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Tutorial 2 - Counting Web Page Hits with SSI
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Installation:
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Part I: Server side installation:
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ftp article02.cgi to your cgi-bin/thecgibin directory
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on telnet change directories to cgi-bin/thecgibin and
enter the command 'perl -wc article02.cgi'.
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you should get the response 'article02.cgi syntax ok'
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now that the program's on your server and compiles cleanly,
you need to set up the data file. use the 'data'
subdirectory of thecgibin.
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create a new file on your text editor and type the number 0
on it and save the file as article02.dat
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now ftp article02.dat into cgi-bin/thecgibin/data
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on telnet say 'perl -wT article02.cgi'
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now look at the contents of
cgi-bin/thecgibin/data/article02.dat. one way would be to
view the file through your ftp client on the server...
you could even download it from your server to your
computer and then look in your text editor. if you ran
the program once it should equal '1', twice should be '2'.
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Part II - installation on the web:
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Now here comes the meat of this article. So far all we've
done is run our program from the command line and update
the count file. Really no different at all from the previous
tutorial. In order to make this program work for the web
we're going to use the SSI facility, i.e. we'll make it a
server side include.
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You'll have to be on a server that not only allows you to
do Perl programming but also has SSI enabled. Assuming you've
already checked that out, let's proceed. We need a webpage,
and to let the server know it's going to use SSI we'll give
it a file extension of SHTML.
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I wrote a simple one below that you can copy:
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Hello Out There
Hello Out There
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Installing your Webpage:
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in your text editor copy the html code above into a
file on your local computer and call it article02.shtml
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ftp article02.shtml to your cgi-bin/thecgibin directory
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now get on a browser window and type in the url
http://www.mysitename/cgi-bin/thecgibin/article02.shtml/
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now go to ftp and look at the data/article02.dat file.
it didn't change, did it? that's because we haven't
changed the chmod yet.
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Setting File Permissions:
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On your ftp client there will be some mechanism for changing
permissions, aka chmod's on the server. The chmod describes
who can do what with any of your files. It is a three by
three matrix and should look something like this:
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owner group public
read
write
execute/list
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When you ftp'd your files you did it after signing on to
the server with a password, which indicated that you were
the file owner. But anyone at all can get to your files
from the web, if you permit them. Which is after all the
beauty of the internet, right? Well, only if you want
them to.
That's where the chmod's come in.
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Let's review what we've done so far. We've written a
Perl program, a data file, and a web page, i.e. an HTML
file to which we've given the extension of SHTML. Why?
So that the server knows our webpage contains a server
side include or SSI which it will execute before
rendering the webpage to the client's browser.
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Now those are the files. How do they work together, and what
permissions are needed? When we're browsing on the web, from
that website's server's point of view we are in the public
column of the chmod matrix. Makes sense, doesn't it?
Whereas when we run from the command line, i.e. telnet, we
are in the owner column of the matrix. Because telnet like
ftp will require us to log on with a userid and password...
that's how it knows we're the owner.
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Have you figured it out yet? In order to execute our
article02.cgi program as an SSI from our article02.shtml webpage
we have to change the chmod so it can be executed by the public.
The appropriate chmod would be 755 which means check off the
boxes for read and execute on all columns and also for
writing for the owner column.
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Now, let's get back to that webpage. Refresh/reload it and see
what you get. Here's what I got:
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cannot open count : Permission deniedBad file descriptorcannot close
count : Bad file descriptor
Hello Out There
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Huh?
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What happened now? If you haven't gotten frustrated yet
then you have a lot more patience than I did when I
installed my first count program on the web. First of all
let's see where that error message came from.
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Could it be.... the open and close statements in our program?
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Yep, that's it alright. Let's look at those lines again.
Back in our program we said:
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print 'cannot open count : ',$! # ready count file
if not open (COUNT, '+
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AND
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print 'cannot close count : ',$!
if not close (COUNT); # done working with count file
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And lo and behold, that's what our webpage says, followed
by the 'Hello Out There' line. Except what's that other
stuff on those error messages? Those are the system error
descriptions coming from where we printed out the
variable '$!'. Perl's full of shorthand builtin variables
like $!.
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Since the chmod was never set properly for the data file
our program wants to write to, the server told us it was
denying permission to open the file for output. Because
when running from a webpage we are executing the program
as public, right?
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Furthermore since the program couldn't open the file it
couldn't close it later either because there was no open
file to close. So now we'll fix this problem by going to
the ftp client and changing the permissions for
data/article02.dat to 666 which means read and write is
enabled for all columns.
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Now back to the browser and refresh the webpage. It should
just say 'Hello Out There' and nothing else. If so then
reload the page a few more times. Ok? Now go to the ftp
client and have a look at that data file on your server.
I've lost count by now myself but it should have added one
for each refresh you made of the page on your browser.
Pretty cool, huh?
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