by David Till

IN THIS CHAPTER

• What Is Perl?

• How Do I Find Perl?

• A Sample Perl Program

• Running a Perl Program

• Line 1 of Your Program: How Comments Work

• Line 2: Statements, Tokens, and <STDIN>

• Line 3: Writing to Standard Output

• Error Messages

• Interpretive Languages Versus Compiled Languages

Welcome to a brief look at Perl 5. In this chapter, you'll learn about the following topics:

• What Perl is and why Perl is useful

• How to get Perl if you do not already have it

• How to run Perl programs

• How to write a simple Perl program

• The difference between interpretive and compiled programming languages

• What an algorithm is and how to develop one

Perl is an acronym, short for Practical Extraction and Report Language. It was designed by Larry Wall as a tool for writing programs in the UNIX environment and is continually being updated and maintained by him.

For its many fans, Perl provides the best of several worlds. For instance:

• Perl has the power and flexibility of a high-level programming language such as C. In fact, as you will see, many of the features of the language are borrowed from C.

• Like shell script languages, Perl does not require a special compiler and linker to turn the programs you write into working code. Instead, all you have to do is write the program and tell Perl to run it. This means that Perl is ideal for producing quick solutions to small programming problems, or for creating prototypes to test potential solutions to larger problems.

• Perl provides all the features of the script languages sed and awk, plus features not found in either of these two languages. Perl also supports a sed-to-Perl translator and an awk-to-Perl translator.

In short, Perl is as powerful as C but as convenient as awk, sed, and shell scripts.

As you'll see, Perl is very easy to learn. Indeed, if you are familiar with other programming languages, learning Perl is a snap. Even if you have very little programming experience, Perl can have you writing useful programs in a very short time. If you pick up a copy of Teach Yourself Perl 5 in 21 Days (Sams Publishing, 1995), you'll easily learn enough about Perl to be able to solve many problems.

To find out whether Perl already is available on your system, take the following steps:

If you are currently working in a UNIX programming environment, check to see whether the file /usr/local/bin/perl exists.

• If you are working in any other environment, check the place where you normally keep your executable programs, or check the directories accessible from your PATH environment variable.

If you do not find Perl in this way, talk to your system administrator and ask whether he has Perl running somewhere else. If you don't have Perl running in your environment, don't despair--read on!

One of the reasons Perl is becoming so popular is that it is available free to anyone who wants it. If you are on the Internet, you can obtain a copy of Perl with File Transfer Protocol (FTP). Following is a sample FTP session that transfers a copy of the Perl distribution. The items shown in boldface type are what you would enter during the session.

If you cannot access the MIT site from where you are, you can get Perl from the following sites via anonymous FTP:

You also can obtain Perl from most sites that store GNU source code, or from any site that archives the Usenet newsgroup comp.sources.unix.

Now that Perl is available on your system, it's time to show you a very simple program that illustrates how easy it is to use Perl. The program shown in Listing 29.1 asks for a line of input and writes it.

1: #!/usr/local/bin/perl

2: $inputline = <STDIN>;

3: print( $inputline );

Here is the output from this listing:

Line 1 is the header comment. Line 2 reads a line of input. Line 3 writes the line of input back to your screen.

The following sections describe how to create and run this program, and they describe it in more detail.

To run the program shown in Listing 29.1, carry out the following actions:

1. Using your favorite editor, type the program and save it in a file called program29_1.

2. Tell the system that this file contains executable statements. To do this in the UNIX environment, enter the following command:

$ chmod +x program29_1

3. Run the program by entering this command:

$ program129_1

When you run program29_1, it waits for you to enter a line of input. After you enter the line of input, program29_1 prints what you entered, as shown here:

$ program29_1

This is my line of input.

This is my line of input.

$ 

If Listing 29.1 is stored in the file program29_1 and run according to the preceding steps, the program should run successfully. If the program doesn't run, one of two things has likely happened:

• The system can't find the file program29_1.

• The system can't find Perl.

If you receive the error message

program29_1 not found

or something similar, your system couldn't find the file program29_1. To tell the system where program29_1 is located, you can do one of two things in a UNIX environment:

• Enter the command ./program29_1, which gives the system the pathname of program29_1 relative to the current directory.

• Add the current directory . to your PATH environment variable. This tells the system to search in the current directory when looking for executable programs such as program29_1.

If you receive the message

/usr/local/bin/perl not found

or something similar, Perl is not installed properly on your machine. Refer to the section "How Do I Find Perl?" earlier in this chapter, for more details.

