Theory: shell programming to explain the theory of conditional statements ----

Preface:
conditional statements also flow control statements, the logic of everyday life

Test conditions

• File test
• Integer test
• Logic test string

The if statement

• if a single branch statement
• if two-branch statement
• if multi-branch statement

A: which test

1.1.1 test command

• Test specific expression is established when the condition is met, the test statement returns 0, otherwise other values

'格式1：test 条件表达式
'格式2：[ 条件表达式 ]

In Format 2, before and after at least one space

1.1.2 file permissions and file type test test

• [ 操作符 文件或目录 ]

1.1.3 Common test operators

• -e: test directory or file exists (Exist)
• -d: test whether a directory (Directory)
• -f: test whether the file (File)
• -r: to test whether the current user has permission to read (Read)
• -w: Testing whether the current user has permission to write (Write)
• -x: Testing whether the current user has permission to perform (eXcute)

[root@localhost opt]# touch test.txt
[root@localhost opt]# mkdir abc
[root@localhost opt]# ls
abc  rh  test.txt  wwwroot
[root@localhost opt]# test -d /opt/abc/
[root@localhost opt]# echo $?
0
[root@localhost opt]# [ -d /opt/abc ]
[root@localhost opt]# echo $?
0
[root@localhost opt]# test -f /opt/abc
[root@localhost opt]# echo $?
1
[root@localhost opt]# test -f /opt/test.txt 
[root@localhost opt]# echo $?
0
[root@localhost opt]# test -f /opt/te.txt
[root@localhost opt]# echo $?
1
[root@localhost opt]# [ -x /opt/abc]
bash: [: 缺少 `]'
[root@localhost opt]# [ -x /opt/abc ]
[root@localhost opt]# ls -al
总用量 16
drwxr-xr-x.  5 root root 156 11月 26 13:51 .
dr-xr-xr-x. 17 root root 224 10月 23 13:41 ..
'drwxr-xr-x.  2 root root   6 11月 26 13:51 abc
drwxr-xr-x.  2 root root   6 3月  26 2015 rh
-rw-r--r--.  1 root root   0 11月 26 13:51 test.txt
[gsy@localhost opt]$ [ -w /opt/abc ]
[gsy@localhost opt]$ echo $?
1
[gsy@localhost opt]$ [ -w /opt/abc ]&&echo "yes"
[gsy@localhost opt]$ echo $?
1
[gsy@localhost opt]$ [ -r /opt/abc ]&&echo "yes"
yes
[gsy@localhost opt]$ echo $?
0
[gsy@localhost opt]$ 

echo $? query on whether the step has founded or 0, non-zero value was not established

[gsy@localhost opt]$ [ -w /opt/abc ]&&echo "yes"

&& is the meaning and said that if both sides are true, will be performed correctly; echo "yes" is clearly correct, that is, if [-w / opt / abc] set up, it will output yes, if not established, it is not outputs yes, this operation can be disguised to verify whether the operation was established

|| or meaning, as long as there is a set up, even for overall; first on, then it will not go see below operating

[root@localhost opt]# [ -d /opt/abc ]|| echo "year"
[root@localhost opt]# echo $?
0
[root@localhost opt]# [ -d /opt/ab ]|| echo "year"
year
[root@localhost opt]# echo $?
0
[root@localhost opt]# [ -d /opt/abc ]|| echo "year"
[root@localhost opt]# echo $?
0

1.2.1 Comparison integer

[ 整数1 操作符 整数2 ]

1.2.2 Common test operators

• -eq: equal (Equal)
• -ne: not equal (Not Equal)
• -gt: greater than (Greater Than)
• -lt: less than (Lesser Than)
• -le: less than or equal to (Lesser or Equal)
• -ge: greater than or equal to (Greater or Equal)

