Conditional statement of Shell script (detailed! There are sample questions hahahaha)

Conditional statements in shell scripts

1. Condition test

1.test command

Test the validity of a specific expression, return 0 if it is true, and return a non-zero value if it is not true

格式1：test 条件表达式
格式2：[ 条件表达式 ] 

Note: There must be at least one space between the two sides of the conditional expression and the brackets

2. File testing

Test file or directory related attributes or whether they have corresponding permissions

[ 操作符 文件或目录 ]

Commonly used test operators

Operator	function
-d	Test whether it is a directory (Directory)
-e	Test whether the directory or file exists (Exist)
-f	Test whether it is a file (File)
-r	Test whether the current user has permission to read (Read)
-w	Test whether the current user has permission to write (Write)
-x	Test whether the current user has permission to execute (eXcute)

[root@localhost ~]#[ -d /mnt ] #测试是否为目录
[root@localhost ~]#echo $? #查看上一条命令的执行结果，返回值为0时表示结果为真
0
[root@localhost ~]#ls
anaconda-ks.cfg        home                  pjlx.sh   survey.sh  文档
backup-2020-12-20.tgz  initial-setup-ks.cfg  prj.sh    test31     下载
cal1.sh                jc.sh                 pxsb.sh   test34     音乐
cal.sh                 kyzjsjx.sh            pxsbx.sh  公共       桌面
ddysjx.sh              lx.sh                 qq.sh     模板
dysjx.sh               mybak.sh              sum1.sh   视频
first.sh               path1.sh              sum.sh    图片
[root@localhost ~]#[ -f first.sh ] #测试first.sh是否为文件
[root@localhost ~]#echo $? #查看上一条命令的执行结果，返回值为0时表示结果为真
0
[root@localhost ~]#[ -d first.sh ] #测试first.sh是否为目录
[root@localhost ~]#echo $? #查看上一条命令的执行结果，返回值为0时表示结果为真
1
[root@localhost ~]#[ -r first.sh ] #测试first.sh是否有读取权限
[root@localhost ~]#echo $? #查看上一条命令的执行结果，返回值为0时表示结果为真
0
[root@localhost ~]#[ -f first.sh ] && echo "yes"
yes #测试first.sh是否为文件，满足条件时输出yes

3. Integer value comparison

format

[ 整数1 操作符 整数2 ]

Common comparison operators

Operator	Role/function
-eq	Equal
-born	-ne: Not Equal
-gt	-gt: Greater Than
-lt	-lt: Less than (Lesser Than)
-the	-le: Less than or equal to (Lesser or Equal)
-give	-ge: Greater or Equal

[root@localhost ~]#free -m #查看内存使用情况，以MB为单位
              total        used        free      shared  buff/cache   available     #总物理内存为1823MB        #空闲内存为1116MB
Mem:           1823         305        1116           9         401        1308
Swap:          4095           0        4095
[root@localhost ~]#free -m | grep "Mem" | awk '{print $4}'
1116 #以MB为单位查看内存，筛选出Mem那行，并打印出第四列内容到屏幕上
[root@localhost ~]#freemem=$(free -m | grep "Mem" | awk '{print $4}')
[root@localhost ~]#echo $freemem 
1116 #把以上那段得出的结果赋值给freemem，然后查看其变量值
[root@localhost ~]#[ $freemem -gt 1024 ] && echo "$freemem"MB
1116MB #如果变量值大于1024，输出其变量值MB，""之中可引用其他变量，此处也用作分隔后面字符

4. Floating point operations

1) bc: Built-in bash calculator

Bash does not support floating-point operations, only integer operations. This is a huge limitation. If you need to perform floating-point operations, you need to use bc (built-in bash calculator) or awk for processing.

Enter bc to enter the calculator, enter quit to enter or directly Ctrl D to exit the calculator.

[root@localhost ~]#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'. 
12 * 2.4
28.8
2.4 * (2 + 9)
26.4
quit

2) Quote bc

Including addition, subtraction, multiplication and division

[root@localhost ~]#v1=$(echo "2.4*2"|bc)
[root@localhost ~]#echo $v1
4.8
[root@localhost ~]#v2=$(echo "3.6/1.5"|bc)
[root@localhost ~]#echo $v2
2
[root@localhost ~]#v2=$(echo "scale=3; 3.6/1.5"|bc)
[root@localhost ~]#echo $v2 #scale=3表示输出结果包含3位小数
2.400
[root@localhost ~]#v3=$(echo "3.6+1.5"|bc)
[root@localhost ~]#echo $v3
5.1
[root@localhost ~]#v3=$(echo "3.6-1.5"|bc)
[root@localhost ~]#echo $v3
2.1

