How to open the AVD manager on Ubuntu Linux from the command line?
First of all to show all devices run the following command from the location - ~/Android/Sdk/tools
/home/user/Android/Sdk/tools> ./emulator -list-avds
then you will see the list of devices id, in my case i got following devices
Nexus_5X_API_28
Nexus_9_API_28
Now just run following command with the device id you want to run.
/home/user/Android/Sdk/tools> ./emulator -avd Nexus_9_API_28