Chapter 12

Using Special Variables

• What Are the Special Variables?
  • Example: Using the DATA File Handle
• Summary
• Review Questions
• Review Exercises

Perl uses quite a few special variables to control various behaviors of fuNCtions. You can use special variables to hold the results of searches, the values of environment variables, and flags to control debugging. In short, every aspect of Perl programming uses special variables.

What Are the Special Variables?

Table 12.1 shows a list of the special variables you can use in your programs. The order of this list is identical to the list in the file PERLVAR.htm, which comes with your Perl distribution. This table lets you quickly find any special variable you may come across in examples or someone else's code.

Table 12.2 puts the variables into different categories so you can see how they relate to one another. This organization is better than Table 12.1 when you are creating your own programs. Some of the categories covered in Table 12.2 have their own chapters. The subheadings in the table point out which chapter you can look at for more information.

Most of these variables are discussed in other chapters of the book, and some of the variables are simple enough to use that you don't need to see examples by this time. However, the DATA file handle and the %ENV associated array deserve some additional mention. They are discussed in the following sections.

Example: Using the DATA File Handle

As you no doubt realize by now, Perl has some really odd features, and the DATA file handle is one of them. This file handle lets you store read-only data in the same file as your Perl script, which might come in handy if you need to send both code and data to someone via e-mail.

When using the DATA file handle, you don't need to open or close the file handle-just start reading from the file handle using the diamond operator. The following simple example shows you how to use the DATA file handle.

Read all the lines that follow the line containing __END__.
Loop through the @lines array, printing each element.
Everything above the __END__ line is code; everything below is data.

@lines = <DATA>;



foreach (@lines) {

    print("$_");



}



__END__

Line one

Line two

Line three

This program displays the following:

Line one

Line two

Line three

Example: Using the %ENV Variable

Environment variables are used by the operating system to store bits of information that are needed to run the computer. They are called environment variables because you rarely need to use them and because they simply remain in the background-just another part of the overall computing environment of your system. When your Perl process is started, it is given a copy of the environment variables to use as needed.

You can change the environment variables, but the changes will not persist after the process running Perl is ended. The changes will, however, affect the current process and any child processes that are started.

You can print out the environment variables by using these lines of code:

foreach $key (keys(%ENV)) {

    printf("%-10.10s: $ENV{$key}\n", $key);

}

On my Windows 95 machine, this program displays the following:

WINBOOTDIR: C:\WINDOWS

TMP       : C:\WINDOWS\TEMP

PROMPT    : $p$g

CLASSPATH : .\;e:\jdk\classes;

TEMP      : C:\WINDOWS\TEMP

COMSPEC   : C:\WINDOWS\COMMAND.COM

CMDLINE   : perl -w 12lst01.pl

BLASTER   : A220 I10 D3 H7 P330 T6

WINDIR    : C:\WINDOWS

PATH      : C:\WINDOWS;C:\WINDOWS\COMMAND;C:\PERL5\BIN;

TZ        : GMT-05:00

Only a few of these variables are interesting. The TMP and TEMP variables let you know where temporary files should be placed. The PATH variable lets the system know where to look for executable programs. It will search each directory in the list until the needed file is found. The TZ variable lets you know which time zone the computer is running in.

The most useful variable is probably the PATH statement. By changing it, you can force the system to search the directories you specify. This might be useful if you suspect that another program of the same name resides in another directory. By placing the current directory at the beginning of the PATH variable, it will be searched first and you'll always get the executable you want. For example:

$ENV{"PATH"} = ".;" . $ENV{"PATH"};

A single period is used to refer to the current directory, and a semicolon is used to delimit the directories in the PATH variable. So this statement forces the operating system to look in the current directory before searching the rest of the directories in PATH.

Environment variables can be useful if you want a quick way to pass information between a parent and a child process. The parent can set the variables, and the child can read it.

Summary

This chapter gathered into one location all the special variables used by Perl. Most of the variables have already been discussed in previous chapters, and a few will be discussed in later chapters.

Table 12.1 was organized to follow the PERLVAR.htm document that comes in the Perl distribution, so if you aren't familiar with a variable used in someone else's code, that's the place to look. The variables are basically ordered alphabetically.

Table 12.2 was organized according to fuNCtionality. Some variables are used with files, some with arrays, and so forth.

You saw an example of how to use the DATA file handle to read information from the same file that holds the Perl script.

The %ENV variable was also discussed. This hash is used to hold the environmental variables used mostly by the operating system.

In the next chapter, "Handling Errors and Signals," you learn about how to handle error conditions, use the eval() fuNCtion, and other things dealing with exceptions that can happen while your program runs.

Review Questions

Answers to Review Questions are in Appendix A.

1. What is the $/ variable used for?
2. What file handle is used to avoid a second system call when doing two or more file tests?
3. What will the following program display?
   
   $_ = "The big red shoe";
   m/[rs].*\b/;
   print("$`\n");
4. What variable holds the value of the last match string?
5. What will the following program display?
   
   @array = (1..5);
   $" = "+";
   print("@array\n");
6. What does the following program display?
   @array = ('A'..'E');
   
   foreach (@array) {
   print();
   }
   
   $\ = "\n";
   foreach (@array) {
   print();
   }

Review Exercises

1. Write a program that changes the array element separator used in interpolation of arrays inside double-quoted strings to be a comma instead of a space.
2. Write a program that displays which version of the Perl interpreter you are running.
3. Create a file in your temporary directory. (Hint: use the %ENV special variable.)
4. Write a program that uses the $\ to end each printed element with an ":END" string.
5. Write a program that prints the last record in a file. The records should be variable-length, but each record starts with the string "START:". (Hint: look at the $/ variable.)