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
