Opening Android Studio every time you want to launch emulator is not an easy task in case of having a slow PC. Android Studio consumes RAM and opening Android Studio might make your PC slow.
I assume you have already created virtual device with AVD using Android Studio. Then in order to launch Android emulator, go to Home>Android>Sdk>Emulator (Ubuntu) and open the terminal. Run the following command to see available virtual devices.
In the example given above, I have a virtual device named Pixel_3_API_28. Now run the following command to launch pixel 3 Android emulator.
emulator -avd Pixel_3_API_28
I hope this blog post helps you to run emulator from command line.