Android源码编译完重启电脑后emulator命令报错

编译完Android源码后，使用emulator命令可以打开模拟器，但重启电脑后报错：

emulator: ERROR: You did not specify a virtual device name, and the system
directory could not be found.

If you are an Android SDK user, please use '@' or '-avd '
to start a given virtual device (see -help-avd for details).

Otherwise, follow the instructions in -help-disk-images to start the emulator

解决办法：

source build/envsetup.sh 
lunch sdk-eng

然后再执行

emulator

可以启动模拟器