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There are some simple software libraries to facilitate writing CGI scripts.
cgi-lib.rxx is a REXX
library of functions (available at SLAC by using the REXX
CALL PUTENV 'REXXPATH=/afs/slac/www/slac/www/tool/cgi-rexx'
statement to include the library at execution time)and cgi-lib.pl is a similar library in Perl written by Steve Brenner (there is an executable copy of this libary at SLAC in /afs/slac/g/www/cgi-lib/cgi-lib.pl). NCSA has a very useful set of Perl CGI handler subroutines that are available via anonymous FTP.Another set of Perl CGI Server Side Scripts written by Brigitte Jellinek is available under Gnu public license. There is also the Source code for www.stanford.edu scripts and programs. There is also an index to Perl WWW programs gathered by Earl Hood. Finally see the Web Development Center.
Since there are security and other risks associated with executing user scripts in a WWW server, the reader may wish to first view a document providing information on a SLAC Security Wrapper for users' CGI scripts. Besides improving security, this wrapper also simplifies the task of writing a CGI script for a beginner.
Before embarking on writing a script, you may also want to check out some rough notes on SLAC Web Utilities Provided by CGI Scripts.
The CGI is an interface for running external programs, or gateways, under an information server. Currently, the supported information servers are HTTP (the Transport Protocol used by WWW) servers.
Gateway programs are executable programs (e.g. UNIX scripts) which can be run by themselves (but you wouldn't want to except for debugging purposes). They have been made executable to allow them to run under various (possibly very different) information servers interchangeably. Gateway programs conforming to this specification can be written in any language, including REXX or Perl, which produces an executable file
QUERY_STRING Environment Variable
QUERY_STRING is defined as anything which follows the first ? in
the URL used to access your gateway. This information could be
added by an HTML
ISINDEX document, or by an HTML
Form (with the GET action). It could also be manually embedded
in an HTML hypertext link, or anchor,
which references your gateway. This string will
usually be an information query, e.g. what the user wants to
search for in databases, or perhaps the encoded results
of your feedback Form. It can be accessed in REXX by using
or in Perl by using
This string is encoded in the standard URL format which changes spaces to +, and encoding special characters with %xx hexadecimal encoding. You will need to decode it in order to use it. You can review the cgi-lib.rxx REXX PROCEDURE DeWeb or the Perl code fragment giving examples of how to decode the special characters.
If your server is not decoding results from a Form, you will also
get the query string decoded for you onto the command line. This
means that the query string will be available in REXX via the
PARSE ARG command,
or in the Perl
For example, if you have a URL
and you use the REXX command
PARSE ARG Arg1 Arg2
Arg1 will contain
"world" (i.e. the + sign is replaced with a space).
If you choose to use the command line to access the input, you need to do
less processing on the data before using it.
PATH_INFO Environment Variable
Much of the time, you will want to send data to your gateways which the client shouldn't muck with. Such information could be the name of the Form which generated the results they are sending.
CGI allows for extra information to be embedded in the URL for
your gateway which can be used to transmit extra context-specific
information to the scripts. This information is usually made
available as "extra" information after the path of your gateway in
the URL. This information is not encoded by the server in any
way. It can be
accessed in REXX by using
or in Perl by using
To illustrate this, let's say I have a CGI script which is
accessible to my server with the name
When I access foo from a
particular document, I want to tell foo that I'm currently in
the English language directory, not the Pig Latin directory. In
this case, I could access my script in an HTML document as:
When the server executes foo, it will give me
/language=english, and my
program can decode this and act accordingly.
The PATH_INFO and the QUERY_STRING may be combined. For example, the
will cause the server to run the script called
It would pass remaining path information
/usr/www/img/map" to htimage in the PATH_INFO
environment variable, and pass "
405,451" in the
QUERY_STRING variable. In this case,
htimage is a
script for implementing active maps supplied with the CERN HTTPD.
