There is a bunch of commands in Nuke that allow the user to create simple UIs for tcl procs.

Here is a list of them and some examples. Best to just copy/paste the snippets of example code into the Nuke shell to see what happens.

message

Shows an info dialog box (with a 'i' and the text message) and waits for the user to hit the OK button.

syntax
message

example:
message "you are using Nuke v$nuke_version"

alert

Pops up an alert box (with a '!' and the text message) and waits for the user to hit the OK button.

syntax:
alert

alert "go home, it's late"