411 lines
9.6 KiB
Markdown
411 lines
9.6 KiB
Markdown
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<h1><center>Shell变量</center></h1>
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作者：行癫（盗版必究）
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------
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## 一：变量概述
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#### 1.什么是变量
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变量来源于数学，是计算机语言中能储存计算结果或能表示值的抽象概念
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变量可以通过变量名访问，在指令式语言中，变量通常是可变的；在某些条件下也是不可变的
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#### 2.变量的规则
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命名只能使用英文字母，数字和下划线，首个字符不能以数字开头
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中间不能有空格，可以使用下划线
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不能使用标点符号
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不能使用bash里的关键字
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## 二：变量分类
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#### 1.自定义变量
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定义变量：变量名=变量值 例如：xingdian=123
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引用变量：$变量名 或 ${变量名}
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查看变量：echo $变量名
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取消变量：unset 变量名
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作用范围：仅在当前shell中有效
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#### 2.环境变量
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定义环境变量：
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方法一 export back_dir2=/home/backup
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方法二 export back_dir1 将自定义变量转换成环境变量
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引用环境变量：$变量名 或 ${变量名}
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查看环境变量：echo $变量名
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取消环境变量：unset 变量名
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变量作用范围：在当前shell和子shell有效
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注意：
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环境变量拥有可继承性：export之后就拥有继承性
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永久生效：写到环境变量脚本，/etc/profile ~/.baserc ~/.bash_profile /etc/bashrc
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案例：
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```shell
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[root@xingdiancloud ~]# vim /etc/profile
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JAVA_HOME=/usr/local/java
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PATH=$JAVA_HOME/bin:$PATH
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export JAVA_HOME PATH
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```
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```shell
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[root@xingdiancloud ~]# vim ~/.bash_profile (只显示部分)
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PATH=$PATH:$HOME/bin:/usr/local/mycat/bin
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```
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/etc/profile
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这是系统最主要的shell设置文件，也是用户登陆时系统最先检查的文件，有关重要的环境变量都定义在此，其中包括PATH,USER,LOGNAME,MAIL,HOSTNAME,HISTSIZE,INPUTRC等。而在文件的最后，它会检查并执行/etc/profile.d/*.sh的脚本
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~/.bash_profile
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这个文件是每位用户的bash环境设置文件，它存在与于用户的主目录中，当系统执行/etc/profile 后，就会接着读取此文件内的设置值。在此文件中会定义USERNAME,BASH_ENV和PATH等环境变量，但是此处PATH除了包含系统的$PATH变量外加入用户的“bin”目录路径
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~/.bashrc
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接下来系统会检查~.bashrc文件，这个文件和前两个文件（/etc/profile 和~.bash_profile）最大的不同是，每次执行bash时，~.bashrc都会被再次读取，也就是变量会再次地设置，而/etc/profile,~./bash_profile只有在登陆时才读取。就是因为要经常的读取，所以~/.bashrc文件只定义一些终端机设置以及shell提示符号等功能，而不是定义环境变量
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~/.bash_login
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如果~/.bash_profile文件不存在，则系统会转而读取~/.bash_login这个文件内容。这是用户的登陆文件，在每次用户登陆系统时，bash都会读此内容，所以通常都会将登陆后必须执行的命令放在这个文件中
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.profile
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如果~./bash_profile ~./bash_login两个文件都不存在，则会使用这个文件的设置内容，其实它的功能与~/.bash_profile相同
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.bash_logout
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如果想在注销shell前执行一些工作，都可以在此文件中设置
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```shell
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[root@xingdiancloud ~]# vi ~.bash_logout
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clear
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仅执行一个clear命令在你注销的时候
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```
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~/.bash_history
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这个文件会记录用户先前使用的历史命令
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注意：
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在/etc/profile.d建立独立的环境变量配置文件
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常用环境变量：USER UID HOME HOSTNAME PWD PATH
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PATH：这个变量存放的是所有命令所在的路径 修改：PATH=$PATH:+目录
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#### 3.位置变量
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$1 $2 $3 $4 $5 $6 $7 $8 $9 $10
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案例：
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```shell
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[root@xingdiancloud sh]# cat xingdian.sh
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#!/bin/bash
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echo "hello $1"
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[root@xingdiancloud sh]# bash xingdian.sh xingdian
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hello xingdian
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```
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4.预定义变量
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```shell
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$0 脚本名
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$* 所有的参数
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$@ 所有的参数
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$# 参数的个数
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$$ 当前进程的PID
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$! 上一个后台进程的PID
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$? 上一个命令的返回值 0表示成功
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```
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案例：