[root@localhost opt]# [ 5 -gt 3 ]&& echo "yes"
yes
[root@localhost opt]# [ 5 >  3 ]&& echo "yes"
yes
[root@localhost opt]# [ 5 <  3 ]&& echo "yes"
yes
[root@localhost opt]# [ 5 \<  3 ]&& echo "yes"
[root@localhost opt]# echo $?
1
[root@localhost opt]# [ 5 \=  3 ]&& echo "yes"
[root@localhost opt]# [ 5 \>  3 ]&& echo "yes"
yes
[root@localhost opt]# [ 3 \=  3 ]&& echo "yes"
yes
[root@localhost opt]# [ 3 ==  3 ]&& echo "yes"
yes
[root@localhost opt]# [ 3 !=  3 ]&& echo "yes"
[root@localhost opt]# echo $?
1
[root@localhost opt]# [ 3 !=  4 ]&& echo "yes"
yes
[root@localhost opt]# [ 3 >=  3 ]&& echo "yes"
bash: [: 3: 期待一元表达式
[root@localhost opt]# [ 3 =>  3 ]&& echo "yes"
bash: [: 3: 期待一元表达式
[root@localhost opt]# [ 3 \>=  3 ]&& echo "yes"
bash: [: >=: 期待二元表达式
[root@localhost opt]# 

[root@localhost opt]# who
root     :0           2019-11-26 08:16 (:0)
root     pts/0        2019-11-26 08:16 (:0)

[root@localhost opt]# 
[root@localhost opt]# who |wc -l
2
[root@localhost opt]# [ $(who |wc -l) -lt 5 ]&& echo "too less"
too less
[root@localhost opt]# [ $(who |wc -l) -ge 2 ]&& echo ">=2"
>=2

$ () Command which then acts as the prime symbol · trans

[root@localhost opt]# free -m
              total        used        free      shared  buff/cache   available
Mem:           1984         686          81           9        1216        1060
Swap:          2047           0        2047
[root@localhost opt]# free -m | grep Mem
Mem:           1984         686          81           9        1216        1060
[root@localhost opt]# free -m | grep Mem | awk '{print $1,$3,$4}'
Mem: 686 81
[root@localhost opt]# free -m | grep Mem | awk '{print $4}'
79
[root@localhost opt]# abc=$(free -m | grep Mem |awk '{ print $4 }')
[root@localhost opt]# echo abc
abc
[root@localhost opt]# echo $abc
77
[root@localhost opt]# [ $abc -gt 1024 ]&& echo "yes"
[root@localhost opt]# [ $abc -gt 50 ]&& echo "yes"
yes

1.3.1 string comparison

• Format 1:

  [ 字符串1 = 字符串2 ]
  [ 字符串1 ！= 字符串2 ]

• Format 2:

  [ -z 字符串 ]

1.3.2 Common test operators

• =: The same string content
• ! =: Different string content! He represents the opposite meaning
• -z: the string is empty
• String marked out with double quotes

1.3.3 demo

[root@localhost opt]# echo $LANG
zh_CN.UTF-8
[root@localhost opt]# [ $LANG = "zh_CN.UTF-8" ]&& echo "yes"
yes
[root@localhost opt]# [ $LANG != "zh_CN.UTF-8" ]&& echo "yes"
[root@localhost opt]# echo $?
1

[root@localhost opt]# [ "男" != "男" ]&& echo "yes"
[root@localhost opt]# [ "男" = "男" ]&& echo "yes"
yes
[root@localhost opt]# [ "男" = "女" ]&& echo "yes"
[root@localhost opt]# [ "男" != "女" ]&& echo "yes"
yes

