day028-Shell自动化编程-判断进阶

1. 特殊变量补充

• $$：获取当前脚本的pid
• $_：获取上一个命令的最后一个参数；在命令行中可以使用Esc+.快捷键

2. 变量扩展-变量子串

• 用于对变量处理、加工

2.1 获取变量字符的长度

[root@aliyun-ubuntu ~]# name=skx
[root@aliyun-ubuntu ~]# echo $name
skx
[root@aliyun-ubuntu ~]# echo ${#name}
3

2.2 给变量设置默认值

• ${para:-word}

# 变量为空
[root@aliyun-ubuntu ~]# echo $name

# 为变量设置默认值，不修改原内容
[root@aliyun-ubuntu ~]# echo ${name:-root}
root
[root@aliyun-ubuntu ~]# echo $name

• ${para:=word}

[root@aliyun-ubuntu ~]# echo $name 

# 为空变量设置默认值，并修改原内容
[root@aliyun-ubuntu ~]# echo ${name:=skx}
skx
[root@aliyun-ubuntu ~]# echo $name
skx

3. 命令

3.1 dirname

• 显示路径前缀

[root@aliyun-ubuntu ~]# dirname /etc/passwd
/etc
[root@aliyun-ubuntu ~]# dirname /etc/ssh/ssh_config
/etc/ssh

3.2 basename

• 显示路径后缀

[root@aliyun-ubuntu ~]# basename /etc/passwd
passwd
[root@aliyun-ubuntu ~]# basename ./snap/lxd/
lxd

3.3 cut

• -c n-m：截取字符串的第n到第m个字符

[root@aliyun-ubuntu ~]# string=1234567890
[root@aliyun-ubuntu ~]# echo $string |cut -c 5
5
[root@aliyun-ubuntu ~]# echo $string |cut -c 5-8
5678

4. 条件测试命令：[]

[ ]或test，用于检查文件属性、字符串比较、数值比较等逻辑判断。它是 Shell 脚本中常见的条件判断语法之一。

• []内部必须用空格分隔

4.1 逻辑运算符

• 逻辑运算符不能在[]内使用
• &&：并且，前面命令执行成功后才执行后面的命令
• ||：或者，前一个命令执行失败后才执行后面的命令

4.2 文件测试

[root@aliyun-ubuntu ~]# [ -f ./passwd.txt ] && echo file || echo 'not file'
file
[root@aliyun-ubuntu ~]# [ -d /etc/ ] && echo dir || echo 'not dir'
dir
[root@aliyun-ubuntu ~]# [ -d /etc/passwd ] && echo dir || echo 'not dir'
not dir

4.3 案例：书写脚本-检查文件类型

[root@aliyun-ubuntu /server/scripts]# cat check_type.sh 
#!/bin/bash
##############################################################
# File Name: check_type.sh
# Version: V1.0
# Author: SunKexu
# Organization: www.oldboyedu.com
# Description:test file type
##############################################################
export LANG=en.US_UTF-8

# vars
file=$1
# command
# check param num
if [ $# -eq 0 ];then
	echo "Usage:$0 file/dir"
	exit 1
fi
# soft link
if [ -h $file ];then
	echo "$file is symbolic"
	exit 0
fi
# file
if [ -f $file ];then
	if [ -x $file ];then
		mode="has permisson"
	else 
		mode="has not permission"
	fi

	if [ -s $file ];then
		size="size not is 0"
	else
		size="size is 0"
	fi
	echo "${file} is file;permission:${mode};size:${size}"
	exit 0
fi
# dir
if [ -d $file ];then
	echo "$file is directory"
	exit 0
fi


echo "$file is other type file"
[root@aliyun-ubuntu /server/scripts]# bash check_type.sh
Usage:check_type.sh file/dir
[root@aliyun-ubuntu /server/scripts]# bash check_type.sh /sbin
/sbin is symbolic
[root@aliyun-ubuntu /server/scripts]# bash check_type.sh ./check_type.sh 
./check_type.sh is file;permission:has not permission;size:size not is 0
[root@aliyun-ubuntu /server/scripts]# bash check_type.sh /usr/   
/usr/ is directory
[root@aliyun-ubuntu /server/scripts]# bash check_type.sh ./test
./test is other type file

4.4 逻辑运算

4.5 案例：书写脚本-检查服务是否正在运行或是开机自启动

[root@aliyun-ubuntu /server/scripts]# cat check_service2.sh 
#!/bin/bash
##############################################################
# File Name: check_service2.sh
# Version: V1.0
# Author: SunKexu
# Organization: www.oldboyedu.com
# Description:
##############################################################

# vars
name=$1
# command
if [ $# -eq 0 ];then
	echo "Usage:$0 server name"
	exit 1
fi

running=`systemctl is-active $name`
enable=`systemctl is-enabled $name`
if [ ${running}="active" -a ${enable}="enabled" ];then
	echo "$name is running and enabled"
else
	echo "$name is not running or enabled"
fi
[root@aliyun-ubuntu /server/scripts]# bash check_service2.sh
Usage:check_service2.sh server name
[root@aliyun-ubuntu /server/scripts]# bash check_service2.sh sshd
sshd is running and enabled

