it does not expand that variable.

My shortcut looks like:

"filterText.commandList": [
    {
        "name": "Add Path to File Name",
        "description": "Add Path to File Name",
        "command": "echo ${fileDirname}/${selectedText}"
    }
],

Suppose I have selected a text example.png in the editor, then run Add Path to File Name, the output is / instead of path/to/file/example.png

What can I do?