Shell programming overview

One 、Shell A term is used to explain

1.1、Kernel

        Linux The kernel is mainly to deal with hardware

1.2、Shell

1. command interpreter (command interpreter)
2. Shell It's a use. C A program written in a language , It is used by users Linux The bridge .Shell It's a command language , Another programming language .
3. Shell It's an application , This application provides an interface , Through this interface, users can access the operating system Nuclear services .

1.3、shell Two main trends

• sh：

1. Bourne shell（sh） ,Solaris,hpux Default shell
2. Bourne again shell（bash） ,Linux System default shell

• csh:  

1. C shell(csh)
2. tc shell(tcsh)

1.4、#! Statement

Tell the system that the program specified by the path is to interpret the script file Shell Program

Two 、Shell Script execution

2.1、 Enter the absolute or relative path of the script

Check whether the file has execution permission , If not , You can give it permission .

• /root/helloworld.sh
• ./helloworld.sh
• It must be an executable file

2.2、bash or sh + Script  

• sh helloworld.sh
• When the script does not x Authority ,root And the file owner can execute normally in this way

2.3、 Add... Before the path of the script ". " or source  

• source helloworld.sh

2.4、 difference  

• The first and the second will open a new one bash, Different bash Variables in cannot be shared .
• The third kind of It's in the same shell What's going on inside

Demo1:  

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Perform process analysis : 

Demo2:

The analysis reason : When you use sh hello.sh perform shell Script words , Because it will recreate a child process when executing this script , Data between processes cannot be shared , So the one you created earlier name= Zhang San is a variable that cannot be recognized . But if you use source hello.sh To execute the script , Because no new process is created during script execution , So it's still possible to name Variable to print .

Then how to solve it ?

The solution lies in the following keywords export

2.5、export 

• You can pass the variables of the current process to the child process to use
• Future configuration profile When All variables must be preceded by export