How do I launch the Android emulator from the command line?

I assume that you have built your project and just need to launch it, but you don't have any AVDs created and have to use command line for all the actions. You have to do the following.

1. Create a new virtual device (AVD) for the platform you need. If you have to use command line for creating your AVD, you can call android create avd -n <name> -t <targetID> where targetID is the API level you need. If you can use GUI, just type in android avd and it will launch the manager, where you can do the same. You can read more about AVD management through GUI and through command line.
2. Run the AVD either by using command emulator -avd <name> or through previously launched GUI. Wait until the emulator fully loads, it takes some time. You can read about additional options here.
3. Now you have to install the application to your AVD. Usually during development you just use the same Ant script you used to build the project, just select install target. However, you can install the application manually using command adb install <path-to-your-APK>.
4. Now switch to emulator and launch your application like on any normal device, through the launcher. Or, as an alternative, you can use the following command: adb shell am start -a android.intent.action.MAIN -n <package>/<activity class>. For example: adb shell am start -a android.intent.action.MAIN -n org.sample.helloworld/org.sample.helloworld.HelloWorld. As a commenter suggested, you can also replace org.sample.helloworld.HelloWorld in the line above with just .HelloWorld, and it will work too.

List all your emulators:

emulator -list-avds

Run one of the listed emulators with -avd flag:

emulator -avd @name-of-your-emulator

where emulator is under:

${ANDROID_SDK}/tools/emulator

I think the best way to reach it via terminal is :

cd ~/Library/Android/sdk/emulator

To run a certain AVD directly:

./emulator -avd {AVD_NAME}

To list your AVDs use :

./emulator -list-avds