3) awk: simple data processing tool

Awk is suitable for processing small text data, its format:

awk '条件类型1 {操作1} 条件类型2 {操作2} ...' filename

Use awk to add, subtract, multiply and divide:

#加法
[root@localhost ~]#f=$(awk 'BEGIN{print 3.6+1.5 }')
[root@localhost ~]#echo $f
5.1
[root@localhost ~]#echo "3.6+1.5=$f"
3.6+1.5=5.1
#减法
[root@localhost ~]#v4=$(awk 'BEGIN{print 3.6-1.5 }')
[root@localhost ~]#echo "3.6-1.5=$v4"
3.6-1.5=2.1
#乘法
[root@localhost ~]#v4=$(awk 'BEGIN{print 3.6*1.5 }')
[root@localhost ~]#echo "3.6*1.5=$v4"
3.6*1.5=5.4
#除法
[root@localhost ~]#v4=$(awk 'BEGIN{print 3.6/1.5 }')
[root@localhost ~]#echo "3.6/1.5=$v4"
3.6/1.5=2.4

Use awk to analyze and process data

Take out the account and login IP, and separate the account and IP with [Tab], as follows

Where $1 is the account number and $3 is the IP

[root@localhost ~]#last -n 3 #利用last取出登录者数据，取出前三行
root     pts/0        192.168.2.1      Wed Dec 23 13:28   still logged in   
reboot   system boot  3.10.0-693.el7.x Wed Dec 23 13:27 - 17:36  (04:09)    
root     pts/0        192.168.2.1      Tue Dec 22 08:34 - down   (08:31)    

wtmp begins Wed Nov 11 19:55:40 2020
[root@localhost ~]#last -n 3 | awk '{print $1 "\t" $3}'
root	192.168.2.1
reboot	boot
root	192.168.2.1

wtmp	Wed

5. String comparison

1) Format

格式1：[ 字符串1 = 字符串2 ] 
      [ 字符串1 != 字符串2 ] 
格式2： [ -z 字符 ]

2) Commonly used test operators

=：字符串内容相同
!=：字符串内容不同， !号表示相反的意思
-z：字符串内容为空
-n：检测字符串是否存在

For example:

[root@localhost ~]#ABC=123
[root@localhost ~]#echo $ABC
123
[root@localhost ~]#[ $ABC = 123 ] && echo "成立" #判断ABC的值是否和字符串123相同，相同输出“成立”
成立
[root@localhost ~]#[ $ABC != 123 ] && echo "成立" || echo "不成立"
不成立 #||逻辑或，上一条命令执行失败的时候执行后面的命令

6. Logic test

1) Format

格式1：[ 表达式1 ] 操作符 [ 表达式2 ]
格式2：命令1 操作符 命令2

2) Commonly used test operators

Operator	function
-a或&&	Logical and, and mean
-o or ||	Logical or, or meaning
!	Logical negation

Note: ①Use -a or -o in the same bracket, and && or || between two brackets

②&&, || operators can normally exist in the [[ ]] conditional judgment structure, but if they appear in the [] structure, an error will be reported

③There must be at least one space between the operator and the expression on both sides, otherwise an error will be reported

For example:

a=3
[ $a -ne 1 ] &&  [$a != 2 ]等同于 [ $a -ne 1 -a $a != 2 ]

[[ $a != 1 && $a != 2 ]]
[[ 5 -lt 9 ]] && echo "true" || echo "false"
[[ 3 -ge 5 ]] && echo "true" || echo "false"

[root@localhost ~]#[ $a -ne 1 ] &&  [ $a !=2 ] #=和2之间无空格导致报错
-bash: [: 3: 期待一元表达式
[root@localhost ~]#[ $a -ne 1 ] &&  [ $a != 2 ]
[root@localhost ~]#a=3
[root@localhost ~]#[ $a -ne 1 ] &&  [ $a != 2 ] #判断a的值不等于1且不等于2
[root@localhost ~]#echo $? #上一个命令成立返回0，不成立返回其他值
0
[root@localhost ~]#[[ $a != 1 && $a != 2 ]]
[root@localhost ~]#echo $?
0
[root@localhost ~]#[[ 5 -lt 9 ]] && echo "true" || echo "false"
true #判断5是否小于9，成立时输出true，||前面的不成立时输出false
[root@localhost ~]#[[ 3 -ge 5 ]] && echo "true" || echo "false"
false #判断3是否大于或等于5，成立输出true，||前面不成立时输出false

