The db.copyDatabase shell command provides a convenient wrapper around the copydb command.

However, there is currently no support for setting the slaveOk flag.

This prevents you from being able to do a database copy from secondaries in a replica set.

At this stage, the only missing flag is slaveOk, however, this doesn't preclude the addition of additional flags into copydb in the future - hence, generic support for optional flags would be very useful.