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```shell
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[root@xingdiancloud sh]# cat test.sh
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#!/bin/bash
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echo "第2个位置参数是$2"
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echo "第1个位置参数是$1"
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echo "第4个位置参数是$4"
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echo "所有参数是: $*"
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echo "所有参数是: $@"
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echo "参数的个数是: $#"
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echo "当前进程的PID是: $$"
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echo '$1='$1
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echo '$2='$2
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echo '$3='$3
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echo '$*='$*
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echo '$@='$@
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echo '$#='$#
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echo '$$='$$
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```
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## 三：变量赋值
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#### 1.显示赋值
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变量名=变量值
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示例：
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```shell
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[root@xingdiancloud ~]# ip1=192.168.1.251
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[root@xingdiancloud ~]# school="BeiJing 1000phone"
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[root@xingdiancloud ~]# today1=`date +%F`
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[root@xingdiancloud ~]# today2=$(date +%F)
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```
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#### 2.键盘读入
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```shell
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read 变量名
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read -p "提示信息: " 变量名
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read -t 5 -p "提示信息: " 变量名 -t 后面跟秒数，定义输入字符的等待时间
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read -n 2 变量名 -n 后跟一个数字，定义输入文本的长度，很实用。
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```
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案例1：
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```shell
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[root@xingdiancloud ~]# vim first.sh
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back_dir1=/var/backup
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read -p "请输入你的备份目录: " back_dir2
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echo $back_dir1
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echo $back_dir2
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[root@xingdiancloud ~]# sh first.sh
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```
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案例2：
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```shell
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[root@xingdiancloud ~]# vim ping2.sh
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#!/bin/bash
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read -p "Input IP: " ip
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ping -c2 $ip &>/dev/null
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if [ $? = 0 ];then
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echo "host $ip is ok"
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else
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echo "host $ip is fail"
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fi
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[root@xingdiancloud ~]# chmod a+x ping2.sh
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[root@xingdiancloud ~]# ./ping.sh
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```
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注意：定义或引用变量时注意事项
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" " 弱引用 可以实现变量和命令的替换
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' ' 强引用 不完成变量替换
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反引 命令替换 等价于 $() 反引号中的shell命令会被先执行
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```shell
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[root@xingdiancloud ~]# school=1000phone
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[root@xingdiancloud ~]# echo "${school} is good"
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1000phone is good
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[root@xingdiancloud ~]# echo '${school} is good'
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${school} is good
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[root@xingdiancloud ~]# touch `date +%F`_file1.txt
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[root@xingdiancloud ~]# touch $(date +%F)_file2.txt
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[root@xingdiancloud ~]# disk_free3="df -Ph |grep '/$' |awk '{print $4}'" 错误
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[root@xingdiancloud ~]# disk_free4=$(df -Ph |grep '/$' |awk '{print $4}')
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[root@xingdiancloud ~]# disk_free5=`df -Ph |grep '/$' |awk '{print $4}
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```
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## 四：变量运算
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#### 1.整数运算
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方法一：expr
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```shell
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[root@xingdiancloud ~]# expr 1 + 2
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[root@xingdiancloud ~]# expr $num1 + $num2 + - \* / %
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```
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方法二：$(())
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```shell
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[root@xingdiancloud ~]# echo $(($num1+$num2)) + - * / %
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[root@xingdiancloud ~]# echo $((num1+num2))
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[root@xingdiancloud ~]# echo $((5-3*2))
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[root@xingdiancloud ~]# echo $(((5-3)*2))
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[root@xingdiancloud ~]# echo $((2**3))
