1.1 A First Perl Program
So, to begin our study of Perl, let us consider a small Perl program.
Do not worry that you are not familiar with all the syntax used here.
The syntax will be introduced more formally as we continue on through
this book. Just try to infer the behavior of the constructs below as
best you can.
For our first Perl program, we will ask the user their username, and
print out a message greeting the user by name.
#!/usr/bin/perl
use strict; # @cc{important pragma}
use warnings; # @cc{another important pragma}
print "What is your username? "; # @cc{print out the question}
my $username; # @cc{``declare'' the variable}
$username = <STDIN>; # @cc{ask for the username}
chomp($username); # @cc{remove ``new line''}
print "Hello, $username.\n"; # @cc{print out the greeting}
# @cc{Now we have said hello to our user}
Let us examine this program line by line to ascertain its meaning. Some
hand-waving will be necessary, since some of the concepts will not be
presented until later. However, this code is simple enough that you need
not yet understand completely what each line is doing.
The first line is how the program is identified as a Perl program. All
Perl programs should start with a line like #!/path/perl.
Usually, it is just #!/usr/bin/perl. You should put this line at
the top of each of your Perl programs.
In the lines that follow, halfway through each line, there is a `#'
character. Everything from the `#' character until the end of the
line is considered a comment. You are not required to comment
each line. In fact, commenting each line is rare. However, you will
find in this text that we frequently put comments on every line, since
we are trying to explain to the reader exactly what each Perl statement
is doing. When you write Perl programs, you should provide comments,
but you need not do so as verbosely as we do in this text.
Note, too, that comments can also occur on lines by themselves. The
last line of the program above is an example of that.
Now, consider the code itself, ignoring everything that follows a
`#' character. Notice that each line (ignoring comments) ends with
a `;'. This is the way that you tell Perl that a statement
is complete. We'll talk more about statements soon; for now, just
consider a statement to be a single, logical command that you give to
Perl.
The first line, use strict, is called a pragma in Perl. It
is not something that "explicitly" gets executed, from your point of
view as the programmer. Instead, a pragma specifies (or changes) the
rules that Perl uses to understand the code that follows. The use
strict; pragma enforces the strictest possible rules for compiling the
code. You should always use this pragma while you are still new to
Perl, as it will help you find the errors in your code more easily.
The second line is another pragma, use warnings. This pragma
tells Perl that you'd like to be warned as much as possible when you
write code that might be questionable. Certain features of Perl can
confuse new (and sometimes even seasoned) Perl programmers. The
use warnings pragma, like use strict, is a way to tell
Perl that you'd like to be warned at run-time when certain operations
seem questionable.
So, you might wonder why two separate pragmas are needed. The reason is
that they are enforced by Perl at different times. The use
strict pragma enforces compile-time constraints on the program source
code. You can even test them without running the program by using
perl -c filename, where filename is the file
containing your program. That option does not run your program, it
merely checks that they syntax of your program is correct. (To remember
this, remember that the letter `c' in `-c' stands for
"check the program".)
By contrast, the use warnings pragma controls run-time
behavior. With use warnings, messages could be printed while
your program runs, if Perl notices something wrong. In addition,
different inputs to the program can cause different messages to be
printed (or suppress such messages entirely).
The third line is the first statement of the program the performs an
action directly. It is a call to Perl's built-in @builtin{print}
function. In this case, it is taking a string (enclosed in double
quotes) as its argument, and sending that string to the standard output,
which is, by default, the terminal, window, or console from which the
program is run.
The next line is a variable declaration. When in @module{strict}
mode (set by the use strict pragma), all variables must be
declared. In this case, Perl's @keyword{my} keyword is used to declare
the variable @scalar{$username}. A variable like @scalar{$username}
that starts with a $ is said to be a scalar variable. For
more information on scalar variables, see 2. Working with Scalars.
For now, just be aware that scalar variables can hold strings.
The next line, $username = <STDIN> is an assignment statement,
which is denoted by the =. The left hand side of the assignment
is that scalar variable, @scalar{$username}, that we declared in the line
before it. Since @scalar{$username} is on the left hand side of the
=, that indicates @scalar{$username} will be assigned a new value
by this assignment statement.
The right hand side of the assignment is a construct that allows us to
get input from the keyboard, the default standard input. @fileh{STDIN}
is called a file handle that represents the standard input. We
will discuss more about file handles later. For now, just remember that
the construct <STDIN>, when assigned to a scalar variable, places
the next line of standard input into that scalar variable.
Thus, at this point, we have the next line of the input (which is
hopefully the username that we asked for), in the @scalar{$username}
variable. Since we got the contents of @scalar{$username} from the
standard input, we know that the user hit return after typing her
username. The return key inserts a special character, called newline,
at the end of the line. The @scalar{$username} variable contains the
full contents of the line, which is not just the user's name, but also
that newline character.
To take care of this, the next thing we do is chomp($username).
Perl's built-in function, @builtin{chomp}, removes any newline characters
that are on the end of a variable. So, after the @builtin{chomp}
operation, the variable @scalar{$username}
The final statement is another @builtin{print} statement. It uses the
value of the @scalar{$username} variable to greet the user with her
name. Note that it is acceptable to use @scalar{$username} inside of
the string to be printed, and the contents of that scalar are included.
This ends our discussion of our small Perl program. Now that you have
some idea of what Perl programs look like, we can begin to look at Perl,
its data types, and its constructs in detail.