If your Form has METHOD="POST" in its FORM tag, your CGI program
will receive the encoded Form input on standard input
stdin in Unix). The server will NOT
send you an EOF on the end of the data, instead you should use the
environment variable CONTENT_LENGTH to determine how much data you
should read from stdin. You can accomplish this in REXX by using
In=CHARIN(,1,GETENV('CONTENT_LENGTH')), or in Perl by using
If you wish to pass the standard input onto another script that you will call later, then you may wish to review the cgi-lib.rxx REXX PROCEDURE ReadPost.
The REXX PROCEDUREs ReadForm together with MethGet and MethPost, all available in cgi-lib.rxx, may be used to simplify the task of reading input from a Form.
Form data is a stream of name=value pairs separated by the ampersand (&) character. Each name=value pair is URL encoded, i.e. spaces are changed into plus signs and some characters are encoded into hexadecimal. To decode the Form data you must first parse the Form data block into separate name=value pairs tossing out the ampersands. Then you must parse each name=value pair into the separate name and value. Use the first equal sign you encounter to split the data. If there is more than one, then something is wrong with the data. Again toss out the equals signs. Finally undo the URL encoding of each name and value.
You can review the REXX or the Perl code fragment giving examples of decoding the Form input.
When using the name and value information in the script, you need to be aware that:
In order to tell the server what kind of document you are sending back, CGI requires you to place a short header on your output. This header is ASCII text, consisting of lines separated by either linefeeds or carriage returns followed by linefeeds. Your script must output at least two such lines before its data will be sent directly back to the client. These lines are used to indicate the MIME type of the following document
Some common MIME types relevant to WWW are:
"text" Content-Type which is used
to represent textual information
in a number of character sets and formatted text description
languages in a standardised manner. The two most likely
text/plain: text with no special formatting
text/html: text with embedded HTML commands
"application" Content-Type, which is used to transmit
application data or binary data. Two frequently used subtypes are:
application/postscript: The data is in
PostScript, and should be fed to a
application/binary: the data is in some unknown
binary format, such as the results of a file transfer.
"image" Content-Type for transmitting still image
(picture) data. There are many possible subtypes, but the ones
most often used on WWW are:
image/gif: an image in the GIF format.
image/xbm: an image in the X Bitmap format.
image/jpeg: an image in the JPEG format.
type/subtype is the MIME type and subtype for your output.
Next, you have to send the second line. With the current specification, THE SECOND LINE SHOULD BE BLANK. This means that it should have nothing on it except a linefeed. Once the server retrieves this line, it knows that you're finished telling the server about your output and will now begin the actual output. If you skip this line, the server will attempt to parse your output trying to find further information about your request and you will become very unhappy.
You can review a
REXX Code Fragment
giving an example of handling the
After these two lines have been outputted, any output to
stdout (e.g. a
REXX SAY command) will be included in the document sent to the client.
This output must be consistent with the
Content-type header. For
example if the header specified
Content-type text/html then the
following output must include HTML formatting such as using
<P> for starting new lines or <PRE>
to remove HTML's
stdout is included in the document sent to the,
diagnostics diagnostics outputted with the SAY command will appear in
You can review a
REXX Code Fragment
giving an example of diagnostic reporting.
If errors are encountered (e.g. no input provided, invalid characters found, too many arguments specified, requested an invalid command to be executed, invalid syntax or undefined variable encountered in the REXX script) the script should provide detailed information on what is wrong etc. It may be very useful to provide information on the settings of various WWW Environment Variables that are set.
The CGIerror, CGIdie and MyURL REXX PROCEDUREs in cgi-lib.rxx provide some assistance for error reporting. In addition review the REXX code fragments using CGIerror and using CGIdie and also typical CGIerror output and CGIdie output.
chmod o+x /u/sf/cottrell/bin/cgi1.rxx
chmod u+x /u/sf/cottrell/bin/cgi1.rxx
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