LINUX CLASSES - PERL PROGRAMMING

Can You Explain Perl Variables to Me?

A variable in a Perl script is a means of referencing a numeric or character value. As in Bash scripts, Perl doesn't require you to declare a type for your variables. Perl figures out by context whether the value should be treated as a number or a character string and will even perform character-to-numeric value conversions when necessary.

To assign or access the value (contents) of a variable, prefix it with a dollar sign. Spaces before or after the equal sign are optional, but if you want to assign a variable that contains character data, you must put quotation marks around the string.

$num=5;
$stuff = "chocolate truffles";
$user = $ENV{USER};

In these examples, Perl assigns 5 to the numeric variable $num, "chocolate truffles" to the variable $stuff, and the value of the USER environment variable to $user.

This is a good time to note that there are several distinct ways to use quotation marks in a Perl script. Let's look at the differences among single quotation marks, double quotation marks, backticks, and the backslash character and then follow up with some examples.

· Single quotation marks, as in the preceding example, will always get you exactly what's inside the quotation marks--any characters that might otherwise have special meaning (like the dollar sign or the backslash) are treated literally.

· Use double quotation marks when you want to assign a string that contains special characters that need to be interpreted.

· The backslash is used to escape (treat literally) a single character (such as $ or *) that might otherwise be treated as a special character.

· Use backticks to indicate that the string is a Linux command that should be executed, with the results assigned to a variable.

Now let's look at some examples that show when to use each method of quoting:

$user = $ENV{USER};
print 'Good Morning $user';
Yields: Good Morning $user

$user = $ENV{USER};
print "Good Morning $user";
Yields: Good Morning hermie

In the first case, the results would probably not be what you wanted. The single quotation marks caused Perl to not treat $user as a variable. In the second case, the results look much better. The double quotation marks allowed Perl to substitute the value of $user in the string.

Here's another example that demonstrates a common error:

$costmsg = "Price is $5.00";
print "$costmsg";
Yields: Price is .00

We thought enough to quote the string, but the dollar sign tells Perl to use the value in the $5 variable, which is as yet undefined. We can easily solve the problem by prefixing the dollar sign with a backslash, as shown here:

$costmsg = "Price is \$5.00";
print "$costmsg";
Actual result: Price is $5.00

Finally, here's an example using backticks to execute a Linux command and capture the results in a Perl variable:

$currdir = `pwd`
print "Current directory is $currdir";
Yields: Current directory is /home/hermie

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Comments (most recent first)

Bob Rankin (24 May 2010, 08:57)

You have another minor problem. The $user variable will not be substituted because it's inside single quotes. Change to double quotes and it'll work. Also, no need to add the "\n" like you did. This will work the same:

print "Good Morning $user \n"

Dennis (24 May 2010, 08:32)

I got already ;
print 'Good Morning $user',"\n"

Dennis (24 May 2010, 08:27)

sorry i meaned \n, to invoke new line.

Dennis (24 May 2010, 08:26)

print 'Good Morning $user';
Yields: Good Morning $user

How to insert here /n in order to be processed?

Bob Rankin (03 May 2010, 14:09)

Quite right! Fixed now...

ben (03 May 2010, 14:00)

I'm new to this, but isn't the second to last line supposed to read:
print "Current directory is $currdir";


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