命令替换
| 方法 | 语法格式 |
| 方法一 | `command` |
| 方法二 | $(command) |
`` 和$()两者是等价的，但推荐初学者使用$()，易于掌握;缺点是极少数UNIX可能不支持
$(())主要用来进行整数运算，包括加减乘除,引用变量前面可以加$，也可以不加$
例子
- 获取系统所有用户并输出
- 根据系统时间计算今年或明年
- 根据系统时间获取今年还剩下多少星期，已经过了多少星期
- 判断 nginx 进程是否存在，若不存在则自动拉起该进程
1）.获取系统所有用户并输出
#!/bin/bash
index=1
for user in `cat /etc/passwd |cut -d ":" -f 1`
do
echo "This is $index user: $user"
index=$(($index+1))
done
输出
This is 1 user: root
This is 2 user: bin
This is 3 user: daemon
This is 4 user: adm
This is 5 user: lp
This is 6 user: sync
This is 7 user: shutdown
This is 8 user: halt
This is 9 user: mail
This is 10 user: operator
This is 11 user: games
This is 12 user: ftp
This is 13 user: nobody
This is 14 user: systemd-network
This is 15 user: dbus
This is 16 user: polkitd
This is 17 user: sshd
This is 18 user: postfix
This is 19 user: chrony
This is 20 user: nscd
This is 21 user: tcpdump
This is 22 user: ntp
This is 23 user: www
This is 24 user: redis
This is 25 user: openvpn
This is 26 user: emqx
This is 27 user: epmd
2）.根据系统时间计算今年或明年
#!/bin/bash
echo "This is $(date +%Y) year";
echo "This is $(($(date +%Y)+1)) year";
输出
This is 2022 year
This is 2023 year
2）.根据系统时间获取今年还剩下多少星期，已经过了多少星期
# example.sh
echo "This year have passed $(date +%j) days";
echo "This year have passed $(date +%V) weeks";
echo "There is $((365-$(date +%j))) days before new year";
echo "There is $(((365-$(date +%j))/7)) weeks before new year";
输出
This year have passed 363 days
This year have passed 52 weeks
There is 2 days before new year
There is 0 weeks before new year
4）.判断 nginx 进程是否存在，若不存在则自动拉起该进程
#!/bin/bash
nginx_process_num=$(ps -ef|grep nginx|grep -v grep|wc -l)
if [ $nginx_process_num -eq 0 ];then
systemctl start nginx
fi
有类型变量
declare 命令和 typeset 命令两者等价declare、typeset 命令都是用来定义变量类型的
declare命令参数表
| 参数 | 含义 |
| -r | 将变量设为只读 |
| -i | 将变量设为整数 |
| -a | 将变量定义为数组 |
| -f | 显示此脚本前定义过的所有函数及内容 |
| -F | 仅显示此脚本前定义过的函数名 |
| -x | 将变量声明为环境变量 |
取消声明的变量：declare +r;declare +i;declare +a;declare +f;declare +F;declare +x;
例子
- 声明变量为只读类型
- 声明变量类型为整型
- 在脚本中显示定义的函数和内容
- 在脚本中显示定义的函数
- 将变量声明为环境变量
- 声明变量为数组
1）.声明变量为只读类型
# example.sh
string="Hello world";
declare -r string
echo "string = $string"
# 再次赋值将无法修改
string="zxcvadsf"
echo "string = $string"
输出
string = Hello world
example7.sh: line 7: string: readonly variable
string = Hello world
2）.声明变量类型为整型
# example.sh
num1=100
num2=10
num3=$num1+$num2
echo $num3
declare -i num3
num3=$num1+$num2
echo "num3 = $num3"
输出
3）.在脚本中显示定义的函数和内容 && 在脚本中显示定义的函数
5）.将变量声明为环境变量
# example7.sh
echo $num4
# bash
> declare -x num4=122
# example7.sh
echo $num4
输出
6）.声明变量为数组
# example.sh
declare -a array
array=("jones" "mike" "kobe" "jordan")
echo "array = ${array[@]}"
echo "array[0] = ${array[0]}"
echo "array[1] = ${array[1]}"
echo "array length = ${#array[@]}"
for v in ${array[@]}
do
echo $v
done
输出
array = jones mike kobe jordan
array[0] = jones
array[1] = mike
array length = 4
jones
mike
kobe
jordan
Bash 数学运算之 expr
语法格式
| 方法 | 语法 |
| 方法一 | expr $num1 operator $num2 |
| 方法二 | $(($num1 operator $num2 )) |
expr 操作符对照表
| 操作符 | 含义 |
| num1 | num2 | num1不为空且非0 ,返回num1 ;否则返回num2 |
| num1 & num2 | num1不为空且非0，返回num1 ;否则返回0 |
| num1 < num2 | num1小于num2 ,返回1 ;否则返回0 |
| num1 <= num2 | num1小于等于num2，返回1 ;否则返回0 |
| num1 = num2 | num1等于num2 ,返回1 ;否则返回0 |
| num1 != num2 | num1不等于num2，返回1 ;否则返回0 |
| num1 > num2 | num1大于num2 ,返回1 ;否则返回0 |
| num1 >= num2 | num1大于等于num2，返回1 ;否则返回0 |
使用 expr 命令时，表达式中的运算符左右必须包含空格，如果不包含空格，将会输出表达式本身:
例子
练习
提示用户输入一个正整数num，然后计算1+2+3+…+num的值;必须对num是否为正整数做判断，不符合应当允许再此输入。
思路
- 通过 expr 操作可判断是否是整数（只有整数才能使用expr）
- 再进行查看操作数是否大 0
# example.sh
#!/bin/bash
while true
do
read -p "pls input a positive number :" num
expr $num + 1 &> /dev/null
if [ $? -eq 0 ]; then
if [ `expr $num \> 0` -eq 1 ]; then
for ((i = 1; i <= $num; i++))
do
sum=`expr $sum + $i`
done
echo "1+2+3...+$num = $sum"
exit
fi
fi
echo "error, input enlegal"
continue
done
输出
> sh example9.sh
pls input a positive number :10
1+2+3...+10 = 55
Bash 数学运算之 bc
- bc 是 bash 内建的运算器,支持浮点数运算
- 内建变量 scale 可以设置,默认为 0
bc操作符对照表
| 操作符 | 含义 |
| num1 + num2 | 求和 |
| num1 - num2 | 求差 |
| num1 * num2 | 求积 |
| num1 / num2 | 求商 |
| num1 % num2 | 求余 |
| num1 ^ num2 | 指数运算 |
> 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'.
scale=4
23.444/5.2
4.5084
3.56+35.4
38.96
