What is the command to list the available avdnames

List all your emulators:

emulator -list-avds

Run one of the listed emulators:

emulator @name-of-your-emulator

where emulator is under:

${ANDROID_SDK}/tools/emulator

Using sdk/tools/emulator

This will list all available avds

emulator -list-avds

get into Android/sdk/tools and run following command

./emulator -list-avds

which will return something like

Nexus_5X_API_P
Nexus_6_API_25

AFAIK avdmanager list avd is what you need.