Description of the requested feature

It is currently not possible to initialize basic variables using the output of a shell command. Basic variables can only be initialized using a literal value assignment:

(defvar current-volume 100)

My current workaround is to first spawn a window containing a widget which references the variable, and then updating the variable externally:

eww open my-bar && eww update current-volume=$(pamixer --get-volume)

It would be a nice addition to be able to initialize basic variables from shell output directly in eww.yuck.

Proposed configuration syntax

This could be accomplished by adding a simple boolean parameter, called for example execute, to the defvar-directive:

(defvar current-volume :execute true `pamixer --get-volume`)

If the parameter is true, the specified string gets executed in a spawned shell, and the shell's output gets assigned to the variable.

Additional context

No response

Edit: fix example code snippet.