1. Introduction to Perl Learning Objectives:
To introduce the features provided by Perl
To learn the basic Syntax & simple Input/Output control in Perl
To learn how to combine Unix Command in Perl using backquotes

2. Introduction to Perl
Perl is a scripting language that makes manipulation of text, files, and processes easy.
Perl is a cross between shell programming and the C programming language.
Smalltalk
(objects) C
(numbers)
Shell programming
(text)
C++
(numbers, objects)
Perl
(text, numbers)
Java
(objects)

3. Introduction to Perl
Perl provides a more concise and readable way to do many tasks compared to C++ and shell scripts.
Perl has replaced shell programming as the most popular programming language for text processing and Unix system administration.
Perl was originally designed under Unix, but now also runs under all operating systems (including Windows).
Many libraries available (e.g. database, internet)
Perl is also a popular language for CGI (for web server) and GUI programming.

4. Introduction to Perl
The perl command on our system invokes Perl 4.0, but we want to use the latest version Perl 5.0.
You can run a perl 5.0 script explicitly with perl5:
$ cat simple
$ perl5 simple $
You can run the script directly if you make the script executable, and the first line is of the following form ( #!/usr/... must start from the first column):
$ chmod +x simple
#!/usr/local/bin/perl5 -w
$ simple

5. Basic Syntax (1)
The -w option tells Perl to produce extra warning messages about potential dangers. Use -w in all your Perl programs for now.
#!/usr/local/bin/perl5 -w
Whitespace doesn't matter in Perl (like C++), except for #!/usr/local/bin/perl5 -w which must start from column 1 on line 1.

6. Basic Syntax (2) All perl statements end in a semicolon ; (like C++)
In Perl, comments begin with # (like shell scripts)
everything after the # to the end of the line is ignored.
# need not be at the beginning of the line.
there are no C++-like multiline comments: /* */

7. Perl Example 1 (1) Here is a “hello world” Perl program:
$ ls -l
-rwxr-xr-x 1 horner cs 52 Mar 2 15:50 hello*
$ cat hello
#!/usr/local/bin/perl5 -w
# comment lines start with the # character
print "Hello world\n";
$ hello
Hello world $
The print command sends the string to the screen, and “\n“ adds a newline.

8. Perl Example 1 (2)
You can optionally use parenthesis around the argument in print:
print ("Hello world\n");
or, if your prefer the C++ function style:
print("Hello world\n");

9. Scalar Variables
A scalar variable can hold a single number or string (like shell variables), including integers and floating-point numbers (unlike shell variables).
Scalar variables begin with “$” followed by a letter, and then possibly more letters, digits, or underscores. (e.g., $n, $n1, $name, $first_name).
Scalar variables are case sensitive.

10. Assigning Scalar Variables
Scalars are assigned using “=“
$scalar = expression;
To assign a value to a scalar variable:
$number = 25;
$name = "Bill Clinton";
Unlike shell scripts, use the “$” both when the variable is used and assigned:
$ cat test1
#!/usr/local/bin/perl5 -w
print "$number $name\n";
$ test1
25 Bill Clinton $

11. Numerical Scalar Variables
Internally, all numerical scalar values are stored as floats (so you don’t have to worry about integer division in Perl like you do in C++).
Perl supports the usual C++ numerical operations:
$a = 25; # $a is now 25
$a += 5; # $a is now 30
$a *= 3; # $a is now 90
$a++; # $a is now 91
--$a; # $a is now 90
$result = ($a + 2) * 3.4; # $result is 312.8

12. User Input (1) Use <STDIN> to get input from the user:
$ cat test2
#!/usr/local/bin/perl5 -w
print "Enter name: ";
$name = <STDIN>;
chomp ($name);
print "How many girlfriends do you have? ";
$number = <STDIN>;
chomp($number);
print "$name has $number girlfriends!\n";
$ test2
Enter name: Bill Clinton
How many girlfriends do you have? more than you
Bill Clinton has more than you girlfriends!

13. User Input (2)
<STDIN> grabs one line of input, including the newline character. So, after:
$name = <STDIN>;
if the user typed “Bill Clinton[ENTER]”, $name will contain: “Bill Clinton\n”.
To delete the newline, the chomp() function takes a scalar variable, and removes the trailing newline if present. (If there is no newline at the end, it does nothing.)
A shortcut to do both operations in one line is:
chomp($name = <STDIN>);

14. Printing Special Symbol - $
As with the shell scripts, use a backslash before $ if you really want to print the dollar sign:
$ cat test4
#!/usr/local/bin/perl5 -w
print "Enter amount: ";
$cost = <STDIN>;
print "The total is: \$$cost";
$ test4
Enter amount: 18.50
The total is $18.50
No need to use chomp() if the newline on $cost can be used when it is printed.

15. Numerical Example #!/usr/local/bin/perl5 -w
$ cat test6
#!/usr/local/bin/perl5 -w
print "Enter height of rectangle: ";
$height = <STDIN>;
print "Enter width of rectangle: ";
$width = <STDIN>;
$area = $height * $width;
print "The area of the rectangle is $area\n";
$ test6
Enter height of rectangle: 10
Enter width of rectangle: 5
The area of the rectangle is 50
Enter height of rectangle: 10.1
Enter width of rectangle: 5.1
The area of the rectangle is 51.51

16. Backquotes: Command Substitution
You can use command substitution in Perl like in shell scripts:
$ whoami
clinton
$ cat test7
#!/usr/local/bin/perl5 -w
$user = `whoami`;
chomp($user);
$num = `who | wc -l`;
chomp($num);
print "Hi $user! There are $num users logged on.\n";
$ test7
Hi clinton! There are users logged on.
Command substitution will usually include a newline, so use chomp().

17. Backquote Example 2 $ cat big1 #!/usr/local/bin/perl5 -w $dir = `pwd`;
chomp($dir);
$big = `ls -l | sort +4 | tail -1 | cut -c55-70`;
chomp($big);
$nline = `wc -l $big | cut -c6-8`;
chomp($nline);
$nword = `wc -w $big | cut -c6-8 `;
chomp($nword);
$nchar = `wc -c $big | cut -c6-8 `;
chomp($nchar);
print "The biggest file in $dir is $big.\n";
print "$big has $nline lines, $nword words, $nchar characters.\n";
$ big1
The biggest file in /homes/horner/111/perl is big1.
big1 has 14 lines, 66 words, 381 characters.