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iakovlev.org

Базы данных и Perl


Содержание


С базами данных в перле можно работать двумя способами: вы можете реализовать свою собственную базу или использовать уже существующую. Можно использовать стандартный пакет DBM,RDBM.

Пакет DBM

Perl comes with a set of database management (DBM) library files. Here's the list of packages that come with the latest version of Perl (5.002):

  • GDBM_File.pm-GNU's database interface files
  • NDBM_File.pm-Berkeley UNIX compatibility
  • ODBM_File.pm-Standard Perl package
  • SDBM_File.pm-Standard Perl package
  • AnyDBM_File.pm-Virtual classes for any of the above database interfaces

The AnyDBM_File package encapsulates the rest of the packages. If you use the AnyDBM file, you'll automatically load one of the standard packages from the previous list. To use the AnyDBM_File package, insert the following statement at the start of your Perl script:

use AnyDBM_File;

There is nothing preventing you from explicitly naming a particular DBM package to override the defaults used by the AnyDBM_file package. To use the GDBM_File explicitly, insert this statement instead:

use GDBM_File;

A DBM, when used with these packages, is a mapping of an associative array to a file on disk. To map the associative array to disk, you use dbmopen() to create or open an existing file, make all the modifications to the associative array, and then close the file with a call to dmbclose().

I covered how to bind variables to associative arrays and how to use the tie() function in Chapter 6, "Binding Variables to Objects." In the future, most dbmopen() function calls will be replaced by, or at least internally work as, tie() function calls. Basically, a DBM file will be tied to an associative array. Subsequent updates to the associative array will be reflected in the file on disk. However, you will come across legacy code that does use the dbmopen() and dbmclose() functions, so it's important to see how these work together. Also, you may find it easier to use these DBM functions to program for quick prototyping than to have to code the callback functions required for the tie() function. After you've read this chapter, you should be able to decide whether to use DBM functions or write the functions for database access using the tie() function.

Here's an example to illustrate how the DBM files work. First, I'll create a small database of some stock symbols and the names of the companies. The file containing this data sample is called sym.txt and is partially shown here:

$ head sym.txt
ASZ  AMSCO INTL Inc
AMSR  AMSERV HEALTHCARE
ASO  AMSOUTH BAncORP
AMTC  AMTEch
AMTL  AMTROL
AVFC  AMVESTORS FIN CORP
AMW  AMWEST INSURAncE GROUP
AMLN  AMYLIN PHARMACEUTICALS
AAC  ANACOMP Inc
Apc  ANADARKO PETROLEUM CORP
$

This file contains several lines of records where each record has two items of data: the first column is the stock ticker symbol followed by the company name. Note that a company name can contain one or more space characters.

Now I'll construct a mini-database of all company names indexed by their stock symbol. To use the DBM utilities for this endeavor, I would use the script shown in Listing 18.1.


Listing 18.1. Creating a DBM file.
 1 #!/usr/bin/perl
 2 # --------------------------------------------------------
 3 # Sample script to create a DBM repository.
 4 # Input file "sym.txt" to the script has the format:
 5 #    SYMBOL  Company Name
 6 # --------------------------------------------------------
 7 #
 8 # Use the default DBM utilites
 9 #
10 use AnyDBM_File;
11 open(SFILE, "sym.txt") || die "\n Cannot open sym.txt $!\n";
12 #
13 # Open the database "ticker". If it does not exist, create it.
14 # Map the hash stocks to this database.
15 #
16 dbmopen(%stocks,"ticker",0666);
17 while (<SFILE>) {
18     chop;
19     ($symbol,@name) =  split(' ',$_);
20
21     $stocks{$symbol} = join(' ',@name);
22     # print " $symbol [ @name ] \n";
23     }
24 #
25 # Close the input file
26 #
27 close(SFILE);
28 #
29 # Commit changes in hash to disk for the next flush.
30 #
31 dbmclose(%stocks);

In Listing 18.1, the AnyDBM_File package is used at line 10 to get the best available package for the system. You can override the type by replacing the AnyDBM_File package with a DBM package of your choice. In line 11, the input file is opened.

Line 16 creates a DBM called ticker and maps it onto an associative array called %stocks{}. Note that you are actually creating two files called ticker.pag. The file permissions given for this open are 0666 so that it's possible for all other users to use this DBM.

In lines 17 through 23, the symbol names are read in, and the values are assigned to the newly created hash, called stocks. Lines 18 and 19 simply take the text input from the sym.txt file and separate it into a symbol and a company name. At line 21 a value is assigned to the associative array using the symbol as the index into the array. Now that this array has been mapped to the disk, the data can be kept in the array even after the script that created this file is long gone.

Note also that the join() function is used in line 21 to piece together the items of names of the companies into one string. Had the join() function not been used, $stocks{$symbol} would contain the number of items of the @names array and not the contents.

In line 31 the dbmclose() function is used to commit the changes to disk. If you do not use dbmclose(), your DBM file modifications are not be saved to disk. The dbmclose() function disconnects (or un-ties) the hash from the file on disk so that any changes made to the hash after the dbmclose() function are made as if to an uninitialized associative array. When the contents of the hash will be flushed to disk depends on the underlying system. As far as the program is concerned, the hash connected to the underlying database is not initialized anymore.

Two files are created with the use of the AnyDBM_File package. These are the two files that were created by the script in Listing 18.1 when I ran it:

-rw-r--r--   2 khusain  users        6608 Mar 26 08:08 scripts/ticker.dir
-rw-r--r--   2 khusain  users        6608 Mar 26 08:08 scripts/ticker.pag

Of course, because you do not have access to the original files that I used in this example, the sizes shown here will be different for the data file that you use. Just note that the size of both the files is the same.

If the AnyDBM_File package had been substituted with the GDBM_File page, we would only get one file:

-rw-r--r--   1 khusain  users        6608 Mar 26 08:06 scripts/ticker

Again, the size of the ticker file will be different depending on the input data file that you use. However, you should note that the size of the ticker file is the same as that of the two files created by the AnyDBM-File package.

Viewing Your Data

The database has now been created on disk. Whether it consists of one file or two is not important in the application that will access the database because all the internals are hidden with the DBM function calls.

In order for us to look at the data in the newly created DBM file, another script needs to be created. The viewing script is shown in Listing 18.2.


Listing 18.2. Viewing the contents of a DBM file.
 1 #!/usr/bin/perl
 2 # --------------------------------------------------------
 3 # Sample script to list contents of a DBM repository.
 4 # --------------------------------------------------------
 5 #
 6 # Use the default DBM utilites
 7 #
 8 use AnyDBM_File;
 9 #
10 # Open the database "ticker". (If it does not exist, create it.)
11 # Map the hash stocks to this database.
12 #
13 dbmopen(%stocks,"ticker",0666);
14 while (($symbol,$name) = each(%stocks)) {
15     print "Symbol [$symbol] for [$name]\n";
16     }
17 #
18 # Keep any changes, if any, in hash back on disk.
19 #
20 dbmclose(%stocks);

Line 13 opens the hash as before. Because the file already exists, there is no need to create it. Lines 14 and 15 are where the contents of the hash $stocks is printed out. Line 20 closes the file.

Adding and Removing Items

It's relatively simple to add a new symbol to the newly created database. To see how to add to the database, refer to Listing 18.3.


Listing 18.3. Adding to the DBM array.
 1 #!/usr/bin/perl
 2 # --------------------------------------------------------
 3 # Sample script to add to contents of a DBM repository.
 4 # --------------------------------------------------------
 5 #
 6 # Use the default DBM utilites
 7 #
 8 use AnyDBM_File;
 9 #
10 # Open the database "ticker". (If it does not exist, create it.)
11 # Map the hash stocks to this database.
12 #
13 dbmopen(%stocks,"ticker",0666);
14 print "\n Symbol: ";
15 # Get the symbol for the company
16 chop  ($symbol = <STDIN>);
17 print "\n Company Name: ";
18  # Get the name of the company
19 chop ($name = <STDIN>);
20 $stocks{$symbol} = $name;
21 # Keep any changes, if any, in hash back on disk.
22 #
23 dbmclose(%stocks);

To delete an item from the %stocks{} associative array, use the delete command. For example, the code in Listing 18.3 can be modified to become a deletion command by adding the delete operator to line 20. The code in line 20 would look like this:

delete $stocks{$symbol};

Operations on a DBM File

The contents of a DBM array can be easily searched using Perl's features. To look for a symbol in an existing DBM, use the Perl search operation with the =~ // syntax. Listing 18.5 illustrates this.


Listing 18.5. Searching the DBM array.
 1 #!/usr/bin/perl
 2 # --------------------------------------------------------
 3 # Sample script to search contents of a DBM repository.
 4 # --------------------------------------------------------
 5 #
 6 # Use the default DBM utilites
 7 #
 8 use AnyDBM_File;
 9 #
10 # Open the database "ticker". (If it does not exist, create it.)
11 # Map the hash stocks to this database.
12 #
13 dbmopen(%stocks,"ticker",0666);
14 print "\n Enter String to Look for: ";
15 $search = <STDIN>;
16 chop $search;
17 printf "\n Searching.\n";
18 $i = 0;
19 while (($symbol,$name) = each(%stocks)) {
20     if ($name =~ /$search/) {
21         $i++;
22         print "$i. [$symbol] has [$name]\n"
23         }
24     }
25 printf "\n ===> $i Records found.\n ";
26 #
27 # Keep any changes, if any, in hash back on disk.
28 #
29 dbmclose(%stocks);

Lines 19 through 24 contain a while loop that uses the each command. The each command is more efficient than using a for loop command because both the key and the indexed value are retrieved with the each command. In the for loop on the keys of an associative array, two separate steps must be taken to retrieve the same information-first to get the key for the loop iteration with a call to the keys(hash) function, and second to get the value based on the retrieved key.

Running the program to look for NAL in the company names produces the following output:

 Enter String to Look for: NAL
 Searching.
1. [ALOG] has [ANALOGIC CORP]
2. [AATI] has [ANALYSIS & TEchNOLOGY]
3. [ANLY] has [ANALYSTS INTL CORP]
4. [APSG] has [APPLIED SIGNAL TEch]
5. [ADI] has [ANALOG DEVICES]

 ===> 5 Records found.

