If you are using macOS, add both Android SDK emulator and tools directories to the path:
Step 1: In my case the order was important, first emulator and then tools.
export ANDROID_SDK=$HOME/Library/Android/sdk
export PATH=$ANDROID_SDK/emulator:$ANDROID_SDK/tools:$PATH
Step 2: Reload you .bash_profile Or .bashrc depending on OS
Step 3: Get list of emulators available:
$emulator -list-avds
Step 4: Launch emulator from the command line and Replace avd with the name of your emulator $emulator @avd
Don’t forget to add the @ symbol.
This was tested with macOS High Sierra 10.13.4 and Android Studio 3.1.2.