5. Shell operation

 

Integer Value Operations

 

 

Use the expr command: only integer operations can be performed, and the calculation result is returned by default

 

 

Format:

 

expr integer 1 operator integer 2 ...

 

Integer values ​​can be provided by variables, directly giving the result of the operation

 

 

+     　　addition expr 43 + 21 , expr $X + $Y
     　　-subtraction　　 expr 43 - 21 , expr $X - $Y 
\ *       multiplication　　expr 43 \* 21 , expr $X \* $Y
 /        division　　expr 43 / 21 , expr $X / $Y
 % take the remainder　expr 43 % 21 , expr $X % $Y　

 

 

example:

#!/bin/bash
#Calculate the sum of any 2 numbers entered by the user from the terminal
read -p " Please enter the first number " num1
read -p " Please enter the second number " num2
sum=`expr $num1 +  $num2`
echo "$num1 + $num2 = $sum"

 

 

 

Use $[] or $(()) expressions

 

 

 

Need to use echo to output the result, the operation type is similar to expr
Multiplication * without escape character \
When using a variable, specify the variable name directly without adding the $ sign

 

 

How to perform arithmetic operations in the shell :

let Arithmetic expression let C=$A+ $B
$[arithmetic expression] C =$[$A+ $B]
$((arithmetic expression)) C =$(($A+ $B))
expr Arithmetic operation expression, 
there should be spaces between operands and operators in the expression, 
and command references should be used    
C=`expr $A + $B`    

 

simplification of expressions

 

Increment and decrement of variables

 

 

 

shorthand expression full expression
i++      　　i=i+1
i--    　　　i=i-1
i*=2    　　i=i*2
i+=2     　i=i+2
i-=2      i=i-2
i%=2     i=i%2

 

 

Notice:

i++ : Participate in other operations first, then operate
 ++i: operate first, then participate in other operations

 

 

example:

 

i=1
echo $((++i))
j=1
echo $((j++))

 

 

 

 

 

Operations such as increment / decrement of variables

 

 

Use the let command

 

Manipulate variable values, only operate, do not output results
To see the results, use the echo command
let: only do the operation without returning the calculation result, suitable for self-addition and self-subtraction operations

 

#!/bin/bash
i=10 
let i-=2
echo $i
let i-=2
echo $i
echo -------------------
i=10 
let i+=2
echo $i
let i+=2
echo $i