The program found all company names with the string NAL in their name. As you can see, within Perl you have the power to create a new database, add or delete items from the database, and list the contents of the database. Using the hash as the map, you can perform many other operations on the hash and reflect the results back to disk.

The DBM files are still a flat database; therefore, in order to create relationships between data items, you still have to do some shuffling around of indexes, filenames, and files. Fairly sophisticated data algorithms are not out of reach, however, if you are careful. For example, the sample $symbol index can also be used as a filename containing historical data. Let's say the %stocks{} DBM array is used as a storage area for the stocks in a portfolio. It's easy to get historical data from the Internet (for example, via CompuServe) for a given stock symbol. Listing 18.5 collects and displays some statistics for the symbols in the sample portfolio.

So that you don't have to type everything via STDIN, the script is designed to use command-line options with the Getopts::Long module. See Chapter 16, "Command-Line Interface with Perl," for more information on how to use the Getopts::Long.pm module. In this example, the -s option is used to specify the stock symbol and the -v option is used to get a complete listing of all the readings.


Listing 18.4. Using the DBM array.
 1 #!/usr/bin/perl
 2 # --------------------------------------------------------
 3 # Sample script to use the DBM array for indexing.
 4 # --------------------------------------------------------
 5 #
 6 # Use the default DBM utilites
 7 #
 8 use AnyDBM_File;
 9 use Getopt::Long;
10 #
11 # Open the database "ticker". (If it does not exist, create it.)
12 # Map the hash stocks to this database.
13 #
14 dbmopen(%stocks,"ticker",0666);
15 #print "\n Enter Symbol to Look for: ";
16 #$search = <STDIN>;
17 #chop $search;
18 GetOptions('stock=s','v!');
19 $search = $opt_stock;
20 $i = 0;
21 $highest = 0;
22 $lowest = 10000;
23 if ($stocks{$search}) {
24     if (-e $search) {
25         open(SDATA,$search) || die "Cannot open $search $!\n";
26         printf "\n Historical data for $search \n" if ($opt_v);
27         while (<SDATA>) {
28             ($day,$hi,$lo,$close) = split('\s',$_);
29             $sum += $close;
30             $i++;
31             if ($hi > $highest) { $highest = $hi; }
32             if ($lo < $lowest) { $lowest = $lo; }
33             write if ($opt_v);
34         }
35         close SDATA;
36         printf  "\n Summay of %d readings on $search", $i;
37         printf "\n Lowest  On File = %f ", $lowest;
38         printf "\n Highest On File = %f ", $highest;
39         printf "\n Average Price   = %f ", $sum / $i;
40         printf "\n";
41     }
42 } else {
43     printf "\n Sorry, I do not track this symbol: $symbol";
44 }
45
46 dbmclose(%stocks);
47 #
48 # Define a clean output format for displaying data
49 #.
50 format STDOUT =
51 @<<<<<<  @####.### @####.### @####.###
52 $day, $hi, $lo, $close
53 .

You print the stocks you are tracking by checking to see if the name exists in the %stocks{} hash. This is done in line 23 with the following statement:

if ($stocks{$search}) {

Once the file is found to exist with the -e flag in line 24, it is opened and read from in lines 27 to 35. The summary is printed in lines 36 through 40. In case the symbol is not listed in the database, an error message is printed in the else clause in line 43. The format for the output via the write statement in line 33 is defined in lines 50 to 53.

In this example, the historical data for a stock is stored in a file called AMAT. Therefore, to invoke the script, use this command:

$ test.pl -s AMAT

Summay of 305 readings on AMAT
Lowest  On File = 18.500000
Highest On File = 59.875000
Average Price   = 38.879393

As you can see, the output from this script is made a bit more presentable with the use of the format statement.

Using DBM in Modules

Because it's possible to use the DBM files from within modules, let's see if the Invest.pm module (covered in Chapter 5, "Object-Oriented Programming in Perl") can be updated to include saving and restoring portfolio information on disk. You will be appending some functions to the original Invest.pm. All the new functions will go at the end of the file unless specified
otherwise.

Some changes need to be made to the Invest.pm module to get it to work with the DBM files. The first change is to include the following statement before the first executable line in the Invest.pm file:

require AnyDBM_File;

The @portfolio array is changed to %portfolio because this array can be mapped directly to disk via a DBM file. The @portfolio array contains references to hashes and not the content of the item in the referenced hash. Therefore, a new scheme has to be incorporated in order to parse the values and then store them to disk. After the values are stored to disk, the reverse operation has to be applied to read them back. Because this sample portfolio is not a large database, the important values can be stored using a colon delimited array. If this example were a very large array, the items could be stored with the pack() function and thus store only binary values.

A new function has to be created to save the contents of the portfolio to disk. Add the following function to the end of the Invest.pm file on or about line 116:

savePortfolio {
my ($this, $filename) = @_;
my %dummy;
my $a;
my ($key, $val);
dbmopen(%dummy,$filename,0666);
while (($key,$val) = each(%portfolio)) {

    $a = "$key:" . $val->{'type'} . ":" .
    $val->{'symbol'} . ":" . $val->{'shares'};
    $dummy{$key} = $a;
    # Debug: print "\n Writing $dummy{$key}";
    }
dbmclose(%dummy);
}

The %dummy hash is used to map to the disk file. Each item in the portfolio hash is parsed for storage in the %dummy hash. The value of $key goes from 0, 1, 2, and up. One string is saved to disk per hash item. Here's the format of the string:

$key:$type:$symbol:$shares

$type can be Stock or Fund. $symbol is the stock or fund symbol and $shares is the number of shares of the stock. Keep in mind that this is only an example-the real data stored would probably have to include date of purchase, purchase price, and so on. In this event, the fields could be appended to the string with colons separating each field. If you want to add purchase price, your field would look like this:

$key:$type:$symbol:$shares:$purchased

To restore the file back from disk, the Invest.pm file will have to read back the same portfolio file and restore all the values in the portfolio array. The function will also have to recognize the difference between the types of items it has to re-create. The restoration is done by this function which is added to the end of the Invest.pm file at about line 132:

sub restorePortfolio {
my ($this, $filename) = @_;
my %dummy;
my ($key, $val);
my ($ndx,$sec,$sym,$shr);
my $a;
local $i1;
dbmopen(%dummy,$filename,0666);
while (($key,$val) = each(%dummy)) {
    $a = $dummy{$key};
    ($ndx,$sec,$sym,$shr) = split(':',$a);
    # print "Read back $ndx,$sec,$sym,$shr \n";
    if ($sec eq 'Fund')
        {
        $i1 = Invest::Fund::new('Invest::Fund',
                'symbol' => "$sym", 'shares' => "$shr");
        }
    else
        {
        $i1 = Invest::Stock::new('Invest::Stock',
                'symbol' => "$sym", 'shares' =>"$shr");
        }
         $this->Invest::AddItem($i1);
         $this->Invest::showPortfolio;
    }
dbmclose(%dummy);
}

To create the sample portfolio, use the shell script shown in Listing 18.6. You might want to edit the code shown in Listing 18.6 to print the reported information in a different format than the one shown here. In this sample script, two stocks are added and then the contents of the portfolio are printed.


Listing 18.6. Creating a sample portfolio.
 1 #!/usr/bin/perl
 2
 3 push(@Inc,'pwd');
 4
 5 use Invest;
 6 # use Invest::Fund;
 7 use Invest::Stock;
 8
 9 #
10 # Create a new portfolio object
11 #
12 $port = new Invest;
13
14 print "\n -- CREATE PORTFOLIO --";
15 $s1 = new Invest::Stock('symbol' => 'AMAT', 'shares' => '400');
16 $s2 = new Invest::Stock('symbol' => 'INTC', 'shares' => '200');
17
18 print "\n Adding stocks ";
19 $port->Invest::AddItem($s1);
20 $port->Invest::AddItem($s2);
21
22 $port->Invest::showPortfolio();
23
24 #
25 #   SAVE THE DATA HERE in the DBM file called myStocks
26 #
27 $port->Invest::savePortfolio("myStocks");

To view the contents of the portfolio, you'll have to write another simple script. This script is shown is Listing 18.7. The file to recover data from is called myStocks. You should be able to see this file in your directory.


Listing 18.7. Listing the portfolio.
 1 #!/usr/bin/perl
 2
 3 push(@Inc,'pwd');
 4
 5 use Invest;
 6 use Invest::Fund;
 7 use Invest::Stock;
 8
 9 $port = new Invest;
10
11 print "\n -- LIST PORTFOLIO --";
12
13 $port->Invest::restorePortfolio("myStocks");
14 $port->Invest::showPortfolio();

Multiple DBM Files

There are occasions when you'll want to have more than one file open for DBM access. You can use a unique stock ticker symbol as the index into several hashes, each of which is then mapped to its own DBM file. For example, the following database code could be used to track stock price information in one DBM file and earnings information in another DBM file. Both DBM files will be indexed via the stock symbol name. Results of analyzing data from both DBM files could be printed using code similar to the following snippet:

foreach $symbol (@listOfsymbols) {
    dbmopen(%earnings,$symbol,0666);
    dbmopen(%prices,$symbol,0666);

    @results = analyzeEarnings(\%earnings,\%prices);
    printResults(\@results);

    dbmclose(%prices);
    dbmclose(%earnings);
    }

The Catch with DBM Utilities

So far I have only covered the standard DBM utilities that come with Perl distribution. For most casual users, these DBM files will be sufficient for their database needs. Unfortunately, when things get complicated, as in the case of relational databases, you might want to reconsider your options with other database solutions. The price tag for DBM utilities is attractive because they're free. However, you just might want to pay someone to acquire a commercial Relational Database Management System (RDBMS).

Second, there is an inherent danger in using DBM utilities that I must warn you about. If you make a mistake in working with your mapped hash and somehow write it to disk with a dbmclose(), guess what? You just wiped out your entire database. This type of faux pas is not hard to do, especially if you are modifying data. Obliteration of your DBM database is generally only recoverable from backup.

