官方定义：
Invoke “. build/envsetup.sh” from your shell to add the following functions to your environment:
– croot: Changes directory to the top of the tree.– m: Makes from the top of the tree.
– mm: Builds all of the modules in the current directory.
– mmm: Builds all of the modules in the supplied directories.
– cgrep: Greps on all local C/C++ files.
– jgrep: Greps on all local Java files.
– resgrep: Greps on all local res/*.xml files.
– godir: Go to the directory containing a file.
即在已经下载好Android源代码android文件夹以下运行：. build/envsetup.sh 或source build/envsetup.sh,就可以開始使用上面命令
