A good CGI script (script)

As a web designer, you create a client's CGI script, which is used by the server-side program to process user input, and the result is returned to the user.

Here you will learn everything about CGI scripts:

• What is a CGI script? How it works
• What does a CGI script output look like?
• How to create a CGI script with parameters or without parameters
• How to create a CGI script that returns a specified response
• How to create a CGI script for input form
• Questions about using CGI scripts
• CGI variables that you can use in scripts

Are CGI scripts not a real script? According to your server's support, they may be a compiled program or batch command file or other executable stuff. For simplicity, we collectively call them scripts.

CGI scripts are used in two ways: ACTION as a form or as a direct link in a page.

Figure 1. From browser to server to script to programRemember to come back!

Here is a brief explanation:

1. A URL points to a CGI script. The URL of a CGI script can appear anywhere like a normal URL.
2. When the server receives the request, executes the script according to the script file pointed to by that URL (note the location and extension of the file).
3. The script performs operations based on input data, including querying the database, calculating numerical values, or calling other programs in the system.
4. The script produces some kind of output that the web server can understand.
5. The server receives the output from the script and passes it back to the browser to let the user know the result.

At Display Date is a connection to a CGI script. Its HTML is as follows:

<A HREF="/cgi-bin/getdate">Display the Date</A>

When you select this connection, your browser will make a request to the server. The server receives this request and calculates the script file name at the URL and executes the script.

This getdate script is executed in UNIX system like this:

#!/bin/sh


echo Content-type: text/plain


echo


/bin/date

Content-type: text/plain


Tue Oct 25 16:15:57 EDT 1994

This way the browser output is as shown in Figure 3.

Figure 3 date script output result.

This is the most basic, the actual situation is much more complicated, and it can be used to understand how browsers, servers and scripts work.

Sure? You just can't do it, can you learn? All right! Let's continue.

But even if you have a web server, this server must be configured specifically for running CGI scripts. That means all your scripts must be placed in a directory called cgi-bin.

Before writing a CGI script, ask your server administrator if you allow you to install and run CGI scripts, and where do they have to place them if they can? Also, you must have a real web server. If it is an FTP or Gopher server, then you cannot use CGI.

If you run on your own server, you must create a directory called cgi-bin and configure your server to recognize this directory as a script directory. You must also remember the following features of CGI scripts:

• Each script is a program, which runs on a system that the browser can request, and uses CPU time and memory when executing. What happens if thousands of these scripts run at the same time? Your system will not bear the load until it crashes.
• If you don't write your CGI script carefully, you will have the potential to let others enter the system that hurts you through your CGI script parameters.

In this study manual, CGI scripts are written in only two languages: UNIX shell and Perl. This shell is suitable for running on any similar UNIX system and is easy to learn, but it is difficult to deal with complex situations. Perl, it is time to use this language, it is free, this language is stable and powerful, similar to C, but it is also difficult to learn.

If you are renting a server, you must allow CGI scripts to be run.

If you have your own server, check how your server manual handles CGI scripts.

This header is actually not part of the text, but an information protocol between the server and the browser, you can't actually see it.

There are three types of headers: Content-type, Location, and Status. Content-type is the most common.

For an explanation of content-type, see the instructions for HTML, a specific encoding you can issue like this:

Content-type: text/html

Table 1. General formats and content-types.

Format

Content-Type

Figure 4. The script results
This is a very simple example, he can prepare the call like this:

<A HREF="/cgi-bin/pinglaura">Is Laura Logged in?</A>

According to the previous explanation, the content of this script is as follows:
#!/bin/sh
 

echo Content-type: text/html
 

echo "<HTML><HEAD>"
 

echo "<TITLE>Is Laura There?</TITLE>"
 

echo "</HEAD><BODY>"
To test if I have logged into the system, use the who command (my login name is assumed to be lemay), and store the result in the variable ION. If I log in, the variable ION will have something, otherwise it will be empty.

ison='who | grep lemay'

if [ ! -z "$ison" ]; then


        echo "<P>Laura is logged in."</P>


else


        echo "<P>Laura isn't logged in."</P>


fi

echo "</BODY></HTML>"

Content-type: text/html


<HTML><HEAD>


<TITLE>Are You There?</TITLE>


</HEAD><BODY>


<P>Laura is not logged in.


</BODY></HTML>

The complete script for this example is as follows:

#!/bin/sh

echo "Content-type: text/html"


echo

echo "<HTML><HEAD>"

echo "<TITLE>Is Laura There?</TITLE>"

echo "</HEAD><BODY>"

ison='who | grep lemay'

if [ ! -z "$ison" ]; then

        echo "<P>Laura is logged in"

else

        echo "<P>Laura isn't logged in"

fi

echo "</BODY></HTML>"

<A HREF="/cgi-bin/myscript?arg1+arg2+arg3">run my script</A>

This method is sometimes called query, because it was used in the search function in the early days.

