The MAVLink Shell is an NSH console that can be accessed via MAVLink over serial (USB/Telemetry) or WiFi (UDP/TCP) links (in particular, on NuttX-based systems like: Pixhawk, Pixracer, etc.).
The shell can be used for running commands and modules, and displaying their output. While the shell cannot directly display the output of modules that it does not start, it can do so indirectly using the dmesg command (dmesg -f & can be used to display the output of other modules and tasks running on the work queue).