结论：
- 不确定是否要设为系统，则先针对当前用户设，写
~/.profile - 确定为系统级，写
/etc/environment，注意无需export - 不推荐写在
~/.bashrc（Ubuntu不推荐，理由见references）
References：
- https://help.ubuntu.com/community/EnvironmentVariables
- https://askubuntu.com/questions/866161/setting-path-variable-in-etc-environment-vs-profile
There are other ways environment variables are set when users log in that depend more heavily on the type of login. For example, you may occasionally have environment variables that get set just for graphical logins or just for SSH-based remote logins. The list above does not cover such cases.
I’ve left out a few files where people sometimes define environment variables, like ~/.bashrc and /etc/bash.bashrc, because they are not generally recommended places to set PATH and it is rare that you should actually use them for this purpose. If you use these files to add directories to PATH, then they will sometimes be added many times and is very confusing when you examine $PATH. (In extreme cases this may slow things down, but usually it’s just a matter of keeping everything clean and understandable.)