If you don't understand these instructions or are still having trouble running Listing 29.1, talk to your system administrator.

Now that you've run your first Perl program, let's look at each line of Listing 29.1 and figure out what it does.

Line 1 of this program is a special line that tells the system that this is a Perl program:

#!/usr/local/bin/perl

Let's break this line down, one part at a time:

• The first character in the line, the # character, is the Perl comment character. It tells the system that this line is not an executable instruction.

• The ! character is a special character; it indicates what type of program this is. (You don't need to worry about the details of what the ! character does. All you have to do is remember to include it.)

• The path /usr/local/bin/perl is the location of the Perl executable on your system. This executable interprets your program; in other words, it figures out what you want to do and then does it. Because the Perl executable has the job of interpreting Perl instructions, it usually is called the Perl interpreter.

If, after reading this, you still don't understand the meaning of the line #!/usr/local/bin/perl, don't worry. The actual specifics of what it does are not important for our purposes in this book. Just remember to include it as the first line of your program, and Perl will take it from there.

As you have just seen, the first character of the line

#!/usr/local/bin/perl

is the comment character, #. When the Perl interpreter sees the #, it ignores the rest of that line.

Comments can be appended to lines containing code, or they can be lines of their own:

$inputline = <STDIN>;    # this line contains an appended comment

# this entire line is a comment

You can--and should--use comments to make your programs easier to understand. Listing 29.2 is the simple program you saw earlier, but it has been modified to include comments explaining what the program does.

1: #!/usr/local/bin/perl

2: # this program reads a line of input and writes the line

3: # back out

4: $inputline = <STDIN>;    # read a line of input

5: print( $inputline );     # write the line out

This is the sample input and output of this program:

$ program29_2

This is a line of input.

This is a line of input.

$

The behavior of the program in Listing 29.2 is identical to that of Listing 29.1 because the code is the same. The only difference is that Listing 29.2 has comments in it.

Note that in an actual program, comments normally are used only to explain complicated code or to indicate that the following lines of code perform a specific task. Because Perl instructions usually are pretty straightforward, Perl programs don't need to have a lot of comments.

#!/usr/local/bin/perl

This line is a special comment line, and it is not treated like the others.

---

Now that you've learned what the first line of Listing 29.1 does, let's take a look at line 2:

$inputline = <STDIN>;

This is the first line of code that actually does any work. To understand what this line does, you need to know what a Perl statement is and what its components are.

The line of code you have just seen is an example of a Perl statement. Basically, a statement is one task for the Perl interpreter to perform. A Perl program can be thought of as a collection of statements performed one at a time.

When the Perl interpreter sees a statement, it breaks the statement into smaller units of information. In this example, the smaller units of information are $inputline, =, <STDIN>, and ;. Each of these smaller units of information is called a token.

Tokens can normally be separated by as many spaces and tabs as you like. For example, the following statements are identical in Perl:

$inputline = <STDIN>;

$inputline=<STDIN>;

$inputline      =     <STDIN>;

Your statements can take up as many lines of code as you like. For example, the following statement is equivalent to the preceding ones:

$inputline

=

<STDIN>

;

The collection of spaces, tabs, and new lines separating one token from another is known as white space.

When programming in Perl, you should use white space to make your programs more readable. The examples in this book use white space in the following ways:

• New statements always start on a new line.

• One blank space is used to separate one token from another (except in special cases, some of which you'll see in this chapter).

As you've seen already, the statement

$inputline = <STDIN>;

consists of four tokens: $inputline, =, <STDIN>, and ;. The following subsections explain what each of these tokens does. The $inputline and = Tokens The first token in line 1, $inputline (at the left of the statement), is an example of a scalar variable. In Perl, a scalar variable can store one piece of information.

The = token, called the assignment operator, tells the Perl interpreter to store the item specified by the token to the right of the = in the place specified by the token to the left of the =. In this example, the item on the right of the assignment operator is the <STDIN> token, and the item to the left of the assignment operator is the $inputline token. Thus, <STDIN> is stored in the scalar variable $inputline.

Scalar variables and assignment operators are covered in more detail in Teach Yourself Perl 5 in 21 Days. The <STDIN> Token and the Standard Input File The next token, <STDIN>, represents a line of input from the standard input file. The standard input file, or STDIN for short, typically contains everything you enter when running a program.

For example, when you run program29_1 and enter

This is a line of input.

the line you enter is stored in the standard input file.

The <STDIN> token tells the Perl interpreter to read one line from the standard input file, where a line is defined to be a set of characters terminated by a new line. In this example, when the Perl interpreter sees <STDIN>, it reads

This is a line of input.

