流程控制之if判断

Posted 2021-06-27 givenchy_yzl

流程控制之if判断

一：单分支if

语法：

if 条件;then
    要执行的命令1
 	要执行的命令2
    要执行的命令3
    ...
fi

# 上述语法可以用一行代码代替
[ 条件信息 ] && xxx

示例

[root@egon test]# cat disk_monitor.sh 
#!/usr/bin/env bash

disk_use=$(df -P |grep '/$' |awk '{print $5}' |awk -F% '{print $1}')
if [ $disk_use -gt 10 ];then
    echo "warning:Not enough hard disk space"
fi
[root@egon test]# . disk_monitor.sh 
warning:Not enough hard disk space

注意：if 测试中还可以执行命令 根据命令的返回值做判断

[root@egon ~]# if cd / ;then echo Y ;fi
Y
[root@egon /]# if grep -q root /etc/passwd ;then echo Y ;fi
Y

二：双分支if

语法

if 条件;then
    要执行的命令1
 	要执行的命令2
    要执行的命令3
    ...
else
    要执行的命令1
 	要执行的命令2
    要执行的命令3
    ...
fi

# 上述语法可以用一行代码代替
[ 条件信息 ] && xxx || xxxx

示例

#!/bin/bash
username='egon'
password='123'
read -p 'user: ' name 
read -p 'passwd: ' passwd

if [ $name = $username -a $passwd = $password ];then
    echo 'login successful'
else
    echo 'username or password err'
fi

三：多分支if

语法：

if 条件;then
    要执行的命令1
 	要执行的命令2
    要执行的命令3
    ...
elif 条件;then
    要执行的命令1
 	要执行的命令2
    要执行的命令3
    ...
elif 条件;then
    要执行的命令1
 	要执行的命令2
    要执行的命令3
    ...
...
else
    要执行的命令1
 	要执行的命令2
    要执行的命令3
    ...
fi

示例1:猜年龄

======================版本1======================
#!/bin/bash
age=87
read -p 'num: ' n

if [ $n -eq $age ];then
    echo 'you get it'
elif [ $n -gt $age ];then
    echo 'too big'
elif [ $n -lt $age ];then
    echo 'too small'
fi

======================版本2======================
#!/bin/bash

read -p ">>> " num

[[ ! $num =~ ^[0-9]+$ ]] && echo "请输入数字" && exit

if [ $num -gt 18 ];then
    echo "too big"
elif [ $num -lt 18 ];then
    echo "too small"
else
    echo "you got it"
fi

示例2：查询成绩

======================版本1======================
#!/bin/bash
read -p 'your score: ' score

if [ $score -ge 90  ];then
    echo '优秀'
elif [ $score -ge 70 -a $score -lt 90 ];then
    echo '良好'
elif [ $score -ge 60 -a $score -lt 70 ];then
    echo '一般'
elif [ $score -lt 60 ];then
    echo '较差'
fi

======================版本2======================
#!/bin/bash

read -p "your score>>> " score

[[ ! $score =~ ^[0-9]+$ ]] && echo "请输入数字" && exit

if [ $score -ge 90 ];then
    echo "优秀"
elif [ $score -ge 70 ];then
    echo "良好"
elif [ $score -ge 60 ];then
    echo "一般"
else
    echo "较差"
fi

示例3：判断是否是数字

read -p "请输入一个数值: " num

while :
do
        if [[ $num =~ ^[0-9]+$ ]];then
                break
        else
                read -p "不是数字，请重新输入数值: " num
        fi
done

echo "你输入的数字是: $num"

四、练习

1、编写脚本，命令行传入一个文件路径，判断文件的类型

read -p "请输入一个文件路径" file
if [ -d $file ]
    then
        echo "$file is directory"
if [ -b $file ]
    then
        echo "file is block"
if [ -f $file ]
    then
        echo "file is regular file"
else
        echo 'unknown'
fi

2、检测指定的主机是否可以ping通
#!/bin/bash
read -p “请输入检测ip” num
ping -c $num >> /dev/null
if [ $? -eq 0 ];then
echo “ok”
else
echo “down”
fi
[root@web1 test]# chmod +x ping.sh
[root@web1 test]# ./ping.sh 10.10.0.1
down

3、判断一个用户是否存在
[root@egon test]# cat check_user.sh

#!/bin/bash

id $1 &> /dev/null
if [ $? -eq 0 ];then
    echo "user $1 exists"
else
    echo "user $1 not exists"
fi
[root@egon test]# chmod +x check_user.sh 
[root@egon test]# ./check_user.sh egon
user egon exists
[root@egon test]# ./check_user.sh xx
user xx not exists

