Linux Learning Notes (common commands)

Specifies the run level

• Run Level Description
       0 off
       a single user
       over two user state without network service
       over three user network service
       4 system is not reserved for use by the user
       5 GUI
       6 restart
  the typical operating levels are 3 and 5, may be used init switch init [012345 ]

Ubuntu set the root password

Ubuntu, the installation is complete, the default root user password is not set, so we have to own to set the root password to set password .CentOS installation.
Command sudo passwd for the first time to enter the password after the current user's password, after that set the root password to use the su command to switch to root user after the setup is complete

Supplementary (forget the root password)

Single-user mode (single-user mode without a password can log in directly), you can change your password.

Common commands

• Help command

1. man get help
   basic syntax
        man [command or configuration file]

   When we are not familiar with the instructions for one when you can use this command to get help

2. help command
   basic syntax
       help command

   Shell built-in commands to get help information

• Class instruction file directory

1. pwd command displays the absolute path to the current working directory of
   basic grammar
       pwd

2. ls command to display the current directory information
   basic syntax
       ls [options] [directory or file]
   Common Options
       -a: Displays the current directory and all files, including hidden
       -l: Displays information in a manner list

3. cd instruction to switch to the specified directory
   basic syntax
       cd [parameters]

   Common parameters
       absolute and relative paths
   cd ~ or cd: to return to their home directory
   cd ... back to the parent directory of the current directory
   cd / root using an absolute path back to the / root directory

4. Create a directory mkdir command
   basic syntax
       mkdir [options] directory to create
   common options
       -p: create multi-level directory

5. rmdir command to delete empty directories
   basic syntax
       rmdir to delete empty directories
   

6. touch command to create an empty file
   basic syntax of
       basic grammar
       touch the file name
   

7. cp command [copy files to the specified directory]
   The basic syntax of
       cp [options] file name specified path
   common options
       -r: recursive copy implemented
   

8. rm command to delete the specified file or directory
   basic syntax
       rm [options] files to delete
   popular options
       -r: recursive delete entire file
       -f: Force deleted without prompt
   

9. mv command to move or rename files and directories
   basic syntax of
       mv old new file name (rename)
       mv file name of the directory (move files)
   

10. cat command to view the contents of the file in read-only mode to open the
    basic syntax of
        cat [Options] to view the file
    Common Options
        -n: display the line number
    

11. more instruction
    more instruction is a text-based editor vi filter, which displays the contents of the text file by page (where a number of shortcut keys see operating instructions) in a full-screen manner
    basic syntax of
        the file name to see more

12. less instructions
    less split-screen instructions to view the file contents, and more similar to, but more powerful than more, support a variety of terminals displayed in the display, load based on the content to be displayed, for large files, the higher the efficiency of
    basic grammar
        less to see the file name

13. > >> instructions and commands
    > output redirection will cover the contents of the original file
    >> does not overwrite the original contents of the file, but the file appended to the end of
    basic grammar (it will create the file if the file does not exist)
         LS the -l> file (write to the list of documents under cover)
         LS -al >> file (ls -l content appended to the end of the file)
    

14. echo command output content to the console
    basic syntax
        echo [options] [output content]
    

15. the beginning of the head command displays the file (the default display the first ten lines)
    basic syntax
        head (ten lines before viewing the file)
        head -n 3 (see the first three lines of the file)
    
    16.tail command to display the file at the end portion (the default display at the end of ten line)
    basic syntax
        tail file (see the end of the ten lines)
        tail -n 3 file (see the end of the three lines)
        tail -f file (updated real-time tracking of all documents)
    

• ln command to create soft links like shortcuts in windows

  The basic syntax
      ln -s [the original file or directory] [soft link name] (the original file to create a soft link)
  When we use the pwd view the path of any course is the directory where the soft link

• View historical history instruction instruction executed in the past

  The basic syntax of
      history (see past history instruction execution)
      ! N (n is the number of instruction execution)
       History n (n see instructions executed in the past)
  

• date The date and time related instruction operations

  1.date command
  basic syntax
      DATE (current time)
      DATE + '% the Y' (display the current year)
      DATE + '% m' (current month)
      DATE + '% D' (currently displayed day)
      DATE + ' % Y-% d% H:% M:% S '( year, month, day, hour display)
      dATE -s string time (set date and time)
  
  2.cal command
  basic syntax of
      CAL [option] (without display option month calendar)
  

• Search Find command

  The basic syntax
      find [search] [Option] (option to use the help command lookup)
  
  grep command and | grep filtering lookup, '|' represents the result of processing the previous command is output to the rear edge of the command processing
  basic syntax
        grep [options] Find what source file (common options: -n display matching lines and line numbers; -i ignore the case of letters)
  

• Compression, decompression

  And instructions 1.gzip gunzip
  gunzip to decompress the file; gzip archive for
  basic syntax
      gzip file (compressed file .gz format) *
      gunzip .gz file (unzip the file)
  
  file 2.tar instruction packing instructions, the final package love you * .tar.gz
  basic grammar
      tar [option] * .tar.gz packaged content (after packing directory, compressed * .tar.gz format, the option to use self-help instruction view)
  

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