CGI Programming Interview Questions and Answers

Q1. What is CGI?
The Common Gateway Interface, or CGI, is a standard for external gateway programs to interface with information servers such as HTTP servers. A plain HTML document that the Web daemon retrieves is static, which means it exists in a constant state: a text file that doesn’t change.
A CGI program, on the other hand, is executed in real-time, so that it can output dynamic information.

Q2. Is CGI a script or a program?
The distinction is semantic. Traditionally, compiled executables(binaries) are called programs, and interpreted programs are usually called scripts. In the context of CGI, the distinction has become even more blurred than before. The words are often used interchangeable.

Q3. What is the difference between an interpreted language and a compiled language?
A compiled language is written and then run through a compiler which checks its syntax and compresses it into a binary executable. Since an interpreted language is not compiled, it must be checked for errors at run-time, which makes it quite a bit slower than a compiled language (like C or Java). Perl is an example of an interpreted language. Remember, though, that just because a language is interpreted does not necessarily mean it is not full-featured, or simplistic. Perl can get very complex and very cryptic, very quickly.

Q4. What is the difference between Java and CGI?

  • CGI is a common gateway interface and it is a protocol that is used to run programs on WWW servers. Whereas, Java is having its own standard APIs to, run the programs on WWW servers.
  • CGI is having very high response when creating the Operating system shell. Whereas, Java is a light platform in giving the response when, creating the shell.
  • CGI is not very scalable and not a secure object oriented platform to use. Whereas, Java is object oriented and very secure.
  • CGI doesn’t have the separation between the presentation and business logic. Whereas, Java defines, the separation in a very clean manner and keep everything logically placed.
  • CGI is platform dependent scripting language whereas, Java is platform independent language.

Q5. When do I need to use CGI?
There are innumerable caveats to this answer, but basically any Webpage containing a form will require a CGI script or program to process the form inputs.

 Q6. How can I run my CGI program ‘live’ in a debugger?
At First, in the CGI code, at it’s start, add “sleep (30);” This will cause the CGI to do nothing for thirty seconds (you may need to adjust this time). Compile the CGI with debuging info (“-g” in gcc) and install the CGI as normal. Next, using your web browser, activate the CGI. It will of course just sit there doing nothing. While it is ‘sleeping’, find it’s PID(ps -a | grep <cgi name>).
Load your debugger and attach to that PID(“attach <pid>” in gdb). You will also need to tell it where to find the symbol definitions (“symbol-file <cgi>” in gdb). Then set a break point after the invocation of the sleep function and you are ready to debug. Do be aware that your browser will eventually timeout if it doesn’t receive anything.

Q7. How can I stop my CGI script reading and writing files as “nobody”?
CGI scripts are run by the HTTPD, and therefore by the UID of the HTTPD process, which is (by convention) usually a special user “nobody”.

Q8. What is a CGI bin directory?
A CGI bin directory is a special directory on the server where CGI scripts are allowed to be executed. Most servers are configured to only allow CGI scripts to be executed from one location, in order to minimize security holes. Poorly written scripts can wreak havoc on a server if allowed to run unchecked – most system admins will want to verify that the script is not doing anything malicious before letting you run it.

Q9. When do I need to use CGI?
There are innumerable caveats to this answer, but basically any Webpage containing a form will require a CGI script or program to process the form inputs.

Q10. Can I do HTTP authentication using CGI?
It depends on which version of the question you asked.
Yes, we can use CGI to trigger the browser’s standard Username/Password dialogue. Send a response code 401, together with a “WWW-authenticate” header including details of the the authentication scheme and realm: e.g. (in a non-NPH script)
Status: 401 Unauthorized to access the document
WWW-authenticate: Basic realm=”foobar”
Content-type: text/plain
Unauthorised to access this document
The use you can make of this is server-dependent, and harder,since most servers expect to deal with authentication before ever reaching the CGI (eg through .www_acl or .htaccess).
Thus it cannot usefully replace the standard login sequence, although it can be applied to other situations, such as re-validating a user –
e.g after a certain timeout period or if the same person may need to login under more than one userid.

Q11. What you can never get in CGI is the credentials returned by the user.
The HTTPD takes care of this, and simply sets REMOTE_USER to the username if the correct password was entered.

Q12. What are the different instances used in CGI overhead?

  • CGI overhead is the combination of HTTP protocol and it is a stateless protocol. CGI has to be initialized every time it is accessed through the browser.
  • Server is using the fork() to create a new process and then there is a little overhead involved in it to keep on adding the overheads when the processes grows.
  • CGI program needs to be initialized before its use. It might involve an overhead in executing some script.
  • CGI is much more involved with the backend that includes the database that takes more time to initialize thus the overhead should not be sustained in these cases.
  • CGI uses scripts that are inefficient to take the resources and the program that uses system() or back tick gets more overhead then not using it.

