A First Step in perl Programming

A computer program is a list of instructions that tell the computer what to do. A program, like any system, can be seen as sequence of
Input → Process → Output

The Hello World program is the common first program used to introduce a programming language, it has the objectives of

  1. Verifying that someone is able to properly run a program
  2. Verifying that a program is able to send output to the user
  3. Verifying that someone is able to enter the source code (program statements) of a program.

perl is an interpreted language, which means that perl is a program itself that reads statements that are written in the perl language, and executes the statements.

Running perl Interactively

If you have perl installed, you can run perl interactively, which means you can enter commands and have perl read and interpret statements as you type them in. Text entered by the user/programmer is shown below in blue.

Example: running perl from a Linux command line
perl
print 'Hello world.';
[Ctrl]-D
Hello World.

Explanation:
The perl command starts the perl interpreter.
perl will not send the user any output or prompt for input; perl is waiting for program instructions.
The first command entered by the user is the perl statement:
print 'Hello world.';
The user enters [Ctrl]-D (control key + the letter D) to let perl know there is no more input. The [Ctrl]-D may show up as ^D on some systems, or may not show up at all. ^D is often used as the end-of-file marker. Here the ^D marks the end of the standard input file, the keyboard. When the user enters ^D perl executes the print statement, sending the output "Hello World." to the screen and perl terminates.

Running perl with a stored program

Most progams will be entered using a text file, where the programs can be stored, edited as needed, and reused. Create a text file named helloWorld.pl and enter the following one line program

With the program saved, perl can be run from the command line with the name of the stored program.

Example: running perl with a program name
perl helloWorld.pl
Hello World.

Explanation:
The perl helloWorld.pl command starts the perl interpreter and perl will immediately begin to process the program in file helloWorld.pl.
perl will not display any program information about itself, nor will it provide a prompt. perl executes the program helloWorld.pl sending the output Hello World. and then terminates.

Making a perl program executable from the command line

When you run perl from the command line, the operating system will search your path for the perl interpreter. In unix/Linux you can find out where a program is located using the which command. To find out where perl has been installed, issue the following Linux command:
In my system, the which command replies with: By adding the following line as the first line of helloWorld.pl, and making the file executable, the operating system will always know to use /usr/bin/perl to execute the program:

Make the program helloWorld.pl executable using the following unix/Linux command:

The program helloWorld.pl can be run from the command line using only the name of the stored program.

Example: running an executable perl program
./helloWorld.pl
Hello World.

Explanation:
When the operating system is told to execute the helloWorld.pl program, it will see by the first line in the file that it should use the perl interpreter, and perl will immediately begin to process the program. perl will not display any program information about itself, nor will it provide a prompt. perl executes the program helloWorld.pl and then terminates.

The ./ before helloWorld.pl in the command tells the operating system to look in 'the current directory' for the program helloWorld.pl