4.6 增强版条件测试命令：[[]]

[[ ]]，比 [] 更强大，支持 &&、||、正则匹配等

• 正则匹配符号：=~,前后要有空格

[root@aliyun-ubuntu /server/scripts]# id=678
[root@aliyun-ubuntu /server/scripts]# [[ "$id" =~ [0-9]+ ]] && echo 1 || echo 2
1
[root@aliyun-ubuntu /server/scripts]# name=skx123
[root@aliyun-ubuntu /server/scripts]# [[ "$name" =~ [0-9]+ ]] && echo 1 || echo 2
1
[root@aliyun-ubuntu /server/scripts]# [[ "$name" =~ ^[0-9]+$ ]] && echo 1 || echo 2
2

4.7 面试题：[]与[[]]的区别？

5. 字符串比较

[root@aliyun-ubuntu /server/scripts]# [ 123=123 ] && echo 1 || echo 2
1
[root@aliyun-ubuntu /server/scripts]# name="skx"
[root@aliyun-ubuntu /server/scripts]# [ ${name}="skx" ] && echo 1 || echo 2
1

5.1 案例：书写脚本-检查服务是否正在运行和开机自启动

[root@aliyun-ubuntu /server/scripts]# cat check_service.sh
#!/bin/bash
##############################################################
# File Name: check_service.sh
# Version: V1.0
# Author: SunKexu
# Organization: www.oldboyedu.com
# Description:check service status
##############################################################
export LANG=en.US_UTF-8

# vars
service=$1

# command
# check parameter num
if [ $# -eq 0 ];then
	echo "Usage:$0 service name"
	exit 1
fi
# check running state
status_run=`systemctl is-active ${service}`
if [ ${status_run}="active" ];then
	echo "${service} is running"
else
	echo "${service} is not running"
fi

# check enabled state
status_enabled=`systemctl is-enabled ${service}`
if [ ${status_enabled}="enabled" ];then
	echo "${service} is enabled"
else
	echo "${service} is not enabled"
fi
#####################################
if [ ${status_run}="active" -a ${status_enabled}="enabled" ];then
	echo "$service is active and enabled"
fi
[root@aliyun-ubuntu /server/scripts]# bash check_service.sh sshd
sshd is running
sshd is enabled
sshd is active and enabled

6. 多分支：if、case

6.1 案例-统计根分区磁盘使用率，60-70输出警告，70-80输出需要处理，80-95输出需要及时处理，965以上输出立刻处理

[root@aliyun-ubuntu /server/scripts]# cat check_disk.sh
#!/bin/bash
##############################################################
# File Name: check_disk.sh
# Version: V1.0
# Author: SunKexu
# Organization: www.oldboyedu.com
# Description:
##############################################################

export LANG=en.US_UTF-8

# vars
usage=`df -h / |awk -F '[ %]' 'NR==2{print $(NF-2)}'`
# if
if [ $usage -gt 60 -a $usage -le 70 ];then
	echo "Warning: Insufficient disk space ${usage}"
elif [ $usage -gt 70 -a $usage -le 80 ];then
	echo "Warning: Insufficient disk space ${usage}"
elif [ $usage -gt 80 -a $usage -le 95 ];then
	echo "Warning: Severe shortage of disk space ${usage}"
elif [ $usage -gt 95 ];then
	echo "Warning: Disk space is about to run out ${usage}"
else
	echo "Disk space is normal"
fi
[root@aliyun-ubuntu /server/scripts]# bash check_disk.sh
Disk space is normal

6.2 案例：判断系统类型

• bash运行脚本是一个子shell程序，里面的变量仅在脚本中生效
• source运行的脚本是在当前Shell环境，可以用来加载指定的变量

[root@aliyun-ubuntu /server/scripts]# cat check_os.sh
#!/bin/bash
##############################################################
# File Name: check_os.sh
# Version: V1.0
# Author: SunKexu
# Organization: www.oldboyedu.com
# Description:
##############################################################

export LANG=en.US_UTF-8
# This file contains system version information
source /etc/os-release
# case
case "$ID" in 
	kylin|rocky|centos) echo "$ID yum insatll package"
		;;
	ubuntu|debian) echo "$ID apt install package"
		;;
	*)
		echo "other system"

esac

[root@aliyun-ubuntu /server/scripts]# bash check_os.sh 
ubuntu apt install package

7. 踩坑记录

1. 条件测试时，变量要加上双引号

变量建议加引号：

• 如果变量可能为空或包含空格，必须加引号：

if [ "$var" = "hello" ]; then  # 正确
if [ $var = "hello" ]; then    # 如果 $var 为空，会报错

8. 思维导图

【金山文档】 思维导图 https://www.kdocs.cn/l/co3I7PtpTYQX