Let's take a different question:

#!/bin/sh


echo "Content-type: text/html"


echo


echo "<HTML><HEAD>"


echo "<TITLE>Are You There?</TITLE>"


echo "</HEAD><BODY>"

ison='who | grep "${1}"'

All the remaining modifications are as follows:

if [ ! -z "$ison" ]; then


        echo "<P>$1 is logged in"


else


        echo "<P>$1 isn't logged in"


fi

echo "</BODY></HTML>"

<A HREF="/cgi-bin/pinglaura">Is Laura Logged in?</A>

<A HREF="/cgi-bin/pinggeneric?john">Is John Logged in?</A>

See the following path information example,:

http://myhost/cgi-bin/myscript/remaining_path_info?arg1+arg2

For example, let's assume that you already have multiple connections to the same script on multiple pages. You can use this path information to display the name of the connected HTML file. In this way, after you finish processing your script, when you send back an HTML file, you can include a connection in the file and send back the user's initial page.

You will learn more path information in the next chapter: useful forms and scripts. Please log out later

Don't be afraid, let's start explaining these situations here.

Location: ../docs/

The path to this file can be a URL or a relative path. All relative paths refer to the location where the script is located. The text in the example is in the docs directory in the current previous directory:

echo Location: ../docs/


echo

Fortunately, it's all easy. You just need to output the following command (after a blank line):

echo Status: 204 No Response


echo

Remember, most forms have two parts: HTML form format; CGI scripts that process form data. This CGI script is called using the tag <FORM> attribute.

This ACTION property contains scripts for processing the form:

<FORM ACTION="/cgi-bin/processorscript">

The method we talked about above is actually GET, which places the data package in the environment variable QUERY_STRING as part of the URL as a whole to pass to the server.

POST does a lot of the same things as GET, the difference is that it passes data separately to the script. Your script gets this data through standard input. (Some web servers are stored in temporary files.) This QUERY_STRING environment variable will no longer be set.

So which method do you use? POST is a safe method, especially if your form has a lot of data. When you use GET, the server allocates the variable QUERY_STRING to all form data, but the amount of storage that variable can be limited. In other words, if you have a lot of data but you use GET, you will lose a lot of data.

If you use POST, you can use as much data as possible because the data is never allocated to a variable.

theName=Ichabod+Crane&gender=male&status=missing&headless=yes

• Each pair of name/value is separated by the & character.
• Each pair of name/values ​​from the form is separated by the = character. If the user does not enter a value to this name, then the name still appears, but has no value (like "name=").
• Any special characters (that are not simple seven-digit ASCIIs, such as Chinese characters) will be encoded in hexadecimal with percentage %. Of course, special characters such as =, &, and % are also included.
• The spaces in the input area will be displayed with a plus sign +.

Here is a decoding program called uncgi, you can/~koreth/. Get the original code and install it in your own cgi-bin directory.

Figure 5. A form that tells me your name.

This input is sent to the script and then sent back the message showing a hello (Figure 6).

What happens if you don't type anything in your name input? See Figure 7.

Figure 6. Results of the name form.

Figure 7. Another result.

<FORM METHOD=POST ACTION="../cgi-bin/form-name">


</FORM>

<FORM METHOD=POST ACTION="../cgi-bin/uncgi/form-name">


</FORM>

The first step in the script is decoding. In this example, we have used uncgi to decode the input data. In fact, this form has been decoded for you. By creating an uncgi directory, once the form is submitted to the server, the server will automatically decode it, so that all the name/values ​​are ready to wait for your use.

Now, the beginning part of an example assumes the following:

echo Content-type: text/html


echo


echo "<HTML><HEAD>"


echo "<TITLE>Hello</TITLE>"


echo "</HEAD><BODY>"


echo "<P>"

The value of this Name element is included in the WWW_theName environment variable. With a simple test command (-z), you can check whether the environment variable is empty or includes the corresponding output value:

if [ ! -z "$WWW_theName" ]; then


    echo "Hello, "


    echo $WWW_theName


else


    echo "You don't have a name?"


fi

echo "</P><P><A HREF="../lemay/">Go Back</A></P>"


echo "</BODY></HTML>"

• The script content is only displayed but not executed.

  Have you configured your server to run CGI scripts correctly? Are your scripts placed in the cgi-bin directory? If your server allows CGI with .cgi extension to run, is this the extension for your script filename?

• Error 500: Server doesn't support POST.

  The answer is still the same as the previous one, and then you execute your CGI using the command line. Can it run normally? Is there any error? .

• Document contains no data.

  Make sure there is a blank line between your header line and the data section.

• Error 500: Bad Script Request.

  Make sure your script is executable (on UNIX, use chmod +x your script.cgi). Before running from the browser, you should run your script from the command line. If the client is win95, you can log in to your server with telnet and execute the command line. Of course, you must understand UNIX commands.

Table 2. CGI environment variables.

Environment variables

significance

Of course, there are also programs that can be decoded when the form is uploaded, and very few.

For use, you usually write this:
＃！/usr/lib/perl

require '';

&ReadParse(*in);

After decoding the form, you can read and process the name/value by following the following:

print $in{'theName'};

If you have multiple name pairs with the same name, separate multiple names with (\0). This will handle your script normally.

Later versions can be very supportive,/cgi-lib/Learn more.

Another CGI address that handles Perl is/cgi_docs.html .

Note: The above program can be edited with ultra edit. Note that the UNIX format must be saved in UNIX format, uploaded, log in with telnet, type perl on the command line to see if there are any bugs, and then call it in the browser. The CGI program must change the attribute to 777 when placing the CGI directory, and the HTML file to be written must also change the attribute to 777.

There are many free cgis on the Internet now, which can basically meet general needs. Please go to this website to check the cgi you want:/cgicollection/

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