Q13. What is the function of CGIWrap in CGI programming?

  • CGIWrap is used as a gateway program that provides CGI scripts in an HTML forms format.
  • This provides the security of the Http server and allows the user to use the Scripts in a better and compatible way.
  • This allows running of the scripts with permissions that can be given by the users and allows putting up security checks to perform on the scripts.
  • This can be accessed using the URL that is being mentioned in a HTML document and it is configured using the user scripts.
  • The scripts are located at ~/public_html/cgi-bin/ directory.

Q14. What is the meaning of rigging?
Rigging is use for if we want to give animation for any object or character then we apply to character or object internal  bone setting(like our bones) that is called rigging. When apply rigging, then we can give proper animation.

Q15. Can I Identify Users/sessions Without Password Protection?
The most usual (but browser-dependent) way to do this is to set a cookie. If you do this, you are accepting that not all users will have a ‘session’.
An alternative is to pass a session ID in every GET URL, and in hidden fields of POST requests. This can be a big overhead unless _every_ page requires CGI in any case.
Another alternative is the Hyper-G[1] solution of encoding a session-id in the URLs of pages returned:


This has the drawback of making the URLs very confusing, and causes any bookmarked pages to generate old session_ids.
Note that a session ID based solely on REMOTE_HOST (or REMOTE_ADDR) will NOT work, as multiple users may access your pages concurrently from the same machine.

Q16. What are the steps involved in configuring a server using CGI programming?

  • CGI program can configure the server but, before configuring there are certain parameters that must be configured and modified.
  • Setting of ServerRoot is a must if any HTTP server is being used to configure the server.
  • The directive in the httpd.conf must be pointing to the directory that is on the server and it is located at /usr/local/etc/httpd.
  • Then run the CGI scripts which are being placed in ScriptAlias directive as this directive is used to map the resources and its file is srm.conf.
  • The directory in which the CGI scripts are placed is as follows:
    /cgi-bin/ /usr/local/etc/httpd/cgi-bin/
  • The user can access the file that he has created by using the following URL on his local server as:
    and the following directory is being executed on the server
  • CGI programs have different directories due to the security reasons and concerns. This way all the programs can be set at one place and the server administrator can have the control over it.

Q17. What does CGI program store?

  • CGI program stores information about the client and it helps the server to search the input string and show the output according to the match.
  • It consists of the information regarding the host server and the user passing the input information.
  • It provides the information with the server name, communication protocol and the name of the software that is running on the system or server.
  • It includes the directive and directory structures that can be matched while executing a program or running a script.
  • It provides various ways to execute the script so that user can understand and the server can execute the input.

Q18. Can I redirect users to another page?
For permanent and simple redirection, use the HTTPD configuration file:it is much more efficient than doing it yourself. Some servers enable you to do this using a file in your own directory (eg Apache) whereas others use a single configuration file (eg CERN).
For more complicated cases (eg process form inputs and conditionally redirect the user), use the “Location:” response header. If the redirection is itself a CGI script, it is easy to URLencode
parameters to it in a GET request, but dont forget to escape the URL!

Q19.  Is there an equivalent of JavaScripts escape() function in Perl?
require CGI;
$escaped = CGI::escape( $normal );

# …or…

sub escape {
my $str = shift || ”;
$str =~ s/([^w.-])/sprintf(“%%%02X”,ord($1))/eg;
$escaped = escape( $normal );

Q20. How can I stop my CGI script reading and writing files as nobody?
CGI scripts are run by the HTTPD, and therefore by the UID of the HTTPD process, which is (by convention) usually a special user “nobody”.There are two basic ways to run a script under your own userid:
(1) The direct approach: use a setuid program.
(2) The double-server approach: have your CGI script communicate
with a second process (e.g. a daemon) running under your userid,which is responsible for the actual file management.

The direct approach is usually faster, but the client-server architecture may help with other problems, such as maintaining integrity of a database.
When running a compiled CGI program (e.g. C, C++), you can make itsetuid by simply setting the setuid bit: e.g. “chmod 4755 myprog.cgi”
For security reasons, this is not possible with scripting languages(eg Perl, Tcl, shell). A workaround is to run them from a setuid program, such as cgiwrap.
In most cases where you’d want to use the client-server approach, the server is a finished product (such as an SQL server) with its own CGI interface.A lightweight alternative to this is Don Libes’ “expect” package.

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