013_Configuration files of bash

1. classfys according to the range in which configuration files take effect.
    1.1 The global configuration file
        /etc/profile :
            /etc/profile.d/*.sh
        /etc/bashrc :
    1.2 The personal configuretion file
        ~/.bash_profile
        ~/.bashrc
2. classfys according to its function
    profile : Provides configuration for shell which logins interactively
        global : /etc/profile :
            /etc/profile.d/*.sh
        personal : ~/.bash_profile
        function :
            1) used to define environment variables
            2) used to run commands and scripts
    bashrc : Provides configuration for shell which logins non-interactively
        global : /etc/bashrc
        personal : ~/.bashrc
        function :
            1) used to define aliases
            2) used to define location variables(本地变量)
3. shell login
    3.1 interactive login
        The order of loading configuration files is /etc/profile --> /etc/profile.d/*.sh --> ~/.bash_profile --> ~/.bashrc --> /etc/bashrc
    3.2 non-interactive login
        The order of loading configuration files is ~/.bashrc --> /etc/bashrc --> /etc/profile.d/*.sh    
4. source : Read and execute commands from FILENAME in the current shell