Shell programming core technology "three"

Today I will share how to write a Shell Script , And master the preparation Shell The necessary knowledge of script .

Want to write a Shell Script , Necessary knowledge required ： 1、 The first thing you need to master is annotation , Annotate with # start , Used to increase script readability ; 2、 The second is parameters , We need to pass in parameters to the script and parse it ; 3、 Finally, function encapsulation , And master how the script is executed and debugged .

notes

1、 First, let's look at the notes ：

Use vim Create a file , You can also use it vs code Wait for other editors ,vs code Syntax highlighting is supported , It's also very good , Input vim /tmp/test_001.sh Instruction created test_001.sh file .

2、 Next, start writing scripts ：

Press i Key to enter edit mode , And enter comments # this is a test script! Used to tell you what this script is for , Of course, comments are not necessary , Just for readability , meanwhile Shell And will not execute comment statements .

Parameters

When the script starts executing , We need to master the default parameter resolution rules of the system . When passing a parameter to a script , How is it parsed ？

1、$0 Indicates the program being executed , That is, the current script ;

2、1、2 Respectively represents the transmitted number 1 And the first 2 Parameters ,Shell Only supported by default 9 Parameters , If you need to support more parameters, you can use shift;

3、@、* Represents all the parameters , But does not contain

4、${#*}、${#@} Indicates the number of parameters ;

5、{*:1:3}、{*:

interpretative statement ：001、@ And * The difference is as follows ：

Use command ：sh /tmp/test_difference.sh The results are as follows ：

You can see that without quotation marks , Both return the parameters passed in , But with quotation marks , here * Take the parameter as a whole string ( Single string ) return ,@ Return each parameter as a string

002、${#*} And ${#@} Can be used as follows ：

The results are as follows ：

It can be seen from the results that both are the number of parameters

003、{*:1:3}、{*:

As shown in the figure above , You know ：{*:1:3} It means taking the front 3 The value of parameters ,{*:

Next, continue to write scripts ：

As shown in the figure , And print a0=0、a1=1 a2=2, as well as @ and *. The command I use to execute the script is ：sh /tmp/test_001.sh, In the execution result 0 Is the current file name , But at this time, the parameter has no value , The number of parameters is empty .

We enter parameters a,b Pass it to the script , You can see that the first parameter in the output result is a, The second parameter is b, This is the parsing rule of parameters .

function

Finally, the function , The function is named () Heel {} The enclosed structure consists of , Function can realize some function encapsulation , At the same time, the function also supports parameter parsing logic similar to script .

As shown in the figure above , Define a name ：helloworld Of function , Pass through in function if Statement to determine the first parameter $1 Is it equal to python, If equal to, print python auto tester. And then call helloworld function , And pass in a name ：python Parameters of , System printout python auto tester.

We can also encapsulate the above functions directly input on the command line , Put in shell Script and call , adopt elif Judge $1 Is it equal to java, If it is equal to java Just print java auto tester.

The execution result of the calling function is as follows ：

Execute and pass in python Parameters , You can see the final print python auto tester.

If the passed parameter is ：java when , Then print out ：java auto tester

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