Commercial databases have a "safer" feel because they provide you with a safety net by keeping alternate backups. You are still your own worst enemy, but it's a little bit harder to destroy all data. In any event, always back up your data.

If you do decide to use a database management system (DBMS) other than the DBM utilities, all is not lost. You can use RDB (a freeware relational database) or other Perl front-ends to popular databases. All of the front packages allow your Perl programs to talk to different databases via the protocol used by a DBI package.

Listing 18.8 presents the final version of the Invest.pm file after all the modifications discussed up to now in this chapter have been made to it.


Listing 18.8. The final version of Invest.pm with DBM support.
  1
  2 package Invest;
  3
  4 push (@Inc,'pwd');
  5 require Exporter;
  6 require Invest::Stock;
  7 require Invest::Fund;
  8 require AnyDBM_File;
  9 @ISA = (Exporter);
 10
 11 =head1 NAME
 12
 13 Invest - Sample module to simulate Bond behaviour
 14
 15 =head1 SYNOPSIS
 16
 17     use Invest;
 18     use Invest::Fund;
 19     use Invest::Stock;
 20
 21     $port = new Invest::new();
 22
 23     $i1 = Invest::Fund('symbol' => 'twcux');    
 24     $i2 = Invest::Stock('symbol' => 'INTC');    
 25     $i3 = Invest::Stock('symbol' => 'MSFT', 'shares' => '10');    
 26
 27      $port->Invest::AddItem($i1);
 28      $port->Invest::AddItem($i2);
 29      $port->Invest::AddItem($i3);
 30
 31      $port->showPortfolio();
 32      $port->savePortfolio("myStocks");
 33
 34 =head1 DESCRIPTION
 35
 36 This module provides a short example of generating a letter for a
 37 friendly neighborbood loan shark.
 38
 39 The code begins after the "cut" statement.
 40 =cut
 41
 42 @EXPORT = qw( new, AddItem, showPortfolio, savePortfolio,
 43         reportPortfolio,
 44         restorePortfolio, PrintMe);
 45
 46 my %portfolio = {};
 47 my $portIndex = 0;
 48
 49 sub Invest::new {
 50         my $this = shift;
 51         my $class = ref($this) || $this;
 52         my $self = {};
 53         bless $self, $class;
 54     $portIndex = 0;
 55     # printf "\n Start portfolio";
 56         return $self;
 57 }
 58
 59 sub Invest::AddItem {
 60     my ($type,$stock) = @_;
 61     $portfolio{$portIndex} = $stock;
 62     # print "\nAdded ".  $stock->{'shares'} . " shares of " . $stock->{'symbol'};
 63     $portIndex++;
 64 }
 65
 66 sub Invest::showPortfolio  {
 67     printf "\n Our Portfolio is:";
 68     my ($key, $i);
 69     while (($key,$i) = each(%portfolio)) {
 70         print "\n ".  $i->{'shares'} . " shares of " . $i->{'symbol'};
 71     }
 72     print "\n";
 73 }
 74
 75 sub Invest::reportPortfolio {
 76     my $hdrfmt = $~;
 77     my $topfmt = $^;
 78     my $pageCt = $=;
 79     my $lineCt = $-;
 80     my $sym;
 81     my $shr;
 82     my ($key, $i);
 83
 84     $~ = "PORT_RPT";
 85     $^ = "PORT_RPT_TOP";
 86
 87 format PORT_RPT_TOP =
 88
 89     Report
 90 STOCK     SHARES
 91 =====   ======
 92 .
 93
 94 format PORT_RPT =
 95 @<<<<   @<<<<
 96 $sym, $shr
 97 .
 98     # note how the code is intermingled with the format!
 99     while (($key,$i) = each(%portfolio)) {
100         $shr = $i->{'shares'};
101         $sym = $i->{'symbol'};
102         write ;
103
104     }
105
106     $= = $pageCt;
107     $- = $lineCt;
108     $~ = $hdrfmt;
109     $^ = $topfmt;
110 }
111
112 sub PrintMe {
113     my $this = shift;
114     print "\n Class : $$this";    
115 }
116
117 sub savePortfolio {
118     my ($this, $filename) = @_;
119     my %dummy;    
120     my $a;
121     my ($key, $val);
122     dbmopen(%dummy,$filename,0666);
123     while (($key,$val) = each(%portfolio)) {
124         $a = "$key:" . $val->{'type'} . ":" . $val->{'symbol'} . ":" . $val->{'shares'};
125         print "\n Writing $key $a";
126         $dummy{$key} = $a;
127         print "\n Writing $dummy{$key}";
128         }
129     dbmclose(%dummy);
130 }
131
132 sub restorePortfolio {
133     my ($this, $filename) = @_;
134     my %dummy;    
135     my ($key, $val);
136     my ($ndx,$sec,$sym,$shr);
137     my $a;
138     local $i1;
139     dbmopen(%dummy,$filename,0666);
140     while (($key,$val) = each(%dummy)) {
141         $a = $dummy{$key};
142         ($ndx,$sec,$sym,$shr) = split(':',$a);
143         # print "Read back $ndx,$sec,$sym,$shr \n";
144         if ($sec eq 'Fund')
145             {
146             $i1 = Invest::Fund::new('Invest::Fund',
147                     'symbol' => "$sym", 'shares' => "$shr");
148             }
149         else
150             {
151             $i1 = Invest::Stock::new('Invest::Stock',
152                     'symbol' => "$sym", 'shares' =>"$shr");
153             }
154              $this->Invest::AddItem($i1);
155              $this->Invest::showPortfolio;
156         }
157     dbmclose(%dummy);
158 }
159
160 1;

Line 8 is where the require statement is used to implement the AnyDBM_FILE support. Lines 17 through 19 use other packages as well.

What Is the DBI Package?

The database interface (DBI) package for Perl is the implementation of the DBI Application Program Interface (API) specification written by Tim Bunce (Tim.Bunce@ig.co.uk). The DBI package API is designed specifically for use with Perl. The set of functions and variables in the DBI package provide a consistent interface to the application using it. The strong point of the DBI package API, in addition to its broad set of available functions, is that it completely isolates the using application from the internal implementation of the underlying database.

The DBI specification exists at various sites in the CPAN archives, but the latest version (v0.6) is not up to date. The best source of information is to look in the source files for a DBI package itself. The entire specification is good for getting an idea of how everything is intended to work together. However, the interface has changed considerably since the specification was released. Check out the file dbispec.v06 in compressed form at ftp.demon.co.uk in the /pub/perl/db directory.

The DBI specification started out as DBperl back in 1992 as a team effort from several Perl enthusiasts. Here are the initial contributors to the specification for each type of database:

infoperl (Informix)Kurt Andersen (kurt@hpsdid.sdd.hp.com)
interperl (Interbase)Buzz Moschetti (buzz@fsrg.bear.com)
oraperl (Oracle)Kevin Stock (kstock@encore.com)
sybperl (Sybperl)Michael Peppler (mpeppler@itf.ch)
sqlperl/ingperl (Ingres)Ted Mellon (dbi-users@fugue.com) and Tim Bunce

The original DBI specification was edited by Kurt Anderson. In 1994, Tim Bunce took over the editing and maintenance of the specification in addition to the DBI and DBD::Oracle package development. The specification and related files are copyrighted by Tim Bunce.

The original specification was edited by Kurt Anderson from the discussions on the mailing list. In 1993, Tim Bunce took over the editing and maintenance of the specification and in 1994 started the development of the DBI and DBD::Oracle modules. The DBI specification and modules are copyrighted by Tim Bunce but are freely available to all with the same terms as Perl. (Tim is the technical director of the software systems house, Paul Ingram Group in Surrey, England. Tim can be reached at Tim.Bunce@ig.co.uk, but DBI related mail should be sent to the dbi-users@fugue.com mailing list.)

The DBI is not related to any one specific database because it serves as an intermediary between a program and one or more DBD::* driver modules. DBD:: modules are drivers written to support a specific database back-end. The DBI:: module manages all installed DBD:: drivers in your system. You can load and use more than one DBD:: module at the same time.

DBD:: modules are written in such a way that they may be copied and customized to suit your specific needs. For example, the DBD::Oracle module served as the starting point for Alligator Descartes, another well-known pioneer in developing database interfaces for Perl, to develop DBD:: modules for other databases. He has written two copyrighted documents on how to develop your own driver from DBD:: modules. These documents are located on the Web site www.hermetica.com in the technologia/DBI directory.

Available Packages

Some of DBI:: packages available on the Internet are listed here; you can get the latest versions of these files from the Internet CPAN sites:

  • DBD-Oracle-0.29.tar.gz for Oracle users
  • DBD-Informix-0.20pl0.tar.gz for Informix database users
  • DBD-QBase-0.03.tar.gz for Quickbase users
  • DBD-mSQL-0.60pl9.tar.gz for mSQL-based databases
  • DBI-0.67.tar.gz for the DBI interface

The interface packages are simply front-ends to the database engine that you must have installed on your machine. For example, in order to use the Oracle DBI package, you'll need the Oracle database engine installed on your system. The installation instructions are located in the README files in the packages themselves. You'll need to have Perl 5.002 installed on your system to get some of the packages to work, especially the DBI module.

The DBI interface is very different than the old, database-specific interfaces provided by oraperl, ingperl, interperl, and so on. To simplify the transition to Perl 5, some DBD:: drivers, such as DBD::Oracle, come with an extra module that emulates the old-style interface. The DBI interface has never been fully defined because it has been constantly evolving. This evolution will take a large step forward with the adoption of the standard ODBC interface as the core of the DBI. Because this redefinition of the DBI interface standard is bound to change the DBI interface, Tim Bunce recommends using stable emulation interfaces, such as oraperl, instead.

RDB

The RDB database utilities for Perl deserve an honorable mention. The RDB package is complete, simple to use, and very easy to set up. The author of this package is Walt Hobbs; he can be reached at hobbs@rand.org.

The source and documentation is found in the file RDB-2.5k.tar.Z in the CPAN modules directories. There is a file named Info.RDB in the package that provides a short overview of how the RDB package organizes its data and what commands are available for you to use. The Info.RDB file also lists the origins of the RDB package, describes how to use it, and provides a sample data file.

