Shell introduction and variable definition

One 、 What is? shell

         shell It's based on C Language development program .shell It's a command language , Another programming language .

Two 、 frequently-used shell

         frequently-used shell Yes bash、sh、zsh. It's usually used bash.

3、 ... and 、shell Variable definition rules

        1. When defining variables , Variable name and equal sign 、 There cannot be a space between the equal sign and the value .
        2. Variable names can only use letters 、 Underline 、 Numbers , And can only start with letters and underscores 
        3. There cannot be spaces between variable names , have access to "_" Instead of .
        4. When there are spaces in the variable value , You need to use double quotes / Quotation marks .

Four 、 Use of variables

       1. When using variables , Need to use $ Symbol , Such as ：    

      Be careful ： When using variables ,{} It's not necessary ,{} The function of is to determine the boundary of variables , When variables are followed by others code when , Need to add {}, In this way, the program can correctly identify the boundary of variables .
    
    2. Define a read-only variable ： Use readonly Read variables , Usage such as ===》readonly  Variable name 
    3. Delete variables ： Use unset  You can delete variables , But you cannot delete read-only variables 

 4. Variable type （ Just to understand ）：
         local variable ： Only in the current shell Use in , Other shell cannot access .
         environment variable ：Linux A variable of its own 
        shell Variable ：shell Special variables for program settings 
         Be careful ： When you want the child process to access the variables of the parent process , Need to use export, Export private variables .

5、 ... and 、shell String in

    1.shell Strings in can be declared with single or double quotation marks .
    2. String declared in single quotation marks , When there are variables in it , Unable to get the value of the corresponding variable .
    3. Strings declared in double quotation marks can use variables and escape characters .

    4. Get string length ：echo ${# String name }
    5. Extract string ：echo ${ String name :m：n}  m： Represents the starting coordinate ,n Represents the extracted length .
    6. String content substitution ：
           6.1 Replace a matching value ：echo ${ Variable name / Original value / The new value }
           6.2 Replace all matching values ：echo ${ Variable name // Original value / The new value }

    7. Check the position of the specified character in the string ：( Be careful ！ This is not a single quote , It's a back quote “`”)

6、 ... and 、 Define an array

    1.shell Only one dimensional arrays are supported 
    2. The grammar is ： Array name =（ value 1  value 2 .... value n）, Space between each value .
    3. Get the specified element in the array ：echo ${ Array name [ Subscript of corresponding element ]}
    4. Gets the length of the array ：echo ${# Array name [*]}

                                                                                                 -------------------------------------------------------- No end for learning