Shell script learning day01

1、 first Shell Program 
1.1  establish Shell file 
     command ：touch hello.sh
1.2  To write Shell Program 
     Definition file declaration 
     #!/bin/bash
     echo "Hello word"
        #!: Indicates the Convention mark , He will tell the system what kind of interpreter this script needs to execute , It is a kind of script embodiment 
        echo： Indicates that the command is used to output text information 
1.3  Modify file permissions 
     View permissions for each part    command  ls -l hello.sh
     Modify file permissions   chmod +x ./hello.sh
1.4  perform Shell Script files 
     command ： ./hello.sh

 2、Shell Script Syntax   notes 
 “#”  Notation 
 Be careful ： stay Shell Script , No multiline comments , Only single line comments 

3、Shell Scripting language - Variable 
3.1  Variable definitions - matters needing attention 
 Attention two ： Variable names and equal signs cannot have spaces ( It is different from our general language )
 Note three ： The first letter of the variable name must be a letter or an underscore 
 Notice four ： Spaces are not allowed in the middle of variable names 
 Pay attention to five ： Punctuation is not allowed 
3.2  A read-only variable 
     keyword  ：readonly( read-only , Can't modify )
3.3  Delete variables 
     grammar ：unset
     Case study ：unset  Variable name 
3.4  Variable type 
    3.4.1  Type 1 ： The local variable 
     Scope the whole bash The process can use 
     grammar ：name="smile2018"
    3.4.2  Type 2 ： local variable 
     Scope ： The current code snippet ( Modifier ：local)
    3.4.3  Type 3 ： environment variable 
     Scope ： At present shell Processes and sub processes 
     grammar ：export name=“hello”
    3.4.4 Type four positional variables 
     name=${
    0}
     age=${
    1}
     sex=${
    2}
     echo " full name :${name}  Age :${age}  Gender :${sex}"
      Execute the script ： 
      ./hello.sh tom 23  male 
      Execution results ：   full name :./hello.sh  Age :tom  Gender :23
                ${
    0} Indicates the name of the script file   Parameter from 1 Start 
    3.4.5  Special variables 
    ${
    0}- Special variables - File name 
    ${
    ?}  Indicates the return value of the execution status of the previous command 
     0: Indicates successful execution 
     1： Program execution result 
     2： Indicates the program status return code (0-255)
      System reservation error (1,2,127)
     echo ${
    0}
     echo ${
    ?}
     $# - Number of parameters 
     $* - parameter list 
     [email protected] - parameter list 
     $$ - Go back to the current shell Conduct ID
     $! - Execute last instruction PID
    $* and [email protected] difference 
    $*  parameter list ： Combine all the parameters into a string 
    [email protected]  parameter list ： String separation 

4、Shell Scripting language - character string 
4.1 character string - Single quotation marks 
4.2 character string - Double quotes 
4.3 character string - Splicing 
 Script code 
     name="Andy"
     age=100
     sex=" male "
     info="${name}${age}${sex}"
     echo ${
    info}
 Execution results 
    Andy100 male 

 Script code 
     name="Andy"
     age=100
     sex=" male "
     info=" full name :"${
    name}"  Age ："${
    age}"  Gender ：${sex}"
     echo ${
    info}
 Execution results 
     full name :Andy  Age ：100  Gender ： male 

4.4 character string - Get string length 
     Grammatical structure :${
    # Variable name }
     Script code 
         name="Andy"
         echo ${
    #name}
     Script results 
            4
4.5  character string - Intercept 
     grammar ：${
     Variable name : Starting position : Interception length }
     Script code 
     Case study 1： Intercept from the fourth string , Intercept 3 individual 
     name="my name is XQ"
     result=${
    name:3:3}
     echo ${
    result}
     Case study 2： From the string No 5 The first one starts to intercept , To the last end 
         The way 1：
             name="my name is XQ"
             length=${
    #name}
             result=${
    name:4:length-1}
             echo ${
    result}
         The way 2：
             name="my name is XQ"
             result=${
    name:4}
             echo ${
    result}
    4.6 character string - Delete 
     Grammar 1 ：$( Variable name # Delete string   Regular expressions )
     effect ： Start with a string ( On the left ), Start matching the string to be deleted 
     Case study 1：
         name="my name is XQ"
         result=${
    name#XQ}
         echo ${
    result}
         reason ： It doesn't match 
     Case study 2：
         name="my name is XQ"
         result=${
    name#my}
         echo ${
    result}
     Case study 3： Find the first of the specified characters , And delete all the previous characters ( Include yourself )
         name="my name is XQ"
         result=${
    name#*X}
         echo ${
    result}
     Grammar II : $( Variable name ## Delete string   Regular expressions )
     effect ： From the end of string ( On the right ) Start matching the string to be deleted 
     Case a 
         name="my name is XQ"
         result=${
    name##*X}
         echo ${
    result}
     summary ： grammar 1 And grammar 2, Are deleted from the left to the right , The search direction is different 
     Grammar 3 ：${
     Variable name % Delete string   Regular expressions }
     Match from the end of the string , Delete the matching string 
     Case study 1： Find the first character ( Match the first )
         Code 1 ：
             name="my name is XQ"
             result=${
    name%Q}
             echo ${
    result}
     Case study 2： Find the first of the specified characters , And delete all the previous characters ( Include yourself )
             name="my name is XQ"
             result=${
    name%m*}
             echo ${
    result}

      Grammar 4 :$( Variable name %% Delete string   Regular expressions )
      Case study 1： Find the last of the specified character , And delete all the previous characters ( Include yourself )
         name="my name is XQ"
         result=${
    name%%m*}
         echo ${
    result}

     summary ： Delete from left to right 
                 #- Indicates that the query direction is from left to right 
                 ##- Indicates that the query direction is from right to left 
         Delete from right to left 
                %- Indicates that the query direction is from right to left 
                %%- Indicates that the query direction is from left to right