好，我自己回答了. 我检查

Ok got answer myself. I check what happens for

whereis pm2
pm2: /opt/nodejs/bin/pm2

然后我检查了

whereis userdown
userdown: /usr/bin/userdown /usr/bin/X11/userdown /opt/nodejs/bin/userdown

在/usr/bin中的

hmm .... 所以我做了

hmm in /usr/bin.... So I did

sudo ln -s /opt/nodejs/bin/pm2 /usr/bin/pm2 

它有效:)