To test the statements within the brackets can be compared

Whether to create / opt / share directory: (yes / no) successfully created

already exists

[root@localhost opt]# ls -al
总用量 16
drwxr-xr-x.  5 root root 156 11月 26 13:51 .
dr-xr-xr-x. 17 root root 224 10月 23 13:41 ..
'drwxr-xr-x.  2 root root   6 11月 26 13:51 abc
drwxr-xr-x.  2 root root   6 3月  26 2015 rh
-rw-r--r--.  1 root root   0 11月 26 13:51 test.txt
[root@localhost opt]# mv test.txt test.sh
[root@localhost opt]# vim test.sh 
#!/bin/bash
read -p "是否创建/opt/share目录：（yes/no）" ack
[ $ack = "yes" ]&& mkdir /opt/share
echo "创建成功"
[root@localhost opt]# sh test.sh 
是否创建/opt/share目录：（yes/no）yes
创建成功
[root@localhost opt]# ls
abc                    share          test.sh         
rh               
[root@localhost opt]# vim test.sh 
#!/bin/bash
read -p "是否创建/opt/demo目录：（yes/no）" ack
[ -d /opt/demo ]&&echo "/opt/demo已经存在" || mkdir /opt/demo && echo "/opt/demo创建成功"
[root@localhost opt]# sh test.sh 
是否创建/opt/demo目录：（yes/no）yes
/opt/demo创建成功
[root@localhost opt]# sh test.sh 
是否创建/opt/demo目录：（yes/no）yes
/opt/demo已经存在
/opt/demo创建成功

Unary operators:

i=1；

i = i ++ is equivalent to i = $ i + 1, representative of first assignment, then jerk, i.e., no re-assignment

i = ++ i before adding another assignment, the result of the assignment again, this time there will be a sense of Gaga

[root@localhost opt]# i=1
[root@localhost opt]# echo $i
1
[root@localhost opt]# i++
bash: i++: 未找到命令...
[root@localhost opt]# i++;
bash: i++: 未找到命令...
[root@localhost opt]# expr i++
i++
[root@localhost opt]# let i++
[root@localhost opt]# echo $i
2
[root@localhost opt]# let i=i++
[root@localhost opt]# echo $i
2
[root@localhost opt]# let i++
[root@localhost opt]# echo $i
3
[root@localhost opt]# let ++i
[root@localhost opt]# echo $i
4
[root@localhost opt]# let i=++i
[root@localhost opt]# echo $i
5
[root@localhost opt]# let i=i++
[root@localhost opt]# echo $i
5
[root@localhost opt]# let i=++i
[root@localhost opt]# echo $i
6
[root@localhost opt]# let i=i+
bash: let: i=i+: 语法错误: 期待操作数 （错误符号是 "+"）
[root@localhost opt]# let i=i++
[root@localhost opt]# echo $i
6
[root@localhost opt]# let i+=2
[root@localhost opt]# echo $i
8
[root@localhost opt]# 
[root@localhost opt]# i\*=2
bash: i*=2: 未找到命令...
[root@localhost opt]# let i\*=2
[root@localhost opt]# echo $i
16
[root@localhost opt]# let i/=2
[root@localhost opt]# echo $i
8
[root@localhost opt]# let i%=2
[root@localhost opt]# echo $i
0
[root@localhost opt]# echo $i
0
[root@localhost opt]# 

Binary operators

a+b=c

Ternary operator

Results Results && condition. 1 2 ||

Results 1 execution condition is satisfied, does not hold the execution result 2

1.4.1 logic test

Format 1:

[ 表达式1 ] 操作符 [ 表达式2 ] ···

Format 2:

命令1 操作符 命令2 ·······

1.4.2 Common test operators

• or -a &&: logical AND, "and" means
• or -o ||: logical OR, "or" means
• ! : No logic, on behalf of inverted meaning

[root@localhost opt]# [ -d /etc ]&& [ -r /etc ]&&echo "you can open /etc this diretory"
you can open /etc this diretory
[root@localhost opt]# [ -d /etc ]|| [ -d /home ]&& echo "ok"
ok
[root@localhost opt]# [ -r /etc ]|| [ -d /home ]&& echo "ok"
ok
[root@localhost opt]# [ -r /etc ]|| [ -r /home ]&& echo "ok"
ok
[root@localhost opt]# [ -f /etc ]|| [ -f /home ]&& echo "ok"
[root@localhost opt]# echo $?
1
[root@localhost opt]# [ ! -f /etc ]|| [ -f /home ]&& echo "ok"
ok

