Four simple arithmetic of shell
Four simple arithmetic of shell
Arithmetic operations: By default, the shell can only support simple integer operations
Calculation content:
- Add (+)
- Less(-)
- Multiply()
- except(/)
- Find the remainder (%)
- power(*)
Four arithmetic symbols
| expression | For example | Description |
|---|---|---|
| $(( )) | echo $((1+1)) | |
| $[ ] | echo $[10-5] | |
| expr | expr 10 / 5 | In the expr program, the value and the operator need to be separated by a space, the multiplication (*) operator needs to be escaped with an escape character (\ ), expr cannot be used for exponentiation |
| let | n=1; let n+=1 is equivalent to let n=n+1 | let n+=2 is equivalent to let n=n+2, let n**=2 cannot be used |
Integer arithmetic
Method 1: expr
[root@localhost shell]# n1=10
[root@localhost shell]# n2=20
[root@localhost shell]# expr $n1 + $n2
30
[root@localhost shell]# expr $n1 + $n2
30
[root@localhost shell]# sum=`expr $n1 + $n2`
[root@localhost shell]# echo $sum
30
Method two: $(())
[root@localhost shell]# n1=10
[root@localhost shell]# n2=20
[root@localhost shell]# echo $((n1*n2))
200
[root@localhost shell]# echo $((n1/n2))
0
[root@localhost shell]# echo $((n1%n2))
10
[root@localhost shell]# echo $((2**10))
1024
Method three: $[]
[root@localhost shell]# echo $[5+2]
7
[root@localhost shell]# echo $[5*2]
10
[root@localhost shell]# num=$[2**10]
[root@localhost shell]# echo $num
1024
Method 4: let
[root@localhost shell]# let n1=1+2
[root@localhost shell]# echo $n1
3
[root@localhost shell]# i=10
[root@localhost shell]# let i++
[root@localhost shell]# echo $i
11
Understood i ++ sum ++ i
- Effect on the value of the variable
[root@maomao ~]# i=1
[root@maomao ~]# let i++
[root@maomao ~]# echo $i
2
[root@maomao ~]# j=1
[root@maomao ~]# let ++j
[root@maomao ~]# echo $j
2
- Effect on the value of the expression
[root@maomao ~]# unset i j
[root@maomao ~]# i=1;j=1
[root@maomao ~]# let x=i++ 先赋值，再运算
[root@maomao ~]# let y=++j 先运算，再赋值
[root@maomao ~]# echo $i
2
[root@maomao ~]# echo $j
2
[root@maomao ~]# echo $x
1
[root@maomao ~]# echo $y
2
i++ is first assignment, then operation
++i is to calculate first, then assign
Instance
# 我们要写一个查看当前内存使用百分比的脚本
#!/bin/bash
mem_used=`free -m|grep Mem|awk '{print$3}'`
mem_total=`free -m |grep Mem |awk '{print $2}'`
mem_percent=$(($mem_used*100/$mem_total))
echo "当前内存使用百分比：$mem_percent%"
[root@maomao shell]# bash free.sh
当前内存使用百分比：32%
```shell
# i++ 一般常和while一起使用 while的变量需要自增
# 比如我们写一个探测本网段哪些主机存活的脚本
#!/bin/bash
i=2
while [ $i -le 254 ]
do
{
ip=192.168.188.$i
ping -c1 -W1 $ip &>/dev/null
if [ $? -eq 0 ];then
echo "$ip up."
fi
}&
let i++
done
wait
echo "all finish..."
Decimal arithmetic
Method one: bc
# 使用bc之前需要先安装
yum -y install bc
[root@maomao 1.20]# echo "2.2*4.4124" |bc
9.7072
# 除法的时候需要通过scale指定小数点后几位
[root@maomao 1.20]# echo "scale=5;10/3" |bc
3.33333
[root@maomao shell]# echo "运算百分值：$(echo "scale=2;2.5*6.1*9.3"|bc)%"
运算百分值：141.82%
The above memory script can also be optimized to the decimal point
#!/bin/bash
mem_used=`free -m|grep Mem|awk '{print$3}'`
mem_total=`free -m |grep Mem |awk '{print $2}'`
mem_percent=$(echo "scale=2; $mem_used*100/$mem_total" |bc)
echo "当前内存使用百分比：$mem_percent%"
[root@maomao shell]# bash free.sh
当前内存使用百分比：32.43%
Method two: awk
# awk可以进行常规的所有运算
[root@maomao scripts]# echo "7.7 3.8"|awk '{print ($1-$2)}'
3.9
[root@maomao 1.20]# echo "8.8 6.6"|awk '{print ($1/$2)}'
1.33333
[root@maomao scripts]# echo "3 9"|awk '{print ($1+3)*$2}'
54
Method three: python
# 可以请python帮忙计算
[root@maomao 1.20]# echo "print 10.0/3" |python
3.33333333333
[root@maomao 1.20]# echo "print 3.1415926*3" |python
9.4247778
shell calculator script
#!/bin/bash
read -p "请输入第一个数字：" num1
read -p "请输入第二个数字：" num2
menu() {
cat <<-EOF
#########################
# 计算器 #
# 1.加法 #
# 2.减法 #
# 3.乘法 #
# 4.除法 #
# 5.求模 #
# 6.求幂 #
# m.菜单 #
# q.退出 #
#########################
EOF
}
menu
jia() {
awk "BEGIN{print $1+$2}"
}
jian() {
awk "BEGIN{print $1-$2}"
}
cheng() {
awk "BEGIN{print $1*$2}"
}
chu() {
awk "BEGIN{print $1/$2}"
}
mo() {
awk "BEGIN{print $1%$2}"
}
mi() {
awk "BEGIN{print $1**$2}"
}
while :
do
read -p "请选择功能：" action
case $action in
m)
clear
menu
;;
1)
jia $num1 $num2
;;
2)
jian $num1 $num2
;;
3)
cheng $num1 $num2
;;
4)
chu $num1 $num2
;;
5)
mo $num1 $num2
;;
6)
mi $num1 $num2
;;
q)
break
;;
"")
;;
*)
echo "错误"
;;
esac
done
[root@maomao jiaoben]# bash jisuanqi1.sh
请输入第一个数字：124.12
请输入第二个数字：2703
#########################
# 计算器 #
# 1.加法 #
# 2.减法 #
# 3.乘法 #
# 4.除法 #
# 5.求模 #
# 6.求幂 #
# m.菜单 #
# q.退出 #
#########################
请选择功能：1
2827.12
请选择功能：2
-2578.88
请选择功能：3
335496
请选择功能：4
0.0459193
请选择功能：5
124.12
请选择功能：6
inf
请选择功能：q