The operators in the RDB packages are Perl scripts that use standard I/O for UNIX to operate on ASCII files. The format for databases in the RDBM package is to store data in rows and columns in text files. Each row contains items separated by tabs, and each row is terminated by a newline character. (The field separator is a tab, and the record separator, therefore, is the newline character.) Each column in the text file has the items' names and format defined as the first two rows.

Because the data is stored in such a format, it's easier to access the data using programs other than those supplied with the RDB package. You can use Perl or awk scripts to get what you want if the RDM programs do not give you what you need. The RDB operators are only Perl scripts that you can use as a basis for writing your own extensions.

The operators on the package include listing by row, listing by column, merging tables, and printing reports. All operators in the RDB package read from standard input and write to standard output.

Consider the following sample data file. There are four columns in the data file. Each column has a heading and type of data associated with it. The comments with # in the front of the line are ignored. The first uncommented row contains the name of fields per column. The row immediately after that stores the type of data. 4N means that LINE and WORD are four-digit wide numbers. (S specifies a string and M stands for month.) A digit by itself is a string; therefore, NAME fields are eight characters wide.

#
# Sample data file for Chapter 18
#
LINE    WORD    BYTE    NAME
4N    4N    5N    8
1128    6300    37140    TS03.dat
 644    3966    24462    TS04.dat
1175    6573    40280    TS05.dat
 968    6042    38088    TS13.dat
 687    3972    24383    TS14.dat
 741    4653    28100    TS16.dat
1621    8804    58396    TS17.dat
1061    6086    39001    TS20.dat
1107    4782    29440    TS21.dat
 846    5839    37442    TS22.dat
1758    8521    54235    TS23.dat
 836    4856    30916    TS24.dat
1084    5742    34816    TS27.dat

The commands to operate on are relatively simple. To sort the table by LINE numbers, you use this command on the test.data file:

sorttbl < test.data LINE > out

The resulting output in the out file is as follows. The format for the output file is an RDB file itself. You can run other RDB operators on it, too!

#
# Sample data file for Chapter 18
#
LINE    WORD    BYTE    NAME
4N    4N    5N    8
 644    3966    24462    TS04.dat
 687    3972    24383    TS14.dat
 741    4653    28100    TS16.dat
 836    4856    30916    TS24.dat
 846    5839    37442    TS22.dat
 968    6042    38088    TS13.dat
1061    6086    39001    TS20.dat
1084    5742    34816    TS27.dat
1107    4782    29440    TS21.dat
1128    6300    37140    TS03.dat
1175    6573    40280    TS05.dat
1621    8804    58396    TS17.dat
1758    8521    54235    TS23.dat

You can get summary information about columns with the summ command. For example, to get summary information for the test.data file for averages, use this command:

summ -m < test.data

Here's the resulting output:

Rows: 13
Min, Avg, Max, Total for LINE: 644, 1050, 1758, 13656
Min, Avg, Max, Total for WORD: 3966, 5856, 8804, 76136
Min, Avg, Max, Total for BYTE: 24383, 36669, 58396, 476699
Min, Avg, Max, Total for NAME: 0, 0, 0, 0

You can join two tables together on a per-column basis to get a merged table. The command to do this is jointbl. The -c option does the merge for you on a per-column basis. Consider the two files p1.dat and p2.dat with a common column of NAME in each file. The merged output is shown with this command:

jointbl -c < p1.dat NAME p2.dat

The use of the jointbl command is shown in the following input/output example:

$ cat p1.dat

$
$ cat p2.dat
#
# P2
#
LINE    NAME
4N    8
1128    TS03.dat
 644    TS04.dat
1175    TS05.dat
 968    TS13.dat
 687    TS14.dat
 741    TS16.dat
1621    TS17.dat
1061    TS20.dat
1107    TS21.dat
 846    TS22.dat
1758    TS23.dat
 836    TS24.dat
1084    TS27.dat
$
$ jointbl -c < p1.dat NAME p2.dat
#
# P1
#
#
# P2
#
NAME    BYTE    LINE
8    5N    4N
TS03.dat    37140    1128
TS04.dat    24462     644
TS05.dat    40280    1175
TS13.dat    38088     968
TS14.dat    24383     687
TS16.dat    28100     741
TS17.dat    58396    1621
TS20.dat    39001    1061
TS21.dat    29440    1107
TS22.dat    37442     846
TS23.dat    54235    1758
TS24.dat    30916     836
TS27.dat    34816    1084

Other more powerful features of this RDB package are listed in the RDB.ps PostScript file. It prints out to a 72-page manual with examples and details on all of the commands available for you. If you do not feel like killing a tree, you can use ghostview to view the file.

Summary

Perl supplies a flat database package in the DBM utilities. The modules allow Perl scripts to map hashes to disk files for storage. For most users, storing data with DBM utilities is sufficient. Some DBD:: and DBI:: modules are available as front-ends to commercial databases such as Oracle, Sybase, Informix, and Quickbase. You need the specific database and database engine installed on your machine in order for the DBD:: code to work. The RDB package provides a text file-based relational database management system. Utilities in the RDB package are a set of Perl programs that operate on rows and columns of data files.

Chapter 20

Introduction to Web Pages and CGI


CONTENTS


This chapter offers a brief introduction to the HyperText Markup Language (HTML) and the Common Gateway Interface (CGI). The information in this chapter provides the basis for the rest of the chapters about Web pages in this book, especially for the topic of writing CGI scripts in Perl. This chapter assumes that you have a cursory knowledge of what the World Wide Web (WWW) is about and how to use a browser.

I also assume you're somewhat familiar with HTML code. Going into more detail about HTML programming would cause us to move too far away from the scope of the book: Perl programming. Therefore, I stick to the very basic HTML elements for text formatting and listing items rather than covering a lot of HTML programming issues.

Reading this one chapter won't make you a Webmaster, but you'll learn enough to create Web pages you can subsequently use in conjunction with Perl scripts. With these basics, you'll be able to easily incorporate other HTML page-layout elements in your documents.

If you are not familiar with HTML or would like more information, don't worry. There are several documents on the Internet that describe how to write HTML pages. For up-to-date documentation on HTML, conduct a search on the keywords HTMLPrimer and html-primer in the Internet search areas.

For more information via printed text books, you might want to consult these titles:

  • Teach Yourself Web Publishing with HTML 3.0 in a Week, Laura Lemay, Sams.net Publishing, 1-57521-064-9, 1996.
  • HTML & CGI Unleashed, John December and Mark Ginsberg, Sams.net Publishing, 0-672-30745-6, 1995.
  • Using HTML, Neil Randall, Que, 0-7897-0622-9, 1995.

HTML, CGI, and MIME

HTML is the de facto standard language for writing Web pages on the Internet. HTML documents are written as text files and are meant to be interpreted by a Web browser. A Web browser displays the data in HTML pages by reading in the tags around the data. Web browsers reside on client machines, and Web server daemons run on Web servers. The protocol used by Web servers and clients to talk to each other is called the HyperText Transfer Protocol (HTTP).

An HTML page contains uniform resource locators (URLs) in addition to the tags. A URL tells the browser where to get certain data. URLs can point to other Web documents, FTP sites, Gopher sites, and even executable programs on the server side. The Common Gateway Interface (CGI) is the standard used to run programs for a client on the server.

A Web server gets a request for action from the browser when the user selects the URL. The request is processed by the server by running a program. The program is often referred to as a CGI script because a lot of programs for handling CGI requests are Perl scripts. The results of the CGI script are sent back to the browser making the request. The browser displays the results back to the user. Results can be in plain text, binary data, or HTML documents.

The browser reading the output from the CGI script has to know the type of input it is receiving. The type of information is sent back as a multipurpose Internet mail-extension (MIME) header. For example, to send back plain text, you use "Content-Type: text/plain\n\n" at the start of the document. To send back HTML data, you use "Content-type: text/html\n\n".

Note
Using two carriage returns after the type of data is very important. The HTML standard requires a blank line after the Content-type string. This is why we have "\n\n" appended to Content-type. In most cases, the "\n\n" will work as intended to produce a blank line for a browser. Sometimes this will not work, and the data being sent back to the browser will not be shown because the server will be handling carriage-returns/line-feeds using the "\r\n" string instead of "\n". To allow for inconsistencies in the way operating systems handle carriage-return/line-feed pairs, you should use the string "\r\n\r\n".

A Simple HTML Document

An HTML document uses markup tags to specify special areas of the text. The format of an HTML document is as follows:

<HTML>
<HEAD>
<TITLE>Title of the page</TITLE>
</HEAD>
<BODY>
           The body of the document.
</BODY>
</HTML>

All text for the HTML document is shown between the <HTML> and </HTML> tags. There can be only two pairs of elements, one pair of <BODY> and </BODY> tags to store the text matter for the HTML document, and the other pair of <HEAD> and </HEAD> tags. The <HEAD> and </HEAD> tags show the document title in the heading section of a viewer. The <TITLE> and </TITLE> tags hold the string for text in the title bar for your browser and are the only required element within the <HEAD> and </HEAD> tags.

Both the <HEAD> and <TITLE> tags are optional. However, for compatibility with some browsers, you should include them. The <BODY> and </BODY> tags are required in all cases. Most HTML tags are paired. So if you have <HEAD>, then you should have </HEAD>. There are exceptions to this rule. For example, the paragraph tag <P> and the line break <BR> tag are used by themselves and do not require any accompanying </P> or </BR> tags. (The </P> tag is sometimes used to terminate a paragraph, but the </BR> tag does not exist.)

Tags are not case sensitive, and any formatting in between the tags is almost always ignored. Therefore, the tag <html> is the same as <HtMl> and <HTML>.

It's the presence of <HTML>, <HEAD>, and <BODY> tags in the page that distinguishes an HTML page from a simple text page. Figure 20.1 presents a sample text page which does not have any formatting on it whatsoever being loaded into an HTML browser.

Figure 20.1: An unformatted document.

All the text shown in Figure 20.1 is aligned in the way that the original text document was set up. In some cases, the text would have been clumped in one long paragraph. Here is the text for the document shown in Figure 20.1:

Futures Contracts in Sugar

Test Test HTML Test HTML

Summary of Contracts available.