II: Structure of the if statement

2.1 single-branch structure

End Analyzing fi, exit 0 normal exit, exit 1 abnormal exit

2.2 pairs of branch structure

More than 2.3 branch structure

Three: if statement application examples

3.1 single-branch if statement

• Judgment mount point directory, if there is automatically created
  

  
  [root@localhost opt]# vim test02.sh
  #!/bin/bash
  dir="/opt/demo02"
  if [ ! -d $dir ]
  then
  mkdir -p $dir
  echo "$dir创建成功"
  fi
  [root@localhost opt]# sh test02.sh 
  /opt/demo02创建成功
  [root@localhost opt]# ls
  [root@localhost opt]# ls
  abc                    share          test.sh         
  rh                     demo02 
  [root@localhost opt]# vim test02.sh
  #!/bin/bash
  dir="/opt/demo02"
  if [ ! -d $dir ]
  then
  mkdir -p $dir
  echo "$dir创建成功"
  else
  echo "$dir已存在"
  fi
  [root@localhost opt]# sh test02.sh 
  /opt/demo02已存在

## 3.2 双分支if语句

- 判断目标主机是否存活，显示检测结果

- ping -c 发送包个数 -i 间隔时间，单位s，-W 等待3s

  $1 位置变量ip地址，把结果混合输出到null中

  上一条结果若是等于0，成立，则输出up，else就会down

![](https://s1.51cto.com/images/blog/201911/26/a02e04631aec4534762c49c2f54bbe34.png?x-oss-process=image/watermark,size_16,text_QDUxQ1RP5Y2a5a6i,color_FFFFFF,t_100,g_se,x_10,y_10,shadow_90,type_ZmFuZ3poZW5naGVpdGk=)

