Linux user management & variables - Linux learning (3)

user system

1. useradd username management
   • /home/username create directory
   • /etc/passwd
   • /etc/shadow User password related files
   • id
2. userdel username -r (adding -r will delete the corresponding home directory)
3. usermod -d can modify the location of the user's home directory userod -d /home/w1 w(usermod -g group2 user2)
4. change Modify the user's lifecycle
5. groupadd groupdel(useradd -g group1 user2)
6. su - username Temporary user switching
7. sudo administrator authorization to normal users. The root user can add command authorization through visudo
8. /etc/passwd

   wilson:x:1000:1000::/home/wilson:/bin/bash
   :x(x代表切换用户需要输入密码，为空则不需要密码):uid:gid:comment:家目录位置:用户登录后的命令解释器  
   （如果设置为 /sbin/nologin 则用户不能登录）
   

Pipelines and Redirection

1. pipes and pipe symbols
   • Like signals, pipes are also one of the ways of process communication
   • Anonymous pipe (pipe symbol) is a communication tool often used in Shell programming
   • The pipe symbol is "|", which transfers the execution result of the previous command to the following command
2. redirect
   • A process will open standard input, standard output, and error output three file descriptors by default.
   • Enter the redirection symbol "<"
   • Output redirection symbols ">" (clear input), ">>" (append), "2>" (only output error content), "&>" (output all content)

   #!/bin/bash
   cat > /path/to/a/file << EOF
   I am $USER
   EOF
   

   The above script will generate a new file, which contains a line of I am xx characters

variable assignment

• Interactive variable assignment read var

• variable name = variable value

  a=123 (There can be no spaces on both sides of the equal sign)

• Use let to assign values ​​to variables

  let a=10+20 (not recommended, because bash's computing performance is very poor)

• assign command to variable

  l=ls

• To assign the command result to a variable, use $() or ``

  letc=$(ls -l /etc)

• The variable value can be included in " " or ' ' by special characters such as spaces

• The content of the output variable is quoted using $, such as echo $var

• variable reference

  ${变量名}
$变量名

• If you need to append content after the variable output result, you need to use the first type, otherwise it will be recognized as another variable name

• The default scope of the variable (only valid in its own scope)

• Use source or . to execute the .sh script in the current environment

  • variable export
    • export (variables can be exported to allow subprocesses to read variables)
  • variable export
    • unset

---

• system environment variable
  • Environment variables: Variables that can be obtained every shell open
    • set (see more variables – predefined convenience and environment variables) env (see all environment variables)
    • View only one variable echo $USER
    • $PS1 console log output format
    • $? Is the previous command executed correctly [return 0 for correct execution, 1 for abnormal execution] $$ pid $0 the name of the current process
    • Parameter position $1 $2 ... ${10}

---

• array type
  • Define the array: IPTS=( 10.0.0.1 10.0.0.2 )
    • There are spaces between brackets and content, and spaces are used to separate elements
  • display all elements of an array
    • echo ${IPTS[@]}
  • Display the number of array elements
    • echo ${#IPTS[@]}
  • display the first element of the array
    • echo ${IPTS[0]}

---

• Special characters: a character not only has literal meaning, but also meta-meaning

  • #note
  • ; semicolon - separates statements
  • \ escape symbol
  • " and ' quotes
    • "Double quotes do not fully quote variables
    • ' Single quotes fully quote the variable
    • ` Backticks - can execute a command

  ---

• Operators (the shell only supports integer arithmetic)

  • assignment operator
    • = assignment
    • unset cancel assignment
    • expr performs arithmetic assignment
      • Symbols and operators must have spaces
  • arithmetic operator
  • numeric constant
  • double parentheses
    • (( a++))

• special symbols

  • quotation marks
  • 'Full quote
  • "Incomplete quote
  • `execute command
  • brackets
    • （）(()) $()
      • Parentheses alone produce a shell
      • Array initialization is also done with parentheses IPS=( 21.23 23.23 )
    • [] [[]]
      • for testing
    • > <input and output
    • echo {0…9} outputs numbers from 0 to 9
    • && ||
    • & Background process

vim, sed, awk editor

• vim is a full text editor sed, awk is a line editor
• sed awk is a non-interactive text editor
• sed awk processes text line by line
• Introduction to sed
  • Read file into memory (pattern space) in units of lines
  • Each script with sed operates on the line
  • Output the line after processing is complete