1. ls

      Prototype: list, meaning list
       Function: Use the list to display all the files in the current folder.
       Usage: ls current folder
   
       ls -a displays all files, including hidden files
       ls -l display in detailed information
       ls -l -a effect stacking (also ls -a -l, ls -la, ls -al)

2. cd

      Prototype: change directory, meaning change directory
       Role: used to switch directories
       Usage: cd directory (relative path or absolute path)
  
       Skill usage:
                       cd.. represents the upper level directory (switch to the upper level directory)
                       cd .. .. represents the current directory

3.pwd

    Prototype: print work directory, meaning print work directory
     Function: print out the current absolute path
     Usage: pwd

4.mkdir

Prototype: make directory, meaning to create a folder
 Role: Create an empty folder
 Usage: mkdir folder name

 Skill usage:
                 mkdir -p cascade to create folders (that is, folders can contain folders)

                 For example: mkdir -p app/app1 folder app contains folder app1

5. mv

Prototype: move, meaning to move
 Role: Move files between directories, modify file names
 Usage: mv source file path (pathname) destination file path (pathname)

6.touch

  Role: Create an empty file
   Usage: touch pathname

7.cp

Prototype: copy, meaning copy
 Role: copy files or folders
 Usage: cp source file pathname target file pathname

 Skill usage:
                 cp -r is used to copy folders
                 cp -f force copy 
 Note: Usually cp -f copies files, cp -rf copies folders

8.rm

Prototype: remove, meaning delete
 Role: used to delete files and folders
 Usage: rm file pathname

 Skill usage:
                 rm -r folder pathname

9.cat

Function: display the content directly under the command line, can also be used to input to the folder

10.man

Role: Query the man manual and get help information

 E.g: 
             man 1 ls 1 means that the query is a linux command
             man 2 xxx 2 means that the query is linux api
             man 3 xxx 3 means that the query is a C library function