Shell condition test symbol and if conditional statement

1. Conditional test operation

(1) test command

• Test whether a specific expression is established, when the condition is established, the return value of the test statement is 0, otherwise it is other numeric （利用$?查看上一条命令是否执行成功）
  format 1: test Conditional expression
  format 2: [Conditional expression]（至少有一个空格最好开头末尾都加一个空格）

(2) File testing

Format: [Operator file or directory]

Commonly used test operators

1. -d: test whether it is a directory (Directory)
2. -e: Test whether the directory or file exists (Exist)
3. -f: test whether it is a file (File)
4. -r: Test whether the current user has permission to read (Read)
5. -w: Test whether the current user has permission to write (Weite)
6. -x: Test whether the current user has permission to execute (eXcute)
   （r w x 和chmod的给的权限是一样的）

[root@localhost ~]# [  -d /media/cdrom  ]
[root@localhost ~]# echo $?
0
`返回0表示条件成立`
[root@localhost ~]# [  -d /media/cdrom/Server  ]
[root@localhost ~]# echo $?
1
`返回1表示条件不成立`
[root@localhost ~]# [ -d /media/cdrom ] && echo "YES  
YES 
`条件成立时返回YES,&&逻辑与，“而且”的意思`

(3) Comparison of integer values

Format: [Integer 1 Operator Integer 2]

Commonly used test operators

1. -eq: equal to (Equal)
2. -ne: Not Equal
3. -gt: Greater Than
4. -lt: Less than (Lesser Than)
5. le: Less than or equal to (Lesser or Equal)
6. -ge: greater than or equal to (Greater or Equal)

[root@localhost ~]# who | wc -l
7
————用户数是否 > 5
[root@localhost ~]# [ $(who | wc -l) -gt 5 ] && echo "aaa" 
aaa                     `（是否大于5，大于的话输出aaa）`
————用户数是否 >= 10
[root@localhost ~]# [ $(who | wc -l) -ge 10 ] && echo "bbb"
`(用户大于10，则输出bbb，用户不大于10，则什么都不输出)`
[root@localhost ~]# aaa=$(free -m | grep -i "mem" | awk '{print $4}')
`(可以一步一步执行命令去查看，$(free -m | grep -i "mem" | awk '{print $4}'),grep -i 不区分大小写）`
————空闲内存是否 < 1024MB     
[root@localhost ~]# [ $aaa -lt 1024 ] && echo "空闲内存为 ${aaa}MB"
空闲内存为 379MB
`（空闲内存小于1024则输出空闲内存为aaa的变量值MB）`

(4) String comparison

Format 1:
[String 1 = String 2] (The 内容相同于condition is satisfied when string 1 is string 2)
[String 1 != String 2] (The 内容不同于condition is satisfied when string 1 is string 2)

Format 2:
[-z String 1] (The 内容为空condition is satisfied when the string is 1 )

Commonly used test operators

1. =: The string content is the same
2. !=: The string content is different (！表示相反的意思)
3. -z: The string content is empty

[root@localhost ~]# echo $LANG 
zh_CN.UTF-8
[root@localhost ~]# [ $LANG !=  "en.US" ] && echo  "Not en.US"
Not en.US              `（内容是否是en.US如果不是就输出 Not en.US）`
[root@localhost ~]# read -p “是否覆盖现有文件（yes/no）？"  ACK
是否覆盖现有文件（yes/no）？yes   `(交互式变量，ACK的变量值是yes)`
[root@localhost ~]# [ $ACK = "yes" ] && echo "覆盖"
覆盖           `（ACK的变量值是否为yes ，是的话输出 覆盖）`

(5) Logic test

Format 1: [Expression 1] Operator [Expression 2] …
Format 2: Command 1 Operator command 2…

Commonly used test operators

1. -a or &&: logical and, meaning "and"
2. -o or ||: logical or, meaning "or"
3. ! : Logical no

(1), view the current user

[root@localhost ~]# echo $USER
Root            `（$USER是系统表量，显示当前用户）`
[root@localhost ~]# [ $USER != "teacher" ]  &&  echo "Not teacher"
Not teacher                 `（当$USER不等于 teacher时 输出Not teacher）`

(2), test whether the directory exists

————如何测试/opt/backup目录是否存在？
 [root@localhost ~]# [ -d /media/cdrom ] && echo "YES" || echo "NO”
`（检查/media/cdrom是否为目录，是的话输出YES，不是的话输出NO,条件成立时||的前一个不成立是后一个）`

3. View task progress

————查询当前的活动进程数，并测试是否小于100？
[root@localhost ~]# [ $(ps aux | wc -l) -lt 100 ] && echo "YES" || echo "NO"
YES              `（小于100的话则是YES，大于100的话则是NO,ps aux 查看静态进程，wc -l统计行数）`

