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Perl Fundamentals Introduction to Perl as a Server-Side Technology
Week 4: Perl - I Perl Fundamentals Introduction to Perl as a Server-Side Technology
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Perl “Practical Extraction and Report Language” Invented by Larry Wall
Wall’s goal: “Easy things should be easy and hard things should be possible.” Eclectic blend of features from several languages Often used as an active Web-server scripting language Responds to an HTTP request by generating the HTML for the response page Typically also saves user data and/or retrieves data for the response. Also popular for building system admin tools COMP/DCOM 461 – CWB
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The Role of Perl in Server-Side Response Handling and Page Generation
Browser Server POST or GET HTTP (Web) Server Form Response Page ValidationResult ValidationRequest URL HTTP Parameters Response Page (HTML) JavaScript Code Perl COMP/DCOM 461 – CWB
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A Simple Perl Script #!/bin/perl
Invoke Perl to handle this script Required HTTP header #!/bin/perl print "Content-type: text/html", "\n"; print "\n"; print "<html> <head> <title>First Perl Script</title></head>\n"; print "<body>\n"; print "Hello World!\n"; print "</body></html>\n"; exit(0); End of HTTP headers End of Content Tell Web server that everything is OK COMP/DCOM 461 – CWB
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Note All double quotes in these slides should be ", not “ and ”, which may be accidentally inserted by PowerPoint COMP/DCOM 461 – CWB
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Using Perl on Tomcat All Perl scripts must be installed in the directory WEB-INF/cgi under your webapp (or webappw) Use .pl as the file extension (Optional) The script file must be readable by “other” – that is, chmod 604 myfile.pl URL (replace my ID with yours): COMP/DCOM 461 – CWB
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A Snippet from WEB-INF/web.xml
<!-- define mapping for the CGI servlet --> <servlet-mapping> <servlet-name>cgi</servlet-name> <url-pattern>/cgi-bin/*</url-pattern> </servlet-mapping> Maps all URLs /cgi-bin/* to the built-in CGI-handling servlet You’ll find such a mapping in the WEB-INF/web.xml file in your webapp directory. COMP/DCOM 461 – CWB
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More from WEB-INF/web.xml
<!-- define the servlet that executes CGI scripts --> <servlet> <servlet-name>cgi</servlet-name> <servlet-class> org.apache.catalina.servlets.CGIServlet </servlet-class> <init-param> <param-name>clientInputTimeout</param-name> <param-value>100</param-value> </init-param> <param-name>debug</param-name> <param-value>6</param-value> <param-name>cgiPathPrefix</param-name> <param-value>WEB-INF/cgi</param-value> <load-on-startup>1</load-on-startup> </servlet> Look in the WEB-INF/cgi directory to find the scripts COMP/DCOM 461 – CWB
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Checking Your Script To “compile” your script to check out its syntax:
perl –c filename To run your script outside of Tomcat: perl filename Beware that the ENV variable will not have any HTTP information, so you won’t have any request parameters available. COMP/DCOM 461 – CWB
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CGI “Common Gateway Interface”
A convention for running external commands to handle HTTP requests. Information about the URL and the HTTP headers is loaded into known environment variables Perl makes them available to programmers in a built-in hash table called ENV. COMP/DCOM 461 – CWB
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Simple Request in Perl #!/bin/perl $query = $ENV{'QUERY_STRING'};
The variable query will become the portion of the URL past the ‘?’. #!/bin/perl $query = $ENV{'QUERY_STRING'}; print "Content-type: text/html", "\n"; print "\n"; print "<html> <head> <title>Simple Query in Perl</title></head>\n"; print "<body>\n"; print "The query string from the URL is $query\n"; print "</body></html>\n"; exit(0); The query variable is “interpolated” into the string. COMP/DCOM 461 – CWB
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First Observations on Perl
Variables are indicated by a $ A variable mentioned in a string is interpolated – its value is inserted into the string. The print function outputs a comma-separated list of strings Use \n to get a newline. Script ends with exit(0); CGI information available as $ENV{'something'} Comments: # comments out the rest of the line See next slides. COMP/DCOM 461 – CWB