[Image] Sugar Contracts
[Image] Sugar Options
[Image] Combination
----------------------------------------------------------------------------

Ordered list of particulars

* Price per cent move of Sugar prices: $1120.00
* Appox min. deposit for contract required by broker: $5000 to $10000.
* Appox min. deposit for option required by broker: $1500 to $3000.
* Appox commissions cost: $35 to $75

----------------------------------------------------------------------------

Some Detailed Information in Description Lists.

[Image] Risks with open contracts
     One cent move equals $1120 in your profits. Therefore a 4 cent move can
     either make you a handsome profit or break your bank. A flood in sugar
     growing area may cause prices to drop sharply. If you are holding a
     long contract, this drop in price will have to be covered at the end of
     the trading day or your position will be liquidated.
[Image] Sugar
     Options cost a fixed amount of money. However, the money spent on an
     option should be treated like insurance. No matter where the price goes
     your loss will be limited to the price of the option. Of course, with
     limiting risk you are also limiting profits.

To make the text more presentable, you can add some HTML tags to the document, as shown in Listing 20.1. First, we'll delimit the paragraphs with a <P> tag and then add some headings to it. HTML provides six levels of headings, numbered H1 through H6. H1 is the top-level heading in a document's hierarchy and H6 is the bottom. Generally, you use H2 headers inside H1 headers, H3 headers inside H2 headers, and so on. Do not skip heading levels unless you have a compelling reason to switch heading styles. Use the tags <H1>Text for heading</H1> for defining a heading.

A sample HTML page is shown in Listing 20.1. See the output in Figure 20.2.

Figure 20.2:Using tags to enhance the appearance of HTML documents.


Listing 20.1. Formatted text.
 1 <HTML>
 2 <HEAD><TITLE>Using Lists</TITLE>
 3 </HEAD>
 4 <BODY><P>
 5 <P>
 6 <H1>Futures Contracts in Sugar</H1>
 7 <P>
 8 <A HREF="../cgi-bin/testing.pl"> Test</A>
 9 <A HREF="query.pl"> Test HTML</A>
10 <A HREF="../cgi-bin/query.pl"> Test HTML</A>
11 <H2>Summary of Contracts available.</H2>
12 <IMG SRC="red-ball-small.gif"> <B>Sugar Contracts</B><BR>
13 <IMG SRC="red-ball-small.gif"> <B>Sugar Options</B><BR>
14 <IMG SRC="red-ball-small.gif"> <B>Combination</B><BR>
15 <HR>
16 <H2> Ordered list of particulars </H2>
17 <UL>  
18 <LI> Price per cent move of Sugar  prices: $1120.00
19 <LI> Appox min. deposit for contract required by broker: $5000 to $10000.
20 <LI> Appox min. deposit for option required by broker: $1500 to $3000.
21 <LI> Appox commissions cost: $35 to $75
22</UL>
23 <P>
24 <H2> Some Detailed Information in Description Lists.  </H2>
25 <P>
26 <DL>
27 <DT><IMG SRC="yellow-ball.gif"> <B>Risks with open contracts </B>
28 <DD> One cent move equals $1120 in your profits. Therefore a 4 cent move
29 can either make you a handsome profit or break your bank. A flood in sugar
30 growing area may cause prices to drop sharply. If you are holding a long
31 contract, this drop in price will have to be covered at the end of the
32 trading day or your position will be liquidated.
33 <DT><IMG SRC="yellow-ball.gif"> <B> Sugar </B>
34 <DD> Options cost a fixed amount of money. However, the money spent on
35 an option should be treated like insurance. No matter where the price
36 goes your loss will be limited to the price of the option. Of course,
37 with limiting risk you are also limiting profits.
38 <DT><IMG SRC="yellow-ball.gif"> <B> Combination of both <B>
39 </BODY></HTML>

Your browser will show a different font for each header style. In this listing, we used two heading levels, <H1> and <H2>, lines 5 and 8. The required <TITLE> is also shown and terminated with the </TITLE> keyword in line 1. The <P> tag is used to start a new paragraph in lines 4, 5, and 7, for example. If the <P> tags were not used, the text would follow in one big clump unless you were beginning a new header or a horizontal line with the <HR> tag. See Line 15 where the <HR> tag is used to draw a horizontal line.

Look at lines 12 through 14. Using <BR> tags, I have created the list of three items using IMG tags.

Note how images are stored and displayed within the document with the <IMG> tag. Basically, the IMG tag specifies where an image will be displayed in the HTML document. The SRC attribute of the IMG tag specifies the URL for the contents of the image. The URL can be a complete (absolute) pathname or a relative pathname. An absolute path would describe the entire URL: http://www.ikra.com/mydirectory/red-ball-small.gif, whereas the relative pathname will assume the default directory for images on the server and pick the image called red-ball-small.gif from that directory. By using image files, you can enhance the appearance of items in a list and provide your own bullets. Consider this reference in line 12:

<IMG SRC="red-ball-small.gif"> <B>Sugar Contracts</B><BR>

The file red-ball-small.gif is shown next to the text Sugar Contracts using the relative URL scheme.

There are several more attributes to an IMG tag allowing you to align text around the image, size the image, provide a mapping of URLs to go to if the user clicks on a section of the image, and so on. Be sure to check the references provided in the "HTML, CGI, and MIME" section for more information on how to use IMG tags.

Formatting Lists

HTML supports the following types of list formats with which you can show information:

  • Unnumbered lists (items in <UL> and </UL> tags) where the items in a list are not numbered.
  • Numbered lists (items in <OL> and </OL> tags) where each item in a list is numbered.
  • Definition lists (items in <DT> and <DD> tags) where each item in the list is a term followed by a descriptive paragraph that applies to that term.

Items in a list are separated by <LI> tags. The </LI> tag is not needed to stop one item and start another. In Listing 20.1, lines 17 to 22 provide an unnumbered list with the use of <UL> and
</UL> tags. To make this a numbered list, simply change the <UL> and </UL> pair to an <OL> and </OL> tag, respectively, and all the items in the list will be numbered.

A definition list has two items: a <DT> tag with a term and the definition of the term with <DD>. The <LI> and <DD> items can contain multiple paragraphs with the use of the <P> tag. Lines 27 through 38 in Listing 20.1 show how to create a simple definitions list.

You can even nest lists within lists. Just remember to end each list completely within the list item of the parent list. See the sample HTML code in Listing 20.2.


Listing 20.2. Nested lists.
 1 <HTML><HEAD><TITLE>Another Lists Example</TITLE></HEAD>
 2 <BODY>
 3 <P><H1>This shows Numbered Lists</H1><P>
 4 Start of List here.
 5 <OL>
 6 <LI> Toulouse
 7      <UL>
 8      <LI> To Barritz
 9      <LI> To Bordeaux
10      <LI> To Marseille
11      <LI> To Paris
12      </UL>
13 <LI> Paris
14      <UL>
15      <LI> To Dijon
16      <LI> To Rennes
17      <LI> To Toulouse
18      </UL>
19 <LI> Rennes
20      <UL>
21      <LI> To Paris
22      <LI> To Cherbourg
23      </UL>
24 <LI> Nice
25      <UL>
26      <LI> To Digne
27      <LI> To Menton
28      </UL>
29 </OL>
30 </BODY>
31 </HTML>

The top level numbered list is started at line 5 and ends in line 29. Each item of the numbered list is inserted in lines 6, 13, 19, and 24. Nested lists are shown as unnumbered lists enclosed in <UL> and </UL> tags in lines 8 to 12, 14 to 18, 20 to 23, and 25 to 28. Each <UL> and </UL> pair is terminated in the space for each item in the top level numbered list.

Using HTML Tags

The basic set of HTML tags such as the <HEAD>, <BODY>, and <TITLE> are almost always supported by all browsers. However, you should keep in mind that not all tags are supported by all Web browsers. If a browser does not support a tag, it just ignores it.

You can emphasize text in the document so that it stands out when displayed by a browser. For example, to underline text, use <U> and </U> to enclose the text. For bold text, enclose the text in <B> and </B> tags. For italics, use <I> and </I>. See the sample HTML code in Listing 20.3.


Listing 20.3. Using HTML tags to change font style.
 1 <HTML>
 2 <HEAD>
 3 <TITLE> IISTV.htmL </TITLE>
 4 </HEAD>
 5 <BODY>
 6 <Center><H1>Time Change for Islam TV Program</H1></Center>
 7 <HR>
 8 <P>
 9 From January 1996, the Islamic Information Service TV (IISTV)
10 Program, will be broadcast <b>every Sunday at 9.00 A.M.
11 on Channel 67 (KHSH TV)</b> in the <U>Houston Metropolitan Area</U>.
12 Islam TV is regularly watched in 5000 homes and is a totally
13 viewer supported program and we need your support.<P>
14 The program is also carried on the <EM>International Cable channel</EM>
15 <STRONG>every Saturday at 9:30 AM.</STRONG>
16 <P>
17 For more information or to send contributions, please contact<P>
18 <hr>
19 <CENTER>
20 <STRONG>
21                              ISGH/IIS-TV <BR>
22 </STRONG>
23                     9336-B Katy Freeway Suite 314<BR>
24                         Houston, Texas 77024. <BR>
25                              713 827 1827<BR>
26 </CENTER>
27 <hr>
28 </BODY>
29 </HTML>

Line 6 defines a centered line of text with the <Center> and </Center> tags. Note how the <H1> and </H1> tags are enclosed along with the text being centered. Line 7 draws a horizontal line. (Remember that HR stands for Horizontal Rule.) The <b> and </b> tags are used in lines 10 and 11 to embolden some text. Since HTML is case insensitive, we could have used the tags <B> and </B> with no loss of functionality. Use of the <EM> and <STRONG> tags is shown in lines 14 and 15, respectively.

Note that the effect of <B> remains in effect until the </B> is seen. So the text being set to a style can be typed in across several lines. In lines 19 through 26, center several lines.

