What is cgi? solution?

What is cgi? I found some cgi information, but I still cannot fully understand it. Who can combine the web development history to talk about the relationship between cgi and web servers (APACHE, iis) and php. What is cgi?
I found some cgi information, but I still cannot fully understand it.
Who can combine the web development history to talk about the relationship between cgi and web servers (APACHE, iis) and php.
Share:
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Common GatewayInterface (CGI) is one of the most important technologies in WWW technology and plays an irreplaceable role. CGI is the interface standard between external applications (CGI programs) and Web servers. it is the procedure for transmitting information between CGI programs and Web servers. CGI specifications allow the Web server to execute external programs and send their output to the Web browser. CGI converts a simple set of static hypermedia documents on the Web into a complete new interactive media.
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Cig is a protocol that describes information exchange between web servers (such as nginx and apache) and application gateway (such as php and python.


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This picture is very simple, but it can explain the problem

Cgi is just a specification, and there is no actual thing (or program) called cgi.

To put it simply, cgi specifies what the web server will input to the (cgi-supported) application and what output the application can process.
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Is a set of specifications.

You need to open a restaurant, but opening a restaurant requires a series of procedures, such as photo taking, purchase channels, house rental, and employee recruitment. This can also be regarded as a set of standards, you can start it.