Shell -- operation of variables

Variable operation

stay Bash Shell Environment , You can only do simple integer operations , Decimal operation is not supported
The operation of integer value is mainly through internal command expr Conduct
There must be at least one space between the operator and the variable .

Operation content ： Add (+)、 reduce (-)、 ride (*)、 except (/)、 Remainder (%)
Operation symbol ：$(())and$[]
Operation command ：expr and let
Computing tools ：bc（ System comes with ）

One 、expr command

expr command （ It can not only calculate , It also supports output to the screen ）

frequently-used Several operators

Symbol	meaning
+	Addition operation
-	Subtraction
*	Multiplication , Be careful not to use only “*” Symbol , Otherwise, it will be treated as a file wildcard
/	Division operations
%	Modular operation , Also known as remainder operation , Used to calculate the remainder after the division of numerical values

give an example

[[email protected] ct]# expr 1 + 1
2
[[email protected] ct]# expr 1+1
1+1
[[email protected] ct]# expr 2 * 2
expr:  Grammar mistakes 
[[email protected] ct]# expr 2 \* 2   // Multiplication * Need to escape ;"\" It's the escape sign 
4
[[email protected] ct]# expr 2 '*' 2  // Multiplication can also be expressed in single quotation marks, but it is not necessary because there is only one character 
4

1、expr Arithmetic

expr It supports not only constants but also variable operations

1. Do not operate in script

[[email protected] ~]# X=35 
[[email protected] ~]# Y=16 
[[email protected] ~]# expr $X + 5
40
[[email protected] ~]# expr $X + $Y 
51
[[email protected] ~]# expr $X - $Y 
19
[[email protected] ~]# expr $X \* $Y 
560 
[[email protected] ~]# expr $X / $Y 
2
[[email protected] ~]# expr $X % $Y 
3

2. Interactive computing

#!/bin/bash
#1. Define the output number 
read -p " Please enter the first number ：" num1
read -p " Please enter the second number ：" num2
#2. Perform addition operations 
expr $num1 + $num2

echo " Summation number ：$sum"

Two 、 Operation of special symbolic variables ：$[]、$(())

$[]and$(()) And echo Used together because it can only calculate and cannot output the result

1、$[] Integer operation

[[email protected]]# echo $[10*10]    //$[] Inside * There is no need to escape 
100
[[email protected]]# echo $[10%8]
2
[[email protected]]# echo $[10/8]
1
[[email protected]]# echo $[10/12]
0
[[email protected]]# echo $[10%12]
10

• $[]changeThe\; amountOfshipmentcount,canprovinceA\; little[]inOf$

[[email protected]]# echo $[$a+$b]
13
[[email protected]]# echo $[a+b]
13
[[email protected]]# echo $[a-b]
7
[[email protected]]# echo $[a*b]
30
[[email protected]]# echo $[a/b]
3
[[email protected]]# echo $[a%b]
1

[[email protected]]# echo $[a+b+c]
38
[[email protected]]# echo $[a+b*c]    // Follow multiplication and division before addition and subtraction , Otherwise, it needs to be enclosed in parentheses to indicate the priority operation 
75
[[email protected]]# echo $[(a+b)*c]
360

2、$(()) Integer operation

[[email protected]]# echo $((1+1))
2
[[email protected]]# echo $((5-2))
3

[[email protected]]# echo $((a-b))
7
[[email protected]]# echo $((b-a))    // There can be negative numbers 
-7

3、 ... and 、let Arithmetic

let The operation of can change the value of the variable itself , But it doesn't show the results , need echo, Other operations can be performed without changing the value of the variable itself

 Value of quantity itself 
[[email protected]]# n=1;let n=n+1;echo $n
2
[[email protected]]# let n+=2  //n=n+2
[[email protected]]# 
[[email protected]]# echo $n
4
[[email protected]]# let n=n**2   // Exponentiation ,4 Of 2 Power 
[[email protected]]# echo $n
16
[[email protected]]# let n++ //n Self adding 1 
[[email protected]]# let n--   //n Self reduction 1

[[email protected]]# echo $a
13
[[email protected]]# echo $[a++]   // First output and then self increase 1, At this time a The value of has changed 
13
[[email protected]]# echo $a
14
[[email protected]]# echo $[++a]   // First of all, increase by yourself 1 Then the output , So the changed value is directly output 
15
[[email protected]]# echo $a
15

Four 、bc operation

Support decimal operations , However, it cannot be used directly in the script, otherwise it will enter the interactive interface , It can be used echo Use in combination with piping

• scale ： You can specify a few decimal places

Such as

scale=3  # After the decimal point 3 digit 
10/3
3.333

• It can be done with bc Use it together

  [[email protected] ~]# a=10
  [[email protected] ~]# b=3
  [[email protected] ~]# echo "$a/$b" | bc     
  3
  [[email protected] ~]# echo "scale=2;$a/$b" | bc
  3.33	
  

1、bc You can also do logical operations

How wonderful 1, False as 0

[[email protected] ~]# echo "2>2" | bc
0
[[email protected] ~]# echo "2==2" | bc
1
[[email protected] ~]# echo "2<2" | bc
0

Expand

Common operational expressions ：

i=$(expr 12 \* 5)
i=$((12 * 5))
i=[12 * 5]
let i=12*5

important

i++  amount to  i=$[$i+1]
i--  amount to  i=$[$i-1]
i+=2  amount to  i=$[$i+2]