In HTML documents you should use <STRONG> and <EM> tags instead of <B> and <I> tags, respectively. There are occasions where using even seemingly innocent tags such as <B> for bolding text or <I> for italicizing text may not work. For example, if a browser is using a font that does not have an italics character set, the <I>text</I> will be printed with its codes; that is, it will be as <I>text</I> and not as text. To take care of some of these issues, should they arise, HTML authors prefer to use the <EM></EM> emphasis tags in place of the <I></I> tags and the <STRONG></STRONG> tags in place of the <B></B>.

An HTML document defines only the display style with the use of markup tags. How a Web browser chooses to display the information is left to the implementation at the Web browser. So the client's browser is responsible for rendering this text and may have a completely different font than what you are using to create the page. Therefore, what you see on your screen will be different from what someone else using a different font may see. For best results, try viewing your HTML page with different browsers. The font on someone else's browser might be completely different. For one thing, users can choose whatever font they desire for their browser. Even if users don't explicitly choose a font, the browser may default to a font that is different from the one you use. For example, one user may use the Internet Explorer from Microsoft with a 10-point font, and another user may use Netscape Navigator with a 12-point font. Each user will see different lengths of paragraphs and text layout.

Simple text formatting with the use of carriage returns in HTML documents does not work. The client browser will implement word wrapping when it sees text. You can force paragraph and line breaks with the use of <P> or <BR> tags. The <BR> tag creates a line break, and does not start a new paragraph like the <P> tag. A <P> tag may be used to lay out text differently using formats and text alignment on different types of browsers. A <BR> tag simply starts a new line without changing the current paragraph format. Not using these tags causes all white space (including spaces, tabs, and blank lines) to be collapsed into one white space. Listings 20.4 and 20.5 provide samples of two HTML files that produce the same output.


Listing 20.4. Headings in HTML files.
 1 <HTML><HEAD>
 2 <TITLE>A simple HTML file</TITLE>
 3 </HEAD>
 4 <BODY>
 5 <H1>This is Header Level 1</H1>
 6 <H2>This is Header Level 2</H2>
 7 <H3>This is Header Level 3</H3>
 8 <H4>This is Header Level 4</H4>
 9 <H5>This is Header Level 5</H5>
10 <H6>This is Header Level 6</H6>
11 This is line 1
12 This is line 2<P>
13 This is line 3<P>
14 This is a broken line with an &ltBR&gt<BR> sign.
15 <P>
16 End of File Here.
17 </BODY>
18 </HTML>


Listing 20.5. The second version of the HTML file shown in Listing 20.4.
 1 <HTML><HEAD><TITLE>A simple HTML file</TITLE>
 2 <HEAD>
 3 <BODY>
 4 <H1>This is Header Level 1</H1> <H2>This is Header Level 2</H2>
 5 <H3>This is Header Level 3</H3> <H4>This is Header Level 4</H4>
 6 <H5>This is Header Level 5</H5> <H6>This is Header Level 6</H6>
 7 This is line 1 This is line 2<P> This is line 3<P>
 8 This is a broken line with an &ltoBR&gt<BR> sign. <P> End of File Here.
 9 </BODY></HTML>

Figure 20.3 shows the output from both listings. Note how heading-level 1 and heading-level 2 are shown in the same font style in this figure. The example shown here uses Netscape as the browser. The results on your browser might be different because each browser displays HTML in the way it chooses. It's a bit like the contents of a box shifting during transport but the weight of the contents has not changed.

Figure 20.3: Using different heading levels.

For the sake of readability, it's best to place headings and paragraphs on separate blank lines along with <P> tags. You can also specify formatting options for paragraph alignment in the <P> tag, as illustrated in Listing 20.5. The format specified for <P> will continue until the next <P> tag. To terminate a particular format you can use the </P> tag.


Listing 20.6. Aligning paragraphs.
 1 <html>
 2 <body>
 3 <TITLE>Aligned Paragraphs</TITLE>
 4 <H1>This shows aligned paragraphs</H1>
 5 <P ALIGN=CENTER>
 6 This line is centered
 7 <P ALIGN=LEFT>
 8 This is aligned off the left margin
 9 <P ALIGN=RIGHT>
10 This is line aligned off the right margin
11 <P>
12 End of File Here.
13 </body>
14 </html>

The output from this listing is shown in Figure 20.4.

Figure 20.4: Using aligned paragraphs.

Preformatted Text

Having the browser format text for you does keep you from a lot of the hassle of tracking line breaks, paragraph formatting, and so on. However, when displaying text that is already laid out (such as source code), the default formatting can wreak havoc on your source listings. For this reason, HTML documents have the preformatted (<PRE>) tag. Using the <PRE> tag turns off the HTML formatting at the browser. Listing 20.8 contains an HTML file that illustrates using this tag. The output is shown in Figure 20.5.

Figure 20.5: Preformatted text.


Listing 20.7. Preformatted text.
 1 <html>
 2 <body>
 3 <TITLE>Yet Another Example</TITLE>
 4 <H1>This shows Raw vs. Preformatted Text</H1>
 5 <H3>Unformatted with the PRE tags </H3>
 6 <PRE>
 7 main(int argc, char *argv[])
 8 {
 9 int command;
10
11      command = get_response();
12      while (command != C_QUIT)
13           {
14           process_command(command);
15           command = get_response();
16           }
17
18 }
19 </PRE>
20 <HR>
21 <H3> Without the PRE tags </H3>
22 <P>
23 main(int argc, char *argv[])
24 {
25 int command;
26
27      command = get_response();
28      while (command != C_QUIT)
29           {
30           process_command(command);
31           command = get_response();
32           }
33
34 }
35
36 </body>
37 </html>

Special Characters in HTML Documents

By now you have seen that the greater than symbol and the less than symbol are used to encode documents. What if you wanted to include them in text that was not in preformatted tags? There are several special characters to encode these special symbols in HTML. In order to include them in a file, you have to enter their codes in the HTML file. Four of the most common special characters in HTML and their codes are

  • &lt; as the escape sequence for <.
  • &gt; as the escape sequence for >.
  • &amp; as the escape sequence for the ampersand (&).
  • &quot; the escape sequence for the double quote (").

So, to show this string in an HTML page, <HELLO>, you would use the text statement &lt;HELLO&gt; in your code. Note that the semicolon is required in each code.

What Is a URL?

URL stands for uniform resource locator. Basically, it's an extension of a file name to include the network address of a "thing" file and where that "thing" may be found. The "thing" your URL is pointing to may be a file, directory, database, image, newsgroup, archive, and so on, on a machine that is residing anywhere on the Internet. It's up to the browser to show that data to you.

A URL consists of three parts:

service://sitename[:port]/path/filename

The service part tells you which service you are trying to access. The sitename is the Internet address of the machine where the service is being sought. The port number is optional, because the default is port 80. The path/filename is the location of the file relative to the root directory for the server.

The services your browser can offer will vary. Here are the most common types of service you can use with a Web browser:

  • Http for perusing an HTML document
  • Gopher for starting a gopher session
  • Ftp for starting an ftp session
  • Telnet for starting a telnet session
  • File for getting a local file
  • News for a Usenet newsgroup

Here are some examples of URLs:

  • ftp://pop.ikra.com/pub/perl5/VRML.pm  This URL specifies a file to get via ftp from the /pub/perl5 directory.
  • http://www.ikra.com/iistv.html  This URL specifies an HTML document on the node www.ikra.com.
  • news://ikra.com/newsgroups/comp.dcom.telecom  This URL specifies a newsgroup to get on ikra.com.

To specify URL links to other documents, use the anchor tags, <A> and </A>. Anchor tags are used to provide links to other documents as well as provide a location in a document that another HTML document can refer to. Please check the references in this chapter if you are not familiar with using HTML anchors. Here's the format for using anchors for creating links to other documents:

<A HREF="URL">Text describing where the URL points to</A>

The HREF token specifies the URL to use. The text between the first ending > and the start of </A> is shown by the browser in a different color. The space between <A and HREF is required.

Here is a sample hypertext reference in an HTML document. For clarity, I refer to the document that specifies the hyperlink as the first document, and the location to which the URL points as the second document. Let's look at the URL:

<A HREF="http://pop.ikra.com/iistv.html">IISTV</A>

The browser showing this link will make the word IISTV show up in a different color than regular text and the link is underlined. Clicking on the word IISTV will cause the browser to get the file iistv.html from www.ikra.com.

You normally use the absolute pathname of the file in a URL. Pathnames always use the standard UNIX syntax (/), not the DOS syntax (\), for delimiting directory names in a path. Relative pathnames are also possible if the anchor being specified happens to reside in the same directory tree on the same machine as the original document.

Use relative links if you are likely to move directories around. Moving the top-level file's location will move the entire tree with it. On the other hand, a full pathname makes an explicit link to the file regardless of where other files are moved to.

Anchors can also be used to move to a particular part within the same document. Suppose you want to set a link from one document (index.html) to a section in another document (text.html). Therefore, in the index.html file, define a tag for the topic in the text.html file like this:

<A NAME = "Daumesnil">Daumesnil Station</A><P>
The station to go to the Zoo from....

In the index.html file, you create the anchor to the text.html file with a named anchor. A named anchor uses the name of the file, followed by the hash mark (#) and the location in the file:

Get off on <A HREF = "text.html#Daumesnil">the Daumesnil stop</A>
and go two stations west to Port Doree.

Now when the user clicks on the words the Daumesnil stop, the browser loads index.html and goes to the place where the tag is placed for Daumesnil. To go to a location within the same file, skip the filename portion and use only the hash mark with the anchor name.

CGI Scripts

A request from a client browser at the Web server is handled by the httpd daemon. If the request is to run a CGI program, the server will run the program for you and return the results of the program back to the client browser. Input to the CGI program being run is sent either via the command line or through environment variables. The CGI program can be in any language, but it has to be able to process input in this way. The output from the program is generally to standard output. Therefore, a CGI program can be a compiled executable, or a Perl, shell, or awk script, and so on. Naturally, because this book is about Perl, the CGI scripts I discuss here will be in Perl.

CGI scripts are similar to the scripts you would write to handle the command-line interface but with one glaring exception: You cannot send command-line options to CGI scripts. CGI uses the command line for other purposes, so it's impossible to send arguments to the script via the command line.

