Andrew Hudson - Fedora™ Unleashed, 2008 edition

TABLE 25.2 String Comparison Operators in Perl

Perl uses compound operators, similar to those used by C or awk, which can be used to combine other operations (such as comparisons or arithmetic) into more complex forms of logic. Table 25.3 shows the compound pattern operators and their behavior.

TABLE 25.3 Compound Pattern Operators in Perl

Perl supports a wide variety of math operations. Table 25.4 summarizes these operators.

TABLE 25.4 Perl Arithmetic Operators

You can also use comparison operators (such as ==or <) and compound pattern operators ( &&, ||, and !) in arithmetic statements. They evaluate to the value 0for false and 1for true.

Perl supports a number of operators that don't fit any of the prior categories. Table 25.5 summarizes these operators.

TABLE 25.5 Other Perl Operators

Except for the comma operator and conditional expression, these operators can also be used with the assignment operator, similar to the way addition ( +) can be combined with assignment ( =), giving +=.

Perl supports string constants that have special meaning or cannot be entered from the keyboard. Table 25.6 shows most of the constants supported by Perl.

TABLE 25.6 Perl Special String Constants

Perl offers two conditional statements, ifand unless, which function opposite one another. if enables you to execute a block of code only if certain conditions are met so that you can control the flow of logic through your program. Conversely, unlessperforms the statements when certain conditions are not met. The following sections explain and demonstrate how to use these conditional statements when writing scripts for Linux.

The syntax of the Perl if/elsestructure is as follows:

if ( condition ) {

statement or block of code

} elsif ( condition ) {

statement or block of code

} else {

statement or block of code

}

condition can be a statement that returns a true or false value.

Truth is defined in Perl in a way that might be unfamiliar to you, so be careful. Every thing in Perl is true except 0(the digit zero), "0"(the string containing the number 0), ""(the empty string), and an undefined value. Note that even the string "00"is a true value because it is not one of the four false cases.

The statement or block of code is executed if the test condition returns a true value. For example, Listing 25.3 uses the if/elsestructure and shows conditional statements using the eqstring comparison operator.

if ($favorite eq "chocolate") {

print "I like chocolate too.\n";

} elsif ($favorite eq "spinach") {

print "Oh, I don't like spinach.\n";

} else {

print "Your favorite food is $favorite.\n";