Linux—environment variables

 

1. Commonly used environment variables

• HOME: The current user's home directory
• PS1: The basic prompt is # for root users and $ for ordinary users.
• LOGNAME: the login name of the user at the time
• PWD: current directory
• MAIL: current user's mail storage directory
• SHELL: current user shell type
• PATH: Determines which directories the shell will search for commands or directories
• TERM: terminal type
• HOSTNAME: The name of the current host
• LANG: Current language.

2. Commonly used environment variable commands

1. echo: The user displays the value of an environment variable.

   如：echo $HOME

    

2. env: Display all environment variables, including system environment variables and custom environment variables.
3. set: You can set the value of an environment variable.
4. export: Set a new environment variable.

   export MYSQL=‘/usr/local/mysql’  #设置临时环境变量；
   env|grep MYSQL      #此命令有输出说明环境变量MYSQL存在了；
   关掉命令行，环境变量即消除。
   

    

5. unset: Clear environment variables.

   unset $MYSQL   #删除环境变量MQSQL
   env|grep MYSQL  #此命令没输出，说明环境变量被清除了
   

    

6. readonly: Set the environment variable to read-only.

   export MYSQL=‘/usr/local/mysql’  #设置临时环境变量；
   readonly MYSQL     #将环境变量MYSQL设置为只读
   unset MYSQL     #发现此变量不能被删除
   MYSQL=‘/usr/local/XX’      #此变量也不能被修改
   

    

3. Environment Variable File

  Divided by the life cycle of variables, Linux variables can be divided into two categories: 
      permanent: the configuration file needs to be modified, and the variable takes effect permanently. 
      Temporary: Just use the export command to declare, the variable becomes invalid when the shell is closed.

  The environment variable files in Linux include:     

1. /etc/profile is effective for all users [permanent] 
2. /ect/bashrc 
3. ~/.bash_profile Effective for a single user [permanent] Generally, the environment variables added by yourself are placed in this file. 

   cat .bash_profile     #显示环境变量
   vim .bash_profile       #编辑此环境变量文件
   source .bash_profile    #修改文件后，运行该命令修改马上生效，不然只能在下次重新进入此用户时生效。
   

    

4. ~/.bashrc
5. ~/.bash_logout