CGI-scripts: [ SLAC Utilities | cgi-lib.rxx | Security Wrapper | Security Concerns ]
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
String=GETENV('QUERY_STRING')
or in Perl by using
$string=$ENV('QUERY_STRING');
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 $ARGV[n] array.
For example, if you have a URL
http://www.slac.stanford.edu/cgi-bin/foo?hello+world and you use
the REXX command PARSE ARG Arg1 Arg2 then Arg1 will
contain "hello" and Arg2 will contain
"world" (i.e. the + sign is replaced with a space).
In Perl
$ARGV[1] contains "hello" and $ARGV[2]
contains "world". 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
String=GETENV('PATH_INFO'), or in Perl by using
$string=$ENV('PATH_INFO');
To illustrate this, let's say I have a CGI script which is accessible to my
server with the name foo. 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:
<A
HREF="http://www/cgi-bin/foo/language=english">foo</A>
When the server executes foo, it will give me PATH_INFO of
/language=english, and my program can decode this and act
accordingly.
The PATH_INFO and the QUERY_STRING may be combined. For example, the
URL:http://www/cgi-bin/htimage/usr/www/img/map?404,451
will
cause the server to run the script called htimage. 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.
Standard Input
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
read(STDIN,$in,$ENV{'CONTENT_LENGTH'});
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 subtypes are:
text/plain: text with no special formatting requirements.
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 PostScript
interptreter.
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. Content-type:
type/subtype 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 Content-type
information.
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 <BR>
or <P> for starting new lines or <PRE> to remove
HTML's automatic formatting.
stdout is included in the document sent to the, diagnostics
diagnostics outputted with the SAY command will appear in the document. 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
command, e.g.
chmod o+x /u/sf/cottrell/bin/cgi1.rxx chmod u+x
/u/sf/cottrell/bin/cgi1.rxx