核心知识点（二）变量的高级用法

命令替换

语法格式

方法	语法格式
方法一	command
方法二	$(command)

小例子一：取出系统中所有的用户并输出

#!/bin/bash

index=1

users=`cat /etc/passwd | cut -d ":" -f 1`

for user in $users

do

    echo "this is $index user:$user"

    index=$((index+1))

done

小例子二： 根据系统时间计算今年或者明年

[wanghui@www example_1]$ echo "This is $(date +%Y) year"

[wanghui@www example_1]$ echo "This is $(($(date +%Y) +1 )) year"    ##时间计算+1

[wanghui@www example_1]$ echo "$((20+30))"     #运算

[wanghui@www example_1]$ num1=20

[wanghui@www example_1]$ num2=70

[wanghui@www example_1]$ echo "$(($num1 + $num2))"    #这样也可以echo "$((num1 + num2))"

小例子三：计算今年过去了多少星期，还剩多少星期

[wanghui@www example_1]$ vim example2.sh

#!/bin/bash

echo "This year has passed $(date +%j) days"

echo "This year has passed $(($(date +%j)/7)) week"

echo "There is $((365 - $(date +%j))) days before new year"

echo "There is $(((365 - $(date +%j))/7)) weeks before new year"

小例子四： 判定Nginx进程是否存在，不存在就自动拉起（https://segmentfault.com/a/1190000009723940）

#!/bin/bash

nginx_process_num=$(ps -ef | grep nginx | grep -v grep | wc -l)

if [ $nginx_process_num -eq 0 ];then

   systemctl restart nginx

fi

变量类型

• declare=typeset

image.png

[wanghui@www shell_script]$ declare -r var2

[wanghui@www shell_script]$ var2="hello java"

---------声明

[wanghui@www shell_script]$ num1=10

[wanghui@www shell_script]$ num2=$num1+10

[wanghui@www shell_script]$ echo $num2

10+10

[wanghui@www shell_script]$ declare -i num3

[wanghui@www shell_script]$ num3=$num1+10

[wanghui@www shell_script]$ echo $num3

[wanghui@www shell_script]$ declare -F   #展示系统函数

[wanghui@www shell_script]$ declare -a array     #声明数组 

[wanghui@www shell_script]$ array=("jones","mike","kobe","jordan")    #定义数组

[wanghui@www shell_script]$ echo ${array[@]}    #列出所有元素

声明环境变量

[wanghui@www shell_script]$ num5=30

[wanghui@www shell_script]$ echo $num5

30

[wanghui@www shell_script]$ vim test1.sh

#!/bin/bash

echo $num5

[wanghui@www shell_script]$ echo $num5

30

[wanghui@www shell_script]$ bash test1.sh #不能引用num5

[wanghui@www shell_script]$ declare -x num5  #声明即可

[wanghui@www shell_script]$ bash test1.sh 

30

取消声明的变量

declare +a

declare +r

declare +i

declare +x

expr计算

image.png
image.png

[wanghui@www shell_script]$ num1=30

[wanghui@www shell_script]$ num2=50

[wanghui@www shell_script]$ expr $num1 \> $num2

0

[wanghui@www shell_script]$ num1=17

[wanghui@www shell_script]$ num2=5

[wanghui@www shell_script]$ expr $num1 + $num2

22

[wanghui@www shell_script]$ num3=$(expr $num1 + $num2)

[wanghui@www shell_script]$ echo $num3

22

[wanghui@www shell_script]$ expr $num1 \* $num2

85

[wanghui@www shell_script]$ expr $num1 / $num2

3

[wanghui@www shell_script]$ expr $num1 % $num2

2

[wanghui@www shell_script]$ num3=$(($num1 + $num2))

[wanghui@www shell_script]$ echo $num3

22

[wanghui@www shell_script]$ num3=$(($num1 * $num2))

[wanghui@www shell_script]$ echo $num3

85

image.png

练习

提示用户输入一个正整数，然后计算1+2+3+4+....num,必须对num是否为正整数做出判断，不符合应当允许其再次输入

[wanghui@www shell_script]$ vim sum.sh

#!/bin/bash

while true

do

    read -p "Please input a positive number:" num

    expr $num + 1 &> /dev/null

    if [ $? -eq 0 ];then

        if [ `expr $num \> 0` -eq 1 ];then

            #echo "Yes,Positive Number"

            for ((i=1;i<=$num;i++))

            do

               sum=`expr $sum + $i`

            done

             echo "1+2+3+...+$num=$sum"

             exit

        fi

    fi

    echo "error input unlegal.."

    continue

done

bc计算

• bc是bash内建的运算器，支持浮点数运算

• 内渐变量scale可以设置，默认是0

image.png

[wanghui@www shell_script]$ bc

bc 1.06.95

Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.

This is free software with ABSOLUTELY NO WARRANTY.

For details type `warranty'. 

1+2

3

3-2

1

3*8

24

9/3

3

10/3

3

scale=2

10/3

3.33

[wanghui@www shell_script]$ echo "scale=9;2333.88/33" | bc    #脚本中使用方法

70.723636363

浮点数运算

#!/bin/bash

read -p "input a number:" num1

read -p "input a number:" num2

echo "scale=2;$num1/$num2" | bc