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Selected CGI Parameters
$ENV{'REQUEST_METHOD'} – GET or POST $ENV{'QUERY_STRING'} – URL after the ‘?’ $ENV{'CONTENT_LENGTH'} – number of bytes in the content (for POST only) $ENV{'SCRIPT_URI'} – URL up through the script filename (excluding any ?-string or #-name) $ENV{'REQUEST_URI'} – Complete URL, excluding #-name $ENV{'HTTP_USER_AGENT'} – Browser identification $ENV{'HTTP_XXXXX'} – HTTP header xxxxx COMP/DCOM 461 – CWB
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Dump of the ENV Values We’ll cover the features of Perl used in this example in more detail later. COMP/DCOM 461 – CWB
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Exercise – Perl Cookie Generator
Write a Perl script that sets a cookie in the browser It should return a page that displays any cookies found in the request Hint: Recall the “Set-Cookie” and “Cookie” HTTP headers COMP/DCOM 461 – CWB
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My Solution Code: Test:
Test: COMP/DCOM 461 – CWB
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Selected Perl Features in More Detail
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Perl Variables $abc – a scalar variable called “abc”
@def – an array variable called “def” %ghi – a hashtable variable called “ghi” scalar: having a single value COMP/DCOM 461 – CWB
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The my Operator Normally variables are global
To get a local variable, use the my operator: my $name = “Anthony”; = (1,2,5,8..20); COMP/DCOM 461 – CWB
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More on Scalars Indicated by a leading $
Think of the $ as meaning “Treat the following as a scalar” Holds one value Can be number or string: $a = "wxy"; $b = 8; Reference using the $ as well: $a = $b; COMP/DCOM 461 – CWB
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More On Arrays @a means treat a as an array
An array holds a list of values: @def = (1, 2, "zzz", ); Each value must be a scalar: $abc = $def[3]; Think: $def[3] means “treat def[3] as a scalar” Arrays count from 0 COMP/DCOM 461 – CWB
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Array Slices @a[2,5,6] is an array consisting of items 2, 5, and 6 taken from array a @a[2..5] picks items 2, 3, 4, and 5 @a = (1, 3, "a", "u", 9); print #prints: a 9 print #prints: a u 9 COMP/DCOM 461 – CWB
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Initializing Arrays List notation: Quoted word notation:
@a = (1, 3, “aaa”, “bbb”, 0); Quoted word notation: @a = qw/1 3 aaa bbb/; Each white-space-separated word becomes a string member of the list. COMP/DCOM 461 – CWB
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The sort Array Function
Block sort {$a - Returns sorted array The block in {…} is a little anonymous subroutine that is evaluated for each comparison $a and $b are the items to compare The value should be positive if $a > $b, negative if $b > $a, and zero if $a equals $b The example given sorts the array as numbers Use { $a cmp $b } for alphanumeric comparison COMP/DCOM 461 – CWB
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More Array Functions reverse @a
produces an array in the reverse order of a produces an array of the members of a that match the pattern Use the same pattern syntax as with JavaScript COMP/DCOM 461 – CWB
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More on Hashtables Officially known as associative arrays
A hashtable holds a set of values accessed by keys that are strings %ghi = ("xyz", 4, "wxy", "q", "vwx", 19); $abc = $ghi{"xyz"}; $ghi{"xyz"} means “treat ghi{"xyz"} as a scalar” Every other item is a key; the other items are values Extract the member with the key “xyz” COMP/DCOM 461 – CWB
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More on Hashtables Two ways to initialize:
Every other item is a key; the other items are values Two ways to initialize: %capitals = (“Ohio”, “Columbus”, “Illinois”, “Springfield”, “Virginia”, “Richmond”); %capitals = (Ohio => “Columbus”, Illinois => “Springfield”, Virginia => “Richmond”); => works like a comma and also implies that the previous word is a string COMP/DCOM 461 – CWB
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Built-in Functions For Hashes
each(%a) – returns a different ($key,$val) list each time it’s called in a loop: while(($k,$v) = each(%a)){ … } keys(%a) – returns a list of the keys values(%a) – returns a list of the values delete $a{'qqq'} – deletes member qqq Note list assignment feature COMP/DCOM 461 – CWB
