Mac OS X，Node由Nvm安装，但不能使用全局安装的模块

The operation system is Mac OS X, After installing Node by using NVM, Node -v shows 9.2. And after installing express globally(npm install -g express), I cannot use express in the command line. If it is used, it will prompt that the module is not defined. Using npm root -g to find the installation of node path, in the lib or bin directory I can see the global installation of the package, but I can't be able to add lib or bin directory in the environment path, because there is no executable file in these two directories. Any suggestions are very helpful, thanks.The operation system is Mac OS X, After install