Can not execute emulator in Ubuntu

Below is the error message:
emulator: ERROR: you must provide the name of a virtual machine to start
>> the emulator.

I tried following steps and the issue disappears.

$ . ./build/envsetup.sh
$ choosecombo
./out/host/darwin-x86/bin/emulator -system ./out/target/product/generic
-kernel ./prebuilt/android-arm/kernel/kernel-qemu -skindir
./development/emulator/skins -skin HVGA