If the Perl interpreter then sees another <STDIN> in a different statement, it reads another line of data from the standard input file. The line of data you read earlier is destroyed unless it has been copied somewhere else.

Because the <STDIN> token is to the right of the assignment operator =, the line

This is a line of input.

is assigned to the scalar variable $inputline. The ; Token The ; token at the end of the statement is a special token that tells Perl that the statement is complete. You can think of it as a punctuation mark that is like a period in English.

Now that you understand what statements and tokens are, consider line 3 of Listing 29.1:

print ($inputline);

This statement refers to the library function that is called print. Library functions, such as print, are provided as part of the Perl interpreter; each library function performs a useful task.

The print function's task is to send data to the standard output file. The standard output file stores data that is to be written to your screen. The standard output file sometimes appears in Perl programs under the name STDOUT.

In this example, print sends $inputline to the standard output file. Because the second line of the Perl program assigns the line

This is a line of input.

to $inputline, this is what print sends to the standard output file and what appears on your screen.

When a reference to print appears in a Perl program, the Perl interpreter calls, or invokes, the print library function. This function invocation is similar to a function invocation in C, a GOSUB statement in BASIC, or a PERFORM statement in COBOL. When the Perl interpreter sees the print function invocation, it executes the code contained in print and returns to the program when print is finished.

Most library functions require information to tell them what to do. For example, the print function needs to know what you want to print. In Perl, this information is supplied as a sequence of comma-separated items located between the parentheses of the function invocation. For example, the statement you've just seen

print ($inputline);

supplies one piece of information that is passed to print: the variable $inputline. This piece of information commonly is called an argument.

The following call to print supplies two arguments:

print ($inputline, $inputline);

You can supply print with as many arguments as you like; it prints each argument starting with the first one (the one on the left). In this case, print writes two copies of $inputline to the standard output file.

You also can tell print to write to any other specified file.

If you incorrectly type a statement when creating a Perl program, the Perl interpreter detects the error and tells you where the error is located.

For example, look at Listing 29.3. This program is identical to the program you've been seeing all along, except that it contains one small error. Can you spot it?

1: #!/usr/local/bin/perl

2: $inputline = <STDIN>

3: print ($inputline);

The output should give you a clue.

$ program29_3

Syntax error in file program29_3 at line 3, next char (

Execution of program29_3 aborted due to compilation errors.

$

When you try to run this program, an error message appears. The Perl interpreter has detected that line 2 of the program is missing its closing ; character. The error message from the interpreter tells you what the problem is and identifies the line on which the problem is located.

As you've seen, running a Perl program is easy. All you need to do is create the program, mark it as executable, and run it. The Perl interpreter takes care of the rest. Languages such as Perl that are processed by an interpreter are known as interpretive languages.

Some programming languages require more complicated processing. If a language is a compiled language, the program you write must be translated into machine-readable code by a special program known as a compiler. In addition, library code might need to be added by another special program known as a linker. After the compiler and linker have done their jobs, the result is a program that can be executed on your machine--assuming, of course, that you have written the program correctly. If not, you have to compile and link the program all over again.

Interpretive languages and compiled languages both have advantages and disadvantages, as mentioned here:

• As you've seen with Perl, it takes very little time to run a program in an interpretive language.

• Interpretive languages, however, cannot run unless the interpreter is available. Compiled programs, on the other hand, can be transferred to any machine that understands them.

As you'll see, Perl is as powerful as a compiled language. This means that you can do a lot of work quickly and easily.

In this chapter you learned that Perl is a programming language that provides many of the capabilities of a high-level programming language such as C. You also learned that Perl is easy to use; basically, you just write the program and run it.

You saw a very simple Perl program that reads a line of input from the standard input file and writes the line to the standard output file. The standard input file stores everything you type from your keyboard, and the standard output file stores everything your Perl program sends to your screen.

You learned that Perl programs contain a header comment, which indicates to the system that your program is written in Perl. Perl programs also can contain other comments, each of which must be preceded by a #.

Perl programs consist of a series of statements, which are executed one at a time. Each statement consists of a collection of tokens, which can be separated by white space.

Perl programs call library functions to perform certain predefined tasks. One example of a library function is print, which writes to the standard output file. Library functions are passed chunks of information called arguments; these arguments tell a function what to do.

The Perl interpreter executes the Perl programs you write. If it detects an error in your program, it displays an error message and uses the error-recovery process to try to continue processing your program. If Perl gets confused, error cascading can occur, and the Perl interpreter might display inappropriate error messages.