(4), test whether to install the software package

————如何测试是否已安装sysstat软件包？
[root@localhost ~]# rpm -q sysstatpackage 
未安装软件包 sysstatpackage
[root@localhost ~]# [ $? -eq 0 ] && echo "Installed." || echo "Not installed."
Not installed.        `($? 上条命令的结果，0为成立，除外都为不成立，这里判定的是未安装所以算不成立)`
`（如果上条命令结果成立，则输出lnstalled，如果不成立则输出Not installed）`

(5) Use conditional testing to write scripts to test whether the service is installed

————编辑脚本rpm_install.sh
***用来判断软件是否安装，如果已安装输出“此软件已安装”，如果未安装则进行安装
[root@localhost ~]# vim rpm_install.sh
#!/bin/bash
***查看rpm软件是否已安装
rpm -q $1 &> /dev/null
***已安装输出结果，未安装则进行安装
[ $? -eq 0 ] && echo "$1 软件已安装"  || yum -y install $1 &> /dev/null
`（/dev/null相当于一个回收站）`
[root@aaa ~]# chmod +x rpm_install.sh   ***（给可执行权限）
[root@aaa ~]# ./rpm_install.sh httpd    ***（验证脚本）
[root@aaa ~]# rpm -q httpd              ***（发现已经安装）
httpd-2.4.6-93.e17.centos.x86_64

The structure of the if statement

1. Single branch structure

if condition test operation; then command sequence
fi,
for example:
if disk has occupied space> 80%; then alarm
fi
（；可以在当前行直接写命令序列，不加；的话需要空到下一行去写）
[if begins, fi ends, then writes the command sequence, if it does not meet the if conditions, then nothing will be done]

Single branch if statement

————判断挂载点目录，若不存在则自动创建
[root@localhost ~]# cat 1.sh
#!/bin/bash
A="/media/cdrom/"
if [ ! -d $A ]
then
`(当目录不存在时执行mkdir -p $A,存在的话不会做任何操作)`
    mkdir -p $A
fi
[root@localhost ~]# chmod +x 1.sh
[root@localhost ~]# ./1.sh

Two, double branch structure

if condition test operation; then command sequence 1
else command sequence 2
fi
For example:
if port 80 is listening; then website service is running
else start httpd service
fi
[if starts, fi ends, if condition test is established, then execute the then command Sequence, otherwise execute the command sequence of else]

Double branch if statement

判断目标主机是否存活，显示检测结果
[root@localhost ~]# cat 2.sh
#!/bin/bash
ping -c 3 -i 0.2 -W 3 $1 &> /dev/null     
`(/dev/null是“黑洞”相当于一个回收站，放进去的数据都会自动删除)`
if [ $? -eq 0 ]
`(判断ping命令的检测结果，分别给出不同提示，利用$1位置变量)`
then
    echo "主机 $1 可以进行通信."
else
    echo "主机 $1 无法进行通信."
fi
[root@localhost ~]# ./2.sh 192.168.4.11
主机 192.168.4.11 可以进行通信.
[root@localhost ~]# ./2.sh 192.168.4.13
主机 192.168.4.13 无法进行通信.
`ping命令的-c、-i、-W 选项分别指定测试包个数、发送间隔、超时时间`

Three, multi-branch structure

if condition test operation 1; then command sequence 1
elif condition test operation 2; then command sequence 2
else; command sequence 3
fi
For example:
if score is between 85 and 100; then judged as excellent
elif score is between 70 and 84; Then judged as qualified
esle; judged as unqualified
fi
[if starts, fi ends, the command sequence of then1 is executed when the conditional test operation 1 is established, and the command sequence of then2 is executed when the conditional test operation 2 is established, and so on elif理论是可以无限写的, but if If the judgment result is not consistent with all the conditional test operations, the else command sequence is executed]

Multi-branch if statement

————判断分数范围，分出优秀、合格、不合格三档
[root@localhost ~]# cat 3.sh
#!/bin/bash
read -p "请输入您的分数（0-100）：" A
if [ $A -ge 85 ] && [ $A -le 100 ] ; then  
`（如果符合这个条件执行这个）`
`（判断分数所在区间，给出不同的分档结果)`
    echo "$A 分！优秀"
elif [ $A -ge 70 ] && [ $A -le 84 ] ; then
`（如果符合这个条件执行这个）`
    echo "$A 分，合格"
else                         `（否则执行这个）`
    echo "$A 分？不合格”
fi
[root@localhost ~]#chmod a+x  3.sh
[root@localhost ~]# ./3.sh
请输入您的分数（0-100）：89
89 分！优秀
[root@localhost ~]# ./3.sh
请输入您的分数（0-100）：75
75 分,合格
[root@localhost ~]# ./3.sh
请输入您的分数（0-100）：60
60 分？不合格