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[root@xingdiancloud ~]# sum=$((1+2)); echo $sum
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```
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方法三：$[]
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```shell
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[root@xingdiancloud ~]# echo $[5+2] + - * / %
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[root@xingdiancloud ~]# echo $[5**2]
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```
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方法四：let
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```
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[root@xingdiancloud ~]# let sum=2+3; echo $sum
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[root@xingdiancloud ~]# let i++; echo $i
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```
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#### 2.小数运算
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使用bc做小数运算，scale指定小数点位数
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加法运算(scale参数无效)
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```shell
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[root@xingdiancloud ~]# echo "5.999 + 5.001"|bc
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6.000
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[root@xingdiancloud ~]# echo "5.111+ 5.1114"|bc
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10.2224
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```
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减法运算(scale参数无效)
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```shell
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[root@xingdiancloud ~]# echo "2.22 - 1.11"|bc
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1.11
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```
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乘法运算
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```shell
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[root@xingdiancloud ~]# echo "5.12 * 5.6000"|bc
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28.6720
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```
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注意：乘积小数点位数默认以乘数中小数点位数最多的为准（不指定scale参数）
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除法运算
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```shell
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[root@xingdiancloud ~]# echo "scale=2;9.898 / 1.11"|bc
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8.91
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[root@xingdiancloud ~]# echo "9.898 / 1.11"|bc
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8
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```
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## 五：扩展
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#### 1.内容的删除
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案例一
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```shell
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[root@xingdian ~]# url=www.sina.com.cn
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[root@xingdian ~]# echo ${#url} 获取变量值的长度
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15
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[root@xingdian ~]# echo ${url} 标准查看
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www.sina.com.cn
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[root@xingdian ~]# echo ${url#*.} 从前往后，最短匹配
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sina.com.cn
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[root@xingdian ~]# echo ${url##*.} 从前往后，最长匹配 贪婪匹配
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cn
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[root@xingdian ~]# url=www.sina.com.cn
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[root@xingdian ~]# echo ${url#a.}
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www.sina.com.cn
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[root@xingdian ~]# echo ${url#*sina.}
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com.cn
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```
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案例二
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```shell
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[root@xingdian ~]# url=www.sina.com.cn
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[root@xingdian ~]# echo ${url}
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www.sina.com.cn
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[root@xingdian ~]# echo ${url%.*} 从后往前，最短匹配
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|||
|
|
www.sina.com
|
|||
|
|
[root@xingdian ~]# echo ${url%%.*} 从后往前，最长匹配 贪婪匹配
|
|||
|
|
www
|
|||
|
|
[root@xingdian ~]# echo $HOSTNAME
|
|||
|
|
xingdian.1000phone.com
|
|||
|
|
[root@xingdian ~]# echo ${HOSTNAME%%.*}
|
|||
|
|
xingdian
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
#### 2.索引及切片
|
|||
|
|
|
|||
|
|
```shell
|
|||
|
|
[root@xingdian ~]# echo ${url:0:5}
|
|||
|
|
0:从头开始
|
|||
|
|
5:到第五个
|
|||
|
|
[root@xingdian ~]# echo ${url:5:5}
|
|||
|
|
[root@xingdian ~]# echo ${url:5}
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
#### 3.变量内容替换
|
|||
|
|
|
|||
|
|
```shell
|
|||
|
|
[root@xingdian ~]# url=www.sina.com.cn
|
|||
|
|
[root@xingdian ~]# echo ${url/sina/baidu}
|
|||
|
|
www.baidu.com.cn
|
|||
|
|
[root@xingdian ~]# url=www.sina.com.cn
|
|||
|
|
[root@xingdian ~]# echo ${url/n/N}
|
|||
|
|
www.siNa.com.cn
|
|||
|
|
[root@xingdian ~]# echo ${url//n/N} 贪婪匹配
|
|||
|
|
www.siNa.com.cN
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
#### 4.自增运算
|
|||
|
|
|
|||
|
|
对变量的值的影响
|
|||
|
|
|
|||
|
|
```shell
|
|||
|
|
[root@xingdian ~]# i=1
|
|||
|
|
[root@xingdian ~]# let i++
|
|||
|
|
[root@xingdian ~]# echo $i
|
|||
|
|
2
|
|||
|
|
[root@xingdian ~]# j=1
|
|||
|
|
[root@xingdian ~]# let ++j
|
|||
|
|
[root@xingdian ~]# echo $j
|
|||
|
|
2
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
对表达式的值的影响
|
|||
|
|
|
|||
|
|
```shell
|
|||
|
|
[root@xingdian ~]# unset i
|
|||
|
|
[root@xingdian ~]# unset j
|
|||
|
|
[root@xingdian ~]#
|
|||
|
|
[root@xingdian ~]# i=1
|
|||
|
|
[root@xingdian ~]# j=1
|
|||
|
|
[root@xingdian ~]#
|
|||
|
|
[root@xingdian ~]# let x=i++ 先赋值，再运算
|
|||
|
|
[root@xingdian ~]# let y=++j 先运算，再赋值
|
|||
|
|
[root@xingdian ~]#
|
|||
|
|
[root@xingdian ~]# echo $i
|
|||
|
|
2
|
|||
|
|
[root@xingdian ~]# echo $j
|
|||
|
|
2
|
|||
|
|
[root@xingdian ~]#
|
|||
|
|
[root@xingdian ~]# echo $x
|
|||
|
|
1
|
|||
|
|
[root@xingdian ~]# echo $y
|
|||
|
|
2
|
|||
|
|
```
|