Environment Configuration Files
When a user logs in, an environment is created for that user automatically. This happens based on four different files where some script code can be specified and where variables can be defined for use by one specific user:
■ /etc/profile: This is the generic file that is processed by all users upon login. --全局环境变量，对所有用户都生效
■ /etc/bashrc: This file is processed when subshells are started. --
--全局环境变量，对所有用户都生效，子shell启动时需要用到的配置文件
■ ~/.bash_profile: In this file, user-specific login shell variables can be defined. --用户自己定义的login shell环境变量
■ ~/.bashrc: In this user-specific file, subshell variables can be defined.
As you have seen, in these files a difference is made between a login shell and a subshell. A login shell is the first shell that is opened for a user after the user has logged in. From the login shell, a user may run scripts, which will start a subshell of that login shell. Bash allows for the creation of a different environment in the login shell and in the subshell but to synchronize settings; by default the subshell settings are included when entering a login shell. --用户自己定义的subshell
~/这个表示在用户自己的目录下。如用户testuser，home目录为testuser，则环境变量配置文件为/home/testuser/.bash_profile