CGI uses environment variables to send your CGI program its parameters. Environment variables are useful in CGI scripts in that any child processes created by a CGI script also inherit the values of the parent's environment variables. Some of the main environment variables used by CGI scripts are listed in Table 20.1. The dagger (†) indicates that the variable might not be supported on all servers, so use it with caution.

Table 20.1. CGI environment variables.

VariableDescription
AUTH_TYPE The authorization type
CONTENT_LENGTH The size in a decimal number of octets (8-bit bytes) of any attached entity
CONTENT_TYPE The MIME type of an attached entity
DOCUMENT_ROOT The root directory for your server's documents†
DOCUMENT_URL The URL for your client's document†
DOCUMENT_NAME *The name of your client's document†
DATE_LOCAL Local to server
DATE_GMT Local to server
GATEWAY_INTERFACE The server's CGI specification version
HTTP_(string) The client's header data
PATH_INFO The path to be interpreted by the CGI script
PATH_TRANSLATED The virtual to physical mapping of the path
QUERY_STRING The URL-encoded search string
REMOTE_ADDR The IP address of the client
REMOTE_HOST The full qualified domain name of client
REMOTE_IDENT The identity data of the client
REMOTE_USER The user ID sent by the client
REQUEST_METHOD The request method sent by the client
SCRIPT_NAME The URL path identifying the CGI script
SERVER_NAME The server name (the host name of a DNS entry)
SERVER_PORT The port at which the request was received
SERVER_ROOT *The root directory for CGI binaries at the server
SERVER_PROTOCOL A request for protocol name and version number
SERVER_SOFTWARE A request for server software name and version number

Let's briefly cover some of these environment variables and how your CGI script uses them. Keep in mind that only a few of these variables are guaranteed to be set at any one invocation. Not all of these variables are even set by all servers, so check your documentation (usually a README file of sorts) if you do not have the ncSA server. This book deals with the ncSA server, which pretty much covers most of these variables.

To make things easier, you probably will not even use most of the environment variables all the time. The most often used ones tend to be QUERY_STRING, REQUEST_METHOD, and PATH_INFO. However, I cover some of the others just so you are aware of them and what they do.

CONTENT_LENGTH

This variable is set to the number of bytes for the attached data to a file. If there is no attached data, this is set to NULL. In Perl, this is interpreted as zero. For example, for a string of "x=10&y=10", the variable is set as CONTENT_LENGTH=9.

CONTENT_TYPE

The CONTENT_TYPE variable is set to the type of MIME entity (data) being sent to the CGI script. If there is no data, this variable is set to NULL, as well. The MIME types are defined in HTTP 1.0 (and later versions). With MIME types you can represent different types of data: video, audio, images, or just plain text. Each type of data has its own format: for example, GIF files are in the GIF format, text files are in ASCII, and so on. For a GIF file, the CONTENT_TYPE variable is set as follows:

CONTENT_TYPE = image/gif

A client may specify a list of the types of acceptable media in its request header. This allows the server some flexibility in how it returns data. Most types of data are registered with the Internet Assigned Numbers Authority (IANA). The most common registered types and subtypes are listed in Table 20.2. Subtypes and types are not case sensitive-so GIF is equivalent to gif.

Table 20.2. MIME registered content types used by HTTP.
TypeSubtypes Description
text plain ASCII text information
image jpeg, gif Supported formats
audio basic Audio information (includes .au format)
video mpegVideo data
application octet-stream Application-specific data
message rfc822, partial,
external-body
An encapsulated message
multipart mixed, alternative,
digest, parallel
Multiple parts of individual data
types

The types shown in Table 20.2 are by no means complete. Servers may support more types and subtypes. The application type describes transmitted application-specific data. This is generally binary data that is not interpreted by the server. Here is the usual setting for such data:

CONTENT_TYPE=application/octet-stream

The text type describes textual data. The primary and default subtype is plain. Here is the general setting for this variable:

CONTENT_TYPE=text/plain.

With the introduction of "rich text" as specified by RFC 1521 it may be necessary to specify the character set being used. RFC 1521 is the Internet standard for defining internet MIME formats. For more information on RFC 1521, check out the HTML page on http://www.cis.ohio-state.edu/htbin/rfc/rfc1521.html.

Therefore, the setting for sending a plain text file via CGI would be the following:

CONTENT_TYPE=text/plain; charset=us-ascii
A blank like here is manadatory per standard

Note that each content type must be followed by a mandatory blank line as required by HTML/CGI specifications. The image Content-Type can also be used to describe image data. The two well-known image types are JPEG and GIF. The type of format for a GIF would be

CONTENT_TYPE=image/gif
A blank like here is manadatory per standard

The video Content-Type describes video data in the MPEG format. For audio, the Content-Type would be in the basic format. In extended subtypes, you'll see an x in front of the subtype.

GATEWAY_INTERFACE

This represents the version of the CGI specification to which the server complies. This variable is set for all HTTP requests.

PATH_INFO

This variable represents additional path information. It describes a resource to be returned by the CGI application once it has completed its execution. The value of the PATH_INFO variable can be a URL, a string provided to the CGI script, or some other client information. It is up to the handling CGI script as to how to use this information (if the information is needed). If this information is a valid URL, the server may interpret the URL before passing it to the CGI application.

QUERY_STRING

The QUERY_STRING variable is defined as anything that follows the first question mark (?) in a URL. This information can be added either by an ISINDEX document or an HTML form (with the GET action). This string is encoded in the standard URL format of changing spaces to +, individual assignments separated by ampersands and encoding special characters with %XX hexadecimal encoding. You will need to decode the value of QUERY_STRING before you can use it.

Assignments to values in an HTML FORM are done in the form x=value. Spaces are converted into plus (+) signs. So an assignment of the form x=hello world, will be converted to x=hello+world.

Individual assignments are separated by ampersands (&). The equal (=) sign, the plus (+) sign, and the ampersand (&) may only be included as encoded values (by using the # operator, for example). Therefore, these assignments, x="hello world", "a=2" and "y=I am here", are encoded as this:

x=hello+world&a=2&y=I+am+here

Input and Output to CGI Scripts

The server is responsible for packing the data and setting the environment variables before running the CGI script. Information being passed to the CGI script can be passed in one of two ways: as part of STDIN (the Perl handle for standard input) or by setting the value in QUERY_STRING. The results from the execution are sent back from the CGI script to STDOUT (the default output file handle for Perl).

When data is passed in via STDIN, the CONTENT_LENGTH and CONTENT_TYPE variables are set to reflect the length and type of data, respectively. This kind of input is the result of the POST method at the client. For the GET method of requesting data, the variable QUERY_STRING is set to the values.

Incoming strings take the form of an input string. Assignments to values are done in the form x=value. Spaces are converted into plus (+) signs. Individual assignments are separated by ampersands (&). The equal (=) sign, the plus (+) sign, and the ampersand (&) may only be included as encoded values (by using the # operator, for example). Therefore, two assignments, x=1 and "y=42 and 32", are encoded as this:

x=1&y=42+and+32

There is a method to send data to a CGI script with the command line, provided you have the support in the server and your data traffic is not large. Command-line interfaces tend to be restricted by the number of bytes available for the interface. The method of passing data via the environment variables is considered safer and is the preferred way of passing data.

A CGI script must always return something back to the client. This is a requirement; otherwise, the client at the other end might hang for a long time. The standard output from the CGI program is the output that the script writes to STDOUT. Output is returned in one of two ways: a nonparsed header output or a parsed header output. In the nonparsed header output, a complete HTTP message is sent back. In the parsed header output, an HTML header and the body of a message is sent with another URL. The latter method is the preferred way of sending data back from a server to a browser.

CGI programs can return a number of document types back to the client. The CGI standard specifies what format of data to expect at the server via the MIME header. The most common MIME types are text/html for HTML, and text/plain for straight ASCII text.

For example, to send back HTML to the client, your output should look like the example shown here:

   Content-type: text/html

   <HTML><HEAD>
   <TITLE>HTML Output from test CGI script</TITLE>
   </HEAD><BODY>
   <H1>Test output</H1>
<P>
     Body of text
<P>
   </BODY></HTML>

Let's look a test script to see how an HTML file can be sent back.

A Test CGI Script

The CGI script in Listing 20.8 shows how to display information about the environment variables in use. The best way to learn is by example-and this sample script will show you how to write a very basic CGI script in Perl.


Listing 20.8. A sample test script.
 1 #!/usr/bin/perl
 2 #  A very common echo script on just about every server
 3 $| = 1;
 4 print "Content-type: text/plain\n\n";
 5 print "CGI/1.0 test script report\n\n";
 6 if ($ENV{'REQUEST_METHOD'} eq "POST") {
 7      $form = <STDIN>;
 8      print "$form \n";
 9 } else {
10 print "argc is $#ARGV \nargv is ";
11 while (@ARGV) {
12      $ARGV=shift;
13      print "$ARGV ";
14 }
15 }
16 print "\n";
17 #
18 print "SERVER_SOFTWARE = $ENV{'SERVER_SOFTWARE'}\n";
19 print "SERVER_NAME = $ENV{'SERVER_NAME'}\n";
20 print "GATEWAY_INTERFACE = $ENV{'GATEWAY_INTERFACE'}\n";
21 print "SERVER_PROTOCOL = $ENV{'SERVER_PROTOCOL'}\n";
22 print "SERVER_PORT = $ENV{'SERVER_PORT'}\n";
23 print "SERVER_ROOT = $ENV{'SERVER_ROOT'}\n";
24 print "REQUEST_METHOD = $ENV{'REQUEST_METHOD'}\n";
25 print "HTTP_AccEPT = $ENV{'HTTP_AccEPT'}\n";
26 print "PATH_INFO = $ENV{'PATH_INFO'}\n";
27 print "PATH = $ENV{'PATH'}\n";
28 print "PATH_TRANSLATED = $ENV{'PATH_TRANSLATED'}\n";
29 print "SCRIPT_NAME = $ENV{'SCRIPT_NAME'}\n";
30 print "QUERY_STRING = $ENV{'QUERY_STRING'}\n";
31 print "QUERY_STRING_UNESCAPED = $ENV{'QUERY_STRING_UNESCAPED'}\n";
32 print "REMOTE_HOST = $ENV{'REMOTE_HOST'}\n";
33 print "REMOTE_IDENT = $ENV{'REMOTE_IDENT'}\n";
34 print "REMOTE_ADDR = $ENV{'REMOTE_ADDR'}\n";
35 print "REMOTE_USER = $ENV{'REMOTE_USER'}\n";
36 print "AUTH_TYPE = $ENV{'AUTH_TYPE'}\n";
37 print "CONTENT_TYPE = $ENV{'CONTENT_TYPE'}\n";
38 print "CONTENT_LENGTH = $ENV{'CONTENT_LENGTH'}\n";
39 print "DOCUMENT_ROOT = $ENV{'DOCUMENT_ROOT'}\n";
40 print "DOCUMENT_URI = $ENV{'DOCUMENT_URI'}\n";
41 print "DOCUMENT_NAME = $ENV{'DOCUMENT_NAME'}\n";
42 print "DATE_LOCAL = $ENV{'DATE_LOCAL'}\n";
43 print "DATE_GMT = $ENV{'DATE_GMT'}\n";
44 print "LAST_MODIFIED = $ENV{'LAST_MODIFIED'}\n";

Line 3 forces the data written to STDOUT (the default) to be sent back to the client immediately. Make it a habit to do this is for all your CGI scripts. Flushing the output is necessary when processing large amounts of data, because the inherent caching on the output buffer may delay the transfer of text from the one shown.