请输入IP地址：

```shell
[root@localhost opt]# vim test02.sh
#!/bin/bash
read -p "请输入IP地址：" addr
ping -c 3 -i 0.2 -W 3 $addr &> /dev/null
if [ $? -eq 0 ]
 then
    echo "$addr is up"
 else
    echo "$addr is down"
fi
[root@localhost opt]# sh test02.sh 
请输入IP地址：192.168.139.132
192.168.139.132 is up
[root@localhost opt]# sh test02.sh 
请输入IP地址：139.168.139.133
139.168.139.133 is down
[root@localhost opt]# 

More than 3.3 branch statement

Otherwise, if elif

exit 1 abnormal exit

[root@localhost opt]# vim fenshu.sh
#!/bin/bash
read -p "请输入您的分数" score
if [ $score -lt 0 ]
 then
   echo "你已经没救了"
elif [ $score -gt 100 ]
 then
   echo "别做梦"
elif [ $score -ge 85 ]
 then
   echo "成绩优秀"
elif [ $score -lt 70 ]
 then
   echo "不及格，还要好好努力啊少年"
else
   echo "成绩合格，不要就此止步，继续努力！"
fi
~                               
[root@localhost opt]# sh fenshu.sh 
请输入您的分数-9
你已经没救了
[root@localhost opt]# sh fenshu.sh 
请输入您的分数101
别做梦
[root@localhost opt]# sh fenshu.sh 
请输入您的分数90
成绩优秀
[root@localhost opt]# sh fenshu.sh 
请输入您的分数60
不及格，还要好好努力啊少年

to sum up:

• Syntax condition test operation
  • Test file, compare an integer value, a string comparison logic test
• The syntax diagram if conditional statement
  • Single branch, double-branch, multi-branch

A simple calculator

#!/bin/bash
read -p "请输入一个整数：" numb1
read -p "请选择你需要的运算；选项：加（+）减（-）乘（x）除（/）取余 （%）" yunsuan
read -p "请输入第二个整数:" numb2
if [ "$yunsuan" = "+" ]
  then
  expr=`expr $numb1 + $numb2`
      echo "$numb1 + $numb2 = $expr"
elif [ "$yunsuan" = "-" ]
  then
  expr=`expr $numb1 - $numb2`
      echo "$numb1 - $numb2 = $expr"
elif [ "$yunsuan" = "x" ]
  then
  expr=`expr $numb1 \* $numb2`
      echo "$numb1 x $numb2 =  $expr"
elif [ "$yunsuan" = "/" ]
  then
  expr=`expr $numb1 / $numb2`
     echo "$numb1 / $numb2 = $expr"
else
  expr=`expr $numb1 % $numb2`
     echo "$numb1 % $numb2 = $expr"  
fi
~                                                                                           
~ 
[root@localhost opt]# sh jisuanqi.sh 
请输入一个整数：10
请选择你需要的运算；选项：加（+）减（-）乘（x）除（/）取余 （%）+
请输入第二个整数:8
10 + 8 = 18
[root@localhost opt]# vim jisuanqi.sh
[root@localhost opt]# sh jisuanqi.sh 
请输入一个整数：10
请选择你需要的运算；选项：加（+）减（-）乘（x）除（/）取余 （%）-
请输入第二个整数:5
10 - 5 = 5
[root@localhost opt]# sh jisuanqi.sh 
请输入一个整数：10
请选择你需要的运算；选项：加（+）减（-）乘（x）除（/）取余 （%）x
请输入第二个整数:2
[root@localhost opt]# sh jisuanqi.sh 
请输入一个整数：17
请选择你需要的运算；选项：加（+）减（-）乘（x）除（/）取余 （%）%
请输入第二个整数:4
17 % 4 = 1
[root@localhost opt]# sh jisuanqi.sh 
请输入一个整数：10
请选择你需要的运算；选项：加（+）减（-）乘（x）除（/）取余 （%）/
请输入第二个整数:5
10 / 5 = 2

Job: track and field finalist out of the list Name Sex scores

[root@localhost opt]# vim tianjingsai.sh 
   1 #!/bin/bash
  2 fnan=/opt/nanzizujuesaimingdan
  3 fnv=/opt/nvzizujuesaimingdan
  4 ftao=/opt/taotaimingdan
  5 if [ ! -f $fnan ]&& [ ! -f $fnv ]&& [ ! -f $ftao ]
  6  then
  7  touch $fnan $fnv $tao
  8 fi
  9 read -p "请输入(格式:姓名 性别 成绩):" xingming  xingbie chengji
 10 if [ $chengji -lt 0 ]
 11 then
 12   echo "???你在输什么??"
 13   exit 1
 14 elif [ $chengji -gt 0 ] && [ $chengji -lt 10 ]
 15 then
 16   echo "你的成绩优秀，可以进入10秒内决赛"
 17     if [ $xingbie = "nan" ]
 18       then
 19       echo "$xingming $xingbie $chengji" >> /opt/nanzizujuesaimingdan
 20     else
 21       echo "$xingming $xingbie $chengji" >> /opt/nvzizujuesaimingdan
 22     fi
 23 else
 24   echo "$xingming $xingbie $chengji" >> /opt/taotaimingdan
 25   echo "再加把劲，下次就是你了"
 26 
 27 fi
~         
[root@localhost opt]# sh tianjingsai.sh 
请输入(格式:姓名 性别 成绩):gsy nan 1
你的成绩优秀，可以进入10秒内决赛
[root@localhost opt]# sh tianjingsai.sh 
请输入(格式:姓名 性别 成绩):zzz nv 6
你的成绩优秀，可以进入10秒内决赛
[root@localhost opt]# sh tianjingsai.sh 
请输入(格式:姓名 性别 成绩):aaa nan -6
???你在输什么??
[root@localhost opt]# sh tianjingsai.sh 
请输入(格式:姓名 性别 成绩):aaa nan 14
再加把劲，下次就是你了
[root@localhost opt]# ls
fenshu.sh             nvzizujuesaimingdan  taotaimingdan  test.sh         wwwroot
nanzizujuesaimingdan  rh                   test02.sh      tianjingsai.sh
[root@localhost opt]# cat nvzizujuesaimingdan 
zzz nv 6
[root@localhost opt]# cat nanzizujuesaimingdan 
gsy nan 1
[root@localhost opt]# cat taotaimingdan 
gsy nan 0.5
aaa nan 14