ba-shell Learning summary

Tags: script

shell What is it? ？

shell It's a command line interpreter , Provides users with a way to linux The kernel sends requests to run the program's interface system level program , User available shell To start up , Hang up , Stop even writing programs

shell classification ：c_shell,z_shell,ba_shell

How the script is executed

• The script needs to #！/bin/bash start
• Scripts require executable permissions + X perhaps sh + xxx.sh perform .

Shell A variable is introduced

• Subsystem variables and user-defined variables ,
• System variables ：$HOME $PWD $SHELL $USERetc.etc.canusesetdisplayintheYesOfsystemsystemchangeThe\; amount“$” Used to represent variable substitution
• User customization ： Variable name （ Do not start with a number , Capitalization )= value (= There must be no space between ） A=10
• Revoke variables ：unset + Variable unset A
• Declare static variables ：readonly Variable , Be careful ： You can't unset
• take The return value of the command Assigned to a variable A=’date’ or A=$(date)
• take shell Variable output is environment variable / Global variables （ The whole picture is visible , Available variables ） export Variable name = value /etc/profile
• Let the modified profile take effect immediately source The configuration file source /etc/profile
• Multiline comment :<<! xxxx ！
• Positional arguments $0 To express an order $1 Represents the first parameter ...$9surfaceinThe\; first9individualginsengCount.$# Indicates the number of parameters
• Operator writing REST1=$((2+3))REST2=$[2+3] REST3= expr 2 + 3

grammar

• conditional
  
  1): = String comparison judgment statement ：
  2): A comparison of two integers
  -lt Less than little

  -le Less than or equal to
  -eq be equal to equal
  -gt Greater than

  -ge Greater than or equal to

  -ne It's not equal to

  3): Judge according to the file authority
  -r Have read permission
  -w Have the right to write
  -x Have the authority to execute

  4): Judge according to the document type
  -f The file exists and is a regular file
  -e File exists

  -d The file exists and is a directory

  if [ "ok" = "ok" ]
  then
  		echo "equal"
  fi
  

  if [ -f /root/shcode/aaa.txt ]
  then
  		echo "exist"
  elif
  		echo "not exist "
  fi
  
  

• case sentence
  

  case $1 in   # Positional arguments 
  "1")
  echo " Monday “ ;; "2") echo " Tuesday " ;; *) echo "other..."
  ;;
  esac #case  Write back 
  

• For loop
  
  

• while loop
  

• read Read console input
  

  read -p " Please enter a num" NUM -t 10
  echo " Input num=$NUM"
  

• Custom function
  
  

  Learn to refer to teacher Han Shunping's video ( Attached video address ）
  https://www.bilibili.com/video/BV1Sv411r7vd?p=107&spm_id_from=pageDriver