Decker does not have the ability to execute shell commands or otherwise interact with the host system without explicit user permission; "shell[]" and similar functions are part of Lilt, which has a similar but distinct set of APIs from Decker itself.
As it happens, there is a way to print to stdout from Decker in the latest source revisions- the "app.print[]" and "app.show[]" functions. Note that this feature is not part of Decker v1.31, the current release at time of writing.
There is presently no mechanism for polling from stdin. The closest analogy might be to use the alert[] function:
alert["please input a string:" "string"]