Shell basic operator -- arithmetic operator

Shell Like any programming language , Support for multiple operators , Include ：

• Arithmetic operator
• Relational operator
• Boolean operator
• String operators
• File test operators

Native bash Simple mathematical operations are not supported , But you can do it with other commands , for example awk and expr,expr The most commonly used .expr Is an expression calculation tool , It can be used to evaluate expressions . Add two numbers ( Note the use of back quotes ` Not single quotes ') example

1. #!/bin/bash
2. val=`expr 2 + 2`
3. echo " The sum of the two is : $val"

Execute the script , The output is as follows ：

1. The sum of the two is :4

Two points attention ： Space between expression and operator , for example 2+2 It is not right , Must be written as 2 + 2, This is different from most programming languages we are familiar with . The complete expression is to be ` ` contain , Note that this character is not a common single quotation mark , stay Esc Key bottom .

Arithmetic operator

The following table lists the common arithmetic operators , Assumed variable a by 10, Variable b by 20：

Operator	explain	give an example
+	Add	`expr $a + $b` The result is 30.
-	Subtraction	`expr $a - $b` The result is -10.
*	Multiplication	`expr $a * $b` The result is 200.
/	division	`expr $b / $a` The result is 2.
%	Remainder	`expr $b % $a` The result is 0.
=	assignment	a=$b Will put the variable b The value is assigned to a.
==	equal . Used to compare two numbers , Same returns true.	[ $a == $b ] return false.
!=	It's not equal . Used to compare two numbers , If not, return to true.	[ $a != $b ] return true.

Be careful ： Conditional expressions should be placed between square brackets , And there should be spaces , for example : [$a$b] It's wrong. , Must be written as [ $a$b ]. Examples of arithmetic operators are as follows ：

1. #!/bin/bash
2. a=10
3. b=20
4. val=`expr $a + $b`
5. echo "a + b : $val"
6. val=`expr $a - $b`
7. echo "a - b : $val"
8. val=`expr $a \* $b`
9. echo "a * b : $val"
10. val=`expr $b / $a`
11. echo "b / a : $val"
12. val=`expr $b % $a`
13. echo "b % a : $val"
14. if[ $a == $b ]
15. then
16. echo "a be equal to b"
17. fi
18. if[ $a != $b ]
19. then
20. echo "a It's not equal to b"
21. fi

Execute the script , The output is as follows ：

1. a + b :30
2. a - b :-10
3. a * b :200
4. b / a :2
5. b % a :0
6. a It's not equal to b

Be careful ：

Multiplication sign (*) There must be a backslash in the front (\) To achieve multiplication ;