If you want to run emulator from command line,

<your-full-path>/emulator -avd 5.1_WVGA_API_28 

For newer version of Android SDK, the emulator path should be something as below:

/<xxx>/Android/sdk/emulator/emulator

For older version of Android SDK, the emulator path is as below:

/<xxx>/Android/sdk/tools/emulator

Try either one of above to see which is your case.
Here is the official document for Android emulator command line usage: https://developer.android.com/studio/run/emulator-commandline