The use of if,case,for in shell scripts

Directory
I. Condition selection: If statement
Two. Conditional judgment: Case statement
Three. For loop

I. Condition selection: If statement

2. Single Branch
   if judgment condition; then
   Branch code with true condition
   Fi
   Example: Judging whether a number equals 10

   #!/bin/bash   read -p ‘输入一个数字‘ num   if [ $num -eq 10 ];then    echo 该数字等于10    fi

4. Dual Branch
   if judgment condition; Then condition is true for the branch code
   Else condition is a false branch code
   Fi
   Example: Judging if a number is greater than 10

           #！/bin/bash         read -p ‘输入一个数字‘ num          if [ $num -gt 10 ];then               echo 该数字大于10          else                echo 该数字不大于10            fi

5. Multiple branches
   if judgment condition 1; Then condition is true for the branch code
   Elif judgment Condition 2; Then condition is true for the branch code
   Elif judgment Condition 3; Then condition is true for the branch code
   Else the above conditions are false branch codes
   Fi
   Example: Judging the range of a number

           #!/bin/bash        read -p ‘输入一个数字‘ num        if [ $num -lt 10 ];then             echo 该数字小于10        elif [ $num -ge 10 -a $num -lt 20  ];then              echo 该数字大于等于10小于20         elif [ $num -ge 20 -a $num -lt 50 ];then              cho 该数字大于等于20小于50         else                   echo 该数字大于等于50        fi      

   Two. Conditional Judgment Case Statement

   • Case $ variable name in
     Condition 1)
     branch 1;;
     Condition 2)
     branch 2;;
     Default condition *)
     Default Branch;;
     Esac
     After each condition followed by) the end of each branch with;; End
     Example
     Write a script that can judge Yes/no, (the case can be recognized, yes nine kinds of possible, no four kinds of possible),

                       #!/bin/bash                    read -p "请输入yes|no: " q                case $q in                    [Yy][Ee][Ss]|[Yy])                   echo "yes";;                   [Nn][Oo])                  echo "no";;                              *)                 echo "请输入正确的格式"            esac

     Three. For loop

     Execution mechanism: Assigns the element in the list to the "variable name" in turn; The loop body is executed once each assignment; Until the elements in the list are exhausted, the loop ends
     For variable name in list;
     Loop body
     Done

   Example 1 calculates the sum of 1 to 10 of all positive integers using a for loop

                  #!/bin/bash                let s=0                 for n in echo {1..10};do                  s=$[$s+$n]                   echo $s                     done

   Example 2 printing a 99 multiplication table with a for loop

                    #!/bin/bash         for i in {1..9};do               for n in `seq 1 $i`;do                echo -n -e " $i"x"$n=$[i*n] "                done                 echo            done（每一个for要对应一个done）

The use of if,case,for in shell scripts