Line 4 returns the MIME type of document back to the client. Note the two carriage returns at the end of the print statement. The client expects an extra blank line.

Line 5 simply prints an output header for the text output. Line 6 checks if the input is coming from the standard input (STDIN) or if it should be gathered from the command line. The command-line parsing is shown only for use in this example. In Chapter 22, "Using HTML FORMs," and Chapter 23, "HTML with Perl Modules," I develop a Perl subroutine to handle this input via the HTML FORMS. Keep in mind that the command-line interface is restrictive when large amounts of data are being passed through. For the moment, this example shows how the input is set in $ARGC and the @ARGV array.

Lines 7 and 8 extract the input stream from <STDIN>, whereas lines 11 through 14 extract incoming arguments from the command-line interface. The rest of the script echoes the values of the environment variables set at the time the shell script is executed. Let's look at the variables in the output shown in Figure 20.6. Note that the output is in text.

Figure 20.6: Environment variables from the test script.

The output in Figure 20.6 does not look very pretty does it? We can send back an HTML document, too. Look at Listing 20.9, where we send back an HTML document. In this listing, the content type is set to "text/HTML" because we are sending back an HTML document. The rest of the script is basically echoing an HTML document back to the client.


Listing 20.9. Echo environment variables with HTML output.
 1 #!/usr/bin/perl
 2 print "Content-type: text/html\n\n";
 3 print <<"HTML";
 4 <HTML>
 5 <BODY> <TITLE>  HTML output </TITLE>
 6 <p>
 7 <H1> The environment variables </H1>
 8 <UL>
 9 <LI>AUTH_TYPE = <B> $ENV{'AUTH_TYPE'}</B>
10 <LI>CONTENT_TYPE = <B> $ENV{'CONTENT_TYPE'}</B>
11 <LI>CONTENT_LENGTH = <B> $ENV{'CONTENT_LENGTH'}</B>
12 <LI>DATE_LOCAL = <B> $ENV{'DATE_LOCAL'}</B>
13 <LI>DATE_GMT = <B> $ENV{'DATE_GMT'}</B>
14 <LI>DOCUMENT_ROOT = <B> $ENV{'DOCUMENT_ROOT'}</B>
15 <LI>DOCUGMENT_URI = <B> $ENV{'DOCUMENT_URI'}</B>
16 <LI>DOCUMENT_NAME = <B> $ENV{'DOCUMENT_NAME'}</B>
17 <LI>GATEWAY_INTERFACE = <B> $ENV{'GATEWAY_INTERFACE'}</B>
18 <LI>HTTP_AccEPT = <B> $ENV{'HTTP_AccEPT'}</B>
19 <LI>LAST_MODIFIED = <B> $ENV{'LAST_MODIFIED'}</B>
20 <LI>PATH_INFO = <B> $ENV{'PATH_INFO'}</B>
21 <LI>PATH = <B> $ENV{'PATH'}</B>
22 <LI>PATH_TRANSLATED = <B> $ENV{'PATH_TRANSLATED'}</B>
23 <LI>QUERY_STRING = <B> $ENV{'QUERY_STRING'}</B>
24 <LI>QUERY_STRING_UNESCAPED = <B> $ENV{'QUERY_STRING_UNESCAPED'}</B>
25 <LI>REMOTE_HOST = <B> $ENV{'REMOTE_HOST'}</B>
26 <LI>REMOTE_IDENT = <B> $ENV{'REMOTE_IDENT'}</B>
27 <LI>REMOTE_ADDR = <B> $ENV{'REMOTE_ADDR'}</B>
28 <LI>REMOTE_USER = <B> $ENV{'REMOTE_USER'}</B>
29 <LI>REQUEST_METHOD = <B> $ENV{'REQUEST_METHOD'}</B>
30 <LI>SCRIPT_NAME = <B> $ENV{'SCRIPT_NAME'}</B>
31 <LI>SERVER_NAME = <B> $ENV{'SERVER_NAME'}</B>
32 <LI>SERVER_SOFTWARE = <B> $ENV{'SERVER_SOFTWARE'}</B>
33 <LI>SERVER_PORT = <B> $ENV{'SERVER_PORT'}</B>
34 <LI>SERVER_PROTOCOL = <B> $ENV{'SERVER_PROTOCOL'}</B>
35 <LI>SERVER_ROOT = <B> $ENV{'SERVER_ROOT'}</B>
36 </UL>
37 </BODY>
38 </HTML>

The output is shown in Figure 20.7. I cover sending and receiving data using these environment variables using Perl in Chapters 22 and 23 in greater detail.

Figure 20.7: HTML equivalent output from Listing 20.9.

Using Frames

The Netscape browser allows you to use frames to define scrollable portions on your screen, as illustrated in Figure 20.8. The main file for this page is shown in Listing 20.10.

Figure 20.8: Using HTML FRAME tags.

For more information on frames, look at the Web site for Netscape, the folks who came up with frames, at this address:

http://www.netscape.com/assist/net_sites/frames.html

Listing 20.10. Using FRAMEs.
 1 <HTML><HEAD><TITLE>Kamran's Home Page </TITLE>
 2 </HEAD>
 3 <BODY>
 4 <FRAMESET ROWS="22%,78%">
 5      <FRAME NAME="TOPICS" SRC="topics.html">
 6      <FRAME NAME="INDEX" SRC="index.html">
 7 </FRAMESET>
 8 <NOFRAME>
 9 <P>
10 For more information, please contact <address>Kamran Husain</address>
11 <a href="mailto:khusain@ikra.com">khusain@ikra.com</a> </BODY></HTML>

The names of the two FRAMEs in Listing 20.10 are set as TOPICS and INDEX. The area that these tags use up on the screen in specified in the FRAMESET tags. The ROWS attribute of the FRAMESET specifies the percentage of screen space that is to be used when rendering each page. For example, the line

<FRAMESET ROWS="22%,78%">

sets aside 22% of the top of the screen for the first frame to be laid out and 78% for the other frame. Had we used COLS instead of ROWS in the specification, like this:

<FRAMESET COLS="22%,78%">

we would be specifying 22% of the screen space for the first frame and 78% for the second frame. More than one frame can be specified as long as the sum of all the percentages assigned for spaces add up to 100. For example, to set aside three columnar spaces on a screen, you could use the following line:

<FRAMESET COLS="22%,22%,*">

The first two frames would be given a column whose width is 22% each of the total space available. The asterisk in the third column's space indicates to the browser to use the rest of the available space for the third column.

Also, like lists, frames can be nested within other frames by nesting a <FRAMESET></FRAMESET> tag pair in other FRAMESET tags. For more information on using FRAMES, check out the reference books listed earlier in this chapter.

In Listing 20.10, the TOPICS frame occupies the 22% top area, and the INDEX frame occupies the bottom 78%. The URLs in the INDEX frame use the name TOPICS of the window and the TARGET option to display text in the TOPICS window frame. See Listing 20.11.


Listing 20.11. Using TARGET in FRAMEs.
 1 <HTML>
 2 <HEAD><TITLE>MPS Inc. Home Page </TITLE> </HEAD>
 3 <P>
 4 <H1>Topics to choose from</H1>
 5 <ul>
 6 <li> <A HREF="mpstrain.html" TARGET="INDEX" >
 7  Training Courses</A>
 8 <li> <A HREF="mpsgraph.html" TARGET="INDEX">
 9  Graphics Applications with C/C++ </A>
10 <li> <A HREF="mpsprog.html"  TARGET="INDEX">
11  UNIX Programming</A>
12 <li> <A HREF="mpsgraph.html#Seismic"  TARGET="INDEX">
13  Seismic Exploration Applications></A>
14 </ul>
15 </B></BODY></HTML>

The TARGET attribute sets the frame that will be used to lay out the contents of what the HREF attribute points to. When you click on any of the links defined in the top row of the HTML page, the contents of the URL are laid out in the bottom frame. Therefore, the top serves as a table of contents, and the bottom serves as the viewing area for the HTML page.

Summary

This chapter has been a whirlwind introduction to writing HTML pages and interfacing with CGI scripts. A book devoted to these topics would probably give you more detailed information. However, you now have enough information to start writing your own Web pages. In Chapters 22 and 23 I cover how to use HTML tables and forms with Perl CGI scripts.

CGI scripts are the workhorse behind the HTML pages. Input is sent from a client via a server to a CGI script and replies are expected back even if commands are not successful. Replies can take the form of text, data, images, HTML pages, and so on. Output from the CGI script is sent to its standard output and is passed back to the client via the server. Arguments to CGI scripts are passed via environment variables because this is an easy way to pass values between child and parent processes. CGI scripts do not have to be written in Perl, but it's easier to manipulate strings in Perl than it is in C or other languages.

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