4、检测httpd软件是否安装，没有的话则安装

[root@egon test]# cat check_httpd.sh
#!/bin/bash

rpm -q httpd &>/dev/null
if [ $? -eq 0 ];then
    echo "已经安装"
else
    echo "正在安装..."
    yum install httpd -y &>/dev/null
fi

5、判断80端口的状态,未开启则重启

[root@egon test]# cat check_port.sh 
#!/bin/bash

netstat -an |grep LISTEN |grep '\\b80\\b' &>/dev/null
if [ $? -eq 0 ];then
    echo "80端口ok"
else 
    echo "80端口down"
    echo "正在重启..."
    systemctl restart httpd &> /dev/null
    if [ $? -eq 0 ];then
        echo "重启成功"
    else
        echo "重启失败"
    fi
fi

6、编写监控脚本，如果
根分区剩余空间小于10%
内存的可用空间小于30%
向用户egon发送告警邮件，邮件的内容包含使用率相关信息
[root@egon test]# cat monitor.sh

#! /bin/bash
# 提取根分区剩余空间
use_disk=`df / | grep / | awk '{print $5}'`
use_percent=`echo $use_disk|cut -d% -f1`

# 提取内存剩余空间
avail_mem=`free | awk 'NR==2{print $NF}'`
total_mem=`free | awk 'NR==2{print $2}'`
avail_percent=`echo "scale=2;$avail_mem/$total_mem"|bc | cut -d. -f2`

# 注意 磁盘提取的数值单位为 kb、 内存提取的单位为 Mb
if [ $use_percent -gt 90 ];then
     echo "邮件内容:根分区已经使用为${user_disk}低于10%，请及时处理！！!" | mail -s "硬盘报警邮件" root
fi

if [ $avail_percent -lt 30 ];then
     echo "邮件内容:内存剩余${free_percent}%,低于30%" | mail -s "内存报警邮件" [e-mail]
fi

测试：

# 查看163邮箱
# [root@egon test]# cat /var/spool/mail/root 

mailx配置

[root@egon ~]# yum install mailx -y
[root@egon ~]# cat /etc/mail.rc
set from=[e-mail] 
set smtp=smtps://smtp.qq.com:465
set smtp-auth-user=[e-mail]
set smtp-auth-password="xxxxxxxxxx"
set smtp-auth=login
set ssl-verify=ignore
set nss-config-dir=/etc/pki/nssdb/

测试

[root@egon ~]# echo "卧槽" | mail -s "报警邮件" [e-mail]
[root@egon ~]# Error in certificate: Peer's certificate issuer has been marked as not trusted by the.

上述报错的解决方式为,依次执行下述命令

mkdir -p /root/.certs/

echo -n | openssl s_client -connect smtp.qq.com:465 | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' > ~/.certs/qq.crt

certutil -A -n "GeoTrust SSL CA" -t "C,," -d ~/.certs -i ~/.certs/qq.crt

certutil -A -n "GeoTrust Global CA" -t "C,," -d ~/.certs -i ~/.certs/qq.crt

certutil -L -d /root/.certs

cd /root/.certs

certutil -A -n "GeoTrust SSL CA - G3" -t "Pu,Pu,Pu" -d ./ -i qq.crt

# 最后出现这句就可以了
Notice: Trust flag u is set automatically if the private key is present.

# 重新修改配置文件的最后一行
[root@egon ~]# cat /etc/mail.rc
set from=[e-mail] 
set smtp=smtps://smtp.qq.com:465
set smtp-auth-user=[e-mail]
set smtp-auth-password="xxxxxxxxxx"
set smtp-auth=login
set ssl-verify=ignore
# set nss-config-dir=/etc/pki/nssdb/  # 改为下面一行
set nss-config-dir=/root/.certs

# 然后重新测试邮件发送即可

7、根据操作系统不同进行yum源优化 centos6 centos7 centos8

[root@egon shell]# cat check_yum.sh 
#!/bin/bash

mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup &>/dev/null
var=$(awk '{print $(NF-1)}' /etc/redhat-release)
os_version=`echo ${var%%.*}`
if [ $os_version -eq 7 ];then
    wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo &>/dev/null
elif [ $os_version -eq 6 ];then
    wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo &>/dev/null
elif [ $os_version -eq 5 ];then
    wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-5.repo &>/dev/null
else
    echo "请检查确认系统版本信息"
fi