Basic Grammar of Linux-shell programming

Characters that use double quotes

Double quotes are an important part of the shell

echo Hello        echo"Hello     World" Hello     World

How to display: Hello "World"
is the following command OK? $ echo "Hello" World ""
Correct method: echo "Hello \" World\ ""

Condition test

Test command
Test expression or [expression]
Test command-supported conditional tests
string comparison
Arithmetic comparison
FILE-related condition testing
Logical operations

"abc"if$str"abc"then  echo"The strings are equal"else  echo"The strings are not equal“ fi

 mkdir temp  if-dthen  echo"temp is a directory"fi

Conditional statements

形式if [ expression ]then    statementselif [ expression ]then    statementselif …else    statementsfi紧凑形式; (同一行上多个命令的分隔符)

Case Statement
How many possible situations occur when Append is executed?

#!/bin/sh   #脚本名: Append  case    $#   1 ) Cat >> $1 ;; 2 ) Cat >> $2  <  $1 ;; *) echo  ;; esac   

21exist1notexistexist1exist2notexist2exist1notexistnotexist

SELECT statement

in itemlistdo    statementsdone作用生成菜单列表

举例：一个简单的菜单选择程序#!/bin/shin Continue Finishdo    case “$itemin        Continue) ;;        break ;;        echo “Wrong choice! Please select again!” ;;    esacdoneQuestion: 用while语句模拟?

Command Combination statement

分号串联command1; command2; …条件组合AND命令表         格式：statement1 && statement2 && statement3 && …OR命令表         格式：statement1 || statement2 || statement3 || …

Function call

形式func(){    statements}局部变量local关键字函数的调用func para1 para2 …返回值return

Yesno() {msg= " $"Def=" $” while true; Do        Echo” ”Echo“$msg”ReadAnswerif[ “$answer” ]; Then         Case“$answer”inchy| y|yes| YES)return 0;; n| N|no|no)return 1;; *)Echo“ ”Echo"Error:invalid response, expected \" Yes\ "or \" no\ "."Continue;;Esac        Else          return $def        fi      Done}

调用函数yesnoif1then    :else    exit1fi

Miscellaneous commands

Break: Exit from For/while/until Loop
Continue: Jumps to the next loop to continue execution
Exit N: Exit script run with exit code "n"
Return: function returns
Export: Exporting variables to the shell to make it a shell environment variable
Set: Setting parameter variables for the shell
unset: Removing a variable or function from the environment
Trap: Specifies the action to be performed after receiving the operating system signal
":" (Colon command): Empty command
“.” (Period command) or source: Executes a command in the current shell

Shell Application Examples

Write a script to implement the user management in Linux system, the specific function requirements are as follows
The script adds a new group of Class1, and then adds 30 users belonging to this group, with the user name in the form stuxx (where xx is from 01 to 30)
Key commands
Groupadd
Useradd
Mkdir
Chown
Chgrp

#!/bin/shI=1Groupadd Class1 while[$i-le -] Doif[$i-le9] ; ThenUsername=stu0${i}ElseUsername=stu${i}fiUseradd$usernamemkdir/home/$usernameChown-r$username/home/$usernameChgrp-r class1/home/$usernamei=$ (($i+1)) Done

Basic Grammar of Linux-shell programming