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Using each in a While Loop
while(($k,$v) = each(%a)){ print "Key: $k Value: $v\n"; } COMP/DCOM 461 – CWB
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The List Assignment Feature
($a, $b, $c) = (900, "cat", 9); Assigns 900 to a “cat” to b 9 to c COMP/DCOM 461 – CWB
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Perl Strings Single quotes: treat the string literally
Double quotes: the string is “interpolated” That is, variables are interpreted and inserted in the string: $a = “World”; $b = “Hello $a”; $b becomes “Hello World”; COMP/DCOM 461 – CWB
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Here Documents A mechanism for creating a long, multi-line string
print <<end; <body> <p>Hello, $userName!</p> </body> end Variables in the here-document are interpolated So the paragraph becomes, say, “Hello, Bill!” Means all text up to the line starting with “end” COMP/DCOM 461 – CWB
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foreach Loops Repeats code for each member of an array Syntax:
foreach my $x { … } Repeats the code in { … } for each member of the array cats Sets the (local) variable x to the corresponding member The my is optional, but makes x a local variable COMP/DCOM 461 – CWB
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Implementation of the Dump of the ENV Hashtable
Code snippit: print "<table>\n"; foreach my $key (keys %ENV) { print "<tr><td>$key</td><td>$ENV{$key}</td>\n"; } print "</table>\n"; The keys function extracts a list of the keys in a hashtable COMP/DCOM 461 – CWB
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Alternative Implementation
Code snippit: print "<table>\n"; while ((my ($key, $val) = each(%ENV))) { print "<tr><td>$key</td><td>$val</td>\n"; } print "</table>\n"; The each function returns a 2-element list: one key and one value COMP/DCOM 461 – CWB
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Exercise – Simple Perl Coding
Write a Perl script to generate a table of values The table should have two columns, representing the keys and values in a hashtable Create the hard-coded hashtable using either type of hash initializer COMP/DCOM 461 – CWB
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Comparisons Numerical comparisons:
<=> and cmp return -1, 0, or 1, depending on whether left side is <, =, or > the right side Numerical comparisons: < <= == != >= > <=> String comparisons: lt le eq ne ge gt cmp if($name eq “Fred”) if($orderNumber == ) COMP/DCOM 461 – CWB
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Comparison Demo http://cs.franklin.edu:8461/brownc/cgi-bin/cmp.pl?4
Code: COMP/DCOM 461 – CWB
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Arrays of Hashtables Each array member is a hashtable
my $theOrderId = "18918"; @orders = ( { orderId => "18994", status => "hold", item => "socks" }, { orderId => "18918", status => "shipped", item => "pork bellies" } ); my $theStatus = "unknown"; for($i = 0; $i $i++) { if ( $orders[$i]{orderId} eq $theOrderId) { $theStatus = $orders[$i]{status}; } print "The status of order $theOrderId is: <b>$theStatus</b>.\n"; Each array member is a hashtable Extract the orderId element from the ith member of the orders array. COMP/DCOM 461 – CWB
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Hashtables of Hashtables
my $theOrderId = "18918"; %orders = ( "18994" => { status => "hold", item => "socks" }, "18918" => { status => "shipped", item => "pork bellies" } ); if(($theStatus = $orders{$theOrderId}{status} )) { print "The status of order $theOrderId is: <b>$theStatus</b>.\n"; else print "The order $theOrderId is unknown\n" Each hashtable member is another hashtable Extract the status member from the orderId member of the orders array. COMP/DCOM 461 – CWB
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Perl References Assigned readings in Anderson-Freed Syntax:
Built-in Functions: COMP/DCOM 461 – CWB
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Homework Assignment This week you will just be getting used to Perl by generating the Order Status page For now, put your order data in an array or other Perl structure that you wish Include at least two orders Write Perl print statements to generate the HTML code like you have in your current page design Insert the order information from the array into the HTML Next week you will add some more Perl functionality Accessing the order data from in a file Using a userID to select a particular user’s data COMP/DCOM 461 – CWB
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