Two, if statement

1. Single branch structure

if  条件测试操作
then  
  命令序列
fi

• Three ways of expression

if [ 3 -gt 2 ]
then
    echo "ok"
fi


if [ 3 -gt 2 ];then echo "ok";fi

[ 3 -gt 2 ] && echo "ok"

2. Double branch if statement

if 条件测试操作
then
  命令序列1
else
  命令序列2
fi

For example:

[root@localhost ~]#vim ping.sh
[root@localhost ~]#chmod +x ping.sh 
[root@localhost ~]#./ping.sh 192.168.100.100
192.168.100.100 off
[root@localhost ~]#./ping.sh 192.168.2.3
192.168.2.3 online
[root@localhost ~]#cat ping.sh
#!/bin/bash
ping -c 3 -i 0.5 -W 2 $1 &> /dev/null
if [ $? -eq 0 ]
then
	echo "$1 online"
else
	echo "$1 off"
fi

-c: the number of packets sent
-i: the interval between sending packets to test
-w: timeout

Send three packets with an interval of 0.5 seconds and a timeout of 2 seconds. All output information will be thrown to the black hole file without timeout. When the execution is successful, $?=0, the IP will be output online, which is online.

Otherwise, the black hole file will be lost after the timeout of 2 seconds and the output IP is closed off

3. Multi-branch if statement

if 条件测试操作1
then
  命令序列1
elif 条件测试操作2
then
  命令序列2
else
  命令序列3
fi

For example:

[root@localhost ~]#vim chengji.sh 
[root@localhost ~]#cat chengji.sh 
#!/bin/bash
#
read -p "请输入您的分数（1-100）: " score
if [ $score -ge 85 ] && [ $score -le 100 ]
then
  echo "优秀！" 

  elif [ $score -ge 75 ] && [ $score -le 84 ]
  then
    echo "还不错，继续努力！"

  elif [ $score -ge 60 ] && [ $score -le 74 ]
  then
    echo "及格，还需加油！"

  elif [ $score -ge 1 ] && [ $score -le 59 ]
  then
    echo "没及格，在错误中进步，你可以的！"
fi

[root@localhost ~]#chmod +x chengji.sh 
[root@localhost ~]#./chengji.sh 
请输入您的分数（1-100）: 98
优秀！
[root@localhost ~]#./chengji.sh 
请输入您的分数（1-100）: 83
还不错，继续努力！
[root@localhost ~]#./chengji.sh 
请输入您的分数（1-100）: 70
及格，还需加油！
[root@localhost ~]#./chengji.sh 
请输入您的分数（1-100）: 52
没及格，在错误中进步，你可以的！

Three, case statement

1.case multi-branch structure

case 变量值 in
模式1)
     命令序列1
     ;;
模式2)
     命令序列2
     ;;
.....
*)
     默认命令序列
esac

note:

• The end of the case line must be the word "in", and each pattern must end with a closing parenthesis ")".
• The double semicolon ";;" indicates the end of the command sequence.
• In the pattern string, square brackets can be used to indicate a continuous range, such as "[0-9]"; the vertical bar symbol "|" can also be used to indicate or, such as "AIB"
• The " )" at the end represents the default mode, which is equivalent to a wildcard.

2. Examples

For example：

[root@localhost ~]#vim chengjicase.sh 
[root@localhost ~]#cat chengjicase.sh 
#!/bin/bash
read -p "请输入您的分数(0-100) : " score
if [ $score -le 100 -o $score -ge 0 ];then
           [[ $score -ge 80 && $score -le 100 ]] && a="great"
           [[ $score -ge 60 && $score -lt 80 ]] && a="standard"
           [[ $score -ge 0 && $score -lt 60 ]] && a="false"
           case $a in
           great)
	           echo " $score分,优秀! "
               ;;
           standard)
               echo " $score分,合格! "
               ;;
           false)
               echo " $score分,不合格! "
               ;;
               *)
               echo "输入有误!"
esac
fi
[root@localhost ~]#chmod +x chengjicase.sh 
[root@localhost ~]#./chengjicase.sh 
请输入您的分数(0-100) : 98
 98分,优秀! 
[root@localhost ~]#./chengjicase.sh 
请输入您的分数(0-100) : 78
 78分,合格! 
[root@localhost ~]#./chengjicase.sh 
请输入您的分数(0-100) : 52
 52分,不合格! 
[root@localhost ~]#./chengjicase.sh 
请输入您的分数(0-100) : 111
输入有误!