Learn to Use Command Line Arguments in Perl Programming

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Perl (11) - Command Line Arguments 740X296

Perl (11) - Command Line Arguments 740X296

Command-Line arguments are inputs to the program, provided by the user on program execution. Using Command-Line Arguments is the subject of today’s article. So, be with us, and you won’t regret it!

The @ARGV Array
Perl has a special array @ARGV that contains the list of command-line arguments given to the program at execution.

Example
The following script prints the command-line arguments provided to it.

When executed, the program will act like this:
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So, the first element of the @ARGV array contains the first argument (if provided), the second element contains the second argument (if provided), and etc.
Consequently, the number of provided arguments to a script is equal to the length of @ARGV array.

Example
Let’s re-write the factorial script to take its input from the command-line instead of prompting the user to enter it.

Let’s see it in action:
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Special Variables
Besides to the default variable $_ (that we introduced when talking about the foreach loop in Article (7) – Loops (1)) and the @ARGV array that stores the command-line arguments, Perl maintains a set of special variables that could be very useful in some situations. In this section, we are going to introduce some of these variables and their uses.

$0 Variable
The $0 variable contains the program name.

Example
Re-write the factorial program to verify that the user has provided exactly one argument. If not, the program should print a message like the following:

Consider the following code:

When executed, this script should give the output:
3

Despite its simplicity, I would like to explain two points:

  • The expression scalar(@ARGV) returns the number of elements in the @ARGV array (which we know that it represents the number of typed arguments). Consequently, the if statement if (scalar (@ARGV) == 1) checks whether the user has provided only one argument.
  • To print the script name in the usage message, we used the $0 variable.

Learn the Basics of C Programming Language

The Default Array @_
The array @_ contains the list of arguments passed to a subroutine. This will be discussed in detail in the next article “Subroutines”.

Using Command-Line Options
Similar to the system commands that accept command-line options (switches) that control the command behavior, Perl scripts could also accept command-line options.

For the Perl interpreter to be able to parse the command-line switches, it needs to be invoked with the –s option. When the interpreter encounters a switch like for example –new, it will expect to see (within the script) a variable called $new. When seen, the variable $new will be set to true. If the command-line switch uses a form like for example –num=23, the Perl interpreter will set the variable $num to 23.

Example
Write a basic calculator program that expects three command-line arguments as follows:

  • An option that determines the required operation such as –add, -sub, -mul, or –div
  • Two numbers that represent the required operands for the arithmetic operation.

 

Consider the following code:

When executed, the script will act as follows:
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As you can see, the shebang uses the –s option to enable interpreting command-line options. When the script is executed with –add option, the variable $add will be true, and hence the if($add) condition will be met, and consequently: its body will be executed.

Summary
In this article, we have talked about Command-Line Arguments.

  • Perl uses a special array @ARGV that stores the list of command-line arguments provided to the program at execution.
  • The variable $0 contains the program name.
  • Perl scripts can use command-line options (switches). To enable parsing the command-line arguments, the Perl interpreter should be invoked with –s option.

In the next article, we are going to talk about subroutines. So, see you there.

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