Learn linux from scratch (1)

Learn linux from the beginning (1)-Linux general technology-Linux technology and application information. The following is a detailed description. Author: z-x-wing
Linux beginners may encounter export commands and are confused from time to time. In fact, export is used to create/define environment variables.

For example, enter:
Export Hello = "hello" (do not quote it)

In this way, an environment variable named Hello is defined, and its value is hello.

When you enter:
Echo $ Hello, the following output is displayed:
Hello

So someone may ask, what is an environmental variable?
Linux is a multi-user system. Each user has its own running environment, and the running environment is defined as a set of environment variables. An environment variable is an object with a specific name. It contains information that one or more applications will use. Common environment variables in linux are as follows:

PATH: determines the directories to which shell will look for commands or programs.
HOME: HOME Directory of the current user
MAIL: refers to the MAIL storage directory of the current user.
SHELL: The Shell used by the current user.
HISTSIZE: the number of records that save historical commands.
LOGNAME: The Login Name of the current user.
HOSTNAME: indicates the host name. If a host name is used by many applications, it is usually obtained from this environment variable.
LANG/LANGUGE: it is a language-related environment variable. Users in multiple languages can modify this environment variable.

Common Environment Variable commands for bash-sh:
1. echo shows the value of an environment variable, for example, echo $ PATH.
2. Set a new environment variable for export, for example, export HELLO = "hello" (no quotation marks are allowed)
3. env display all environment variables
4. set: Display locally defined shell Variables
5. unset clear environment variables such as: unset HELLO
6. readonly: readonly HELLO