Run SBT command on module

I have a sub-project that's aggregated under some root project. Sub project defines commands e.g:

lazy val `sub` = project
  ...
 .settings(
    commands ++= Seq(Command.args("abc", ""){(state,args) =>
          ??? //edited
    })
  )

Normally when command is defined as top-most in the sbt I just run:

sbt> abc

However I'm unable to do this when defined on module:

sbt> sub/abc
[error] Expected ':'
[error] Not a valid key: abc (similar: watch, doc)
[error] sub/abc
[error]             ^

How to run abc at the sub level?