Lesson 1

This module discusses reading and writing to files and communicating with other programs using Perl Pipes. Some of the subjects you will learn about in this module include:

1. How streams work
2. Using file streams
3. Using pipes to communicate with other programs
4. Sending email from Perl

In the process, you will build and experiment with the second part of our class project, the email program.
When you are finished with this module you will understand how to use streams and pipes well enough to write your own programs using these techniques.

Whenever you execute I/O on a file in Perl, you do so through what in Perl is called a filehandle.
A filehandle is an internal name for an external file. It is the job of the open function to make the association between the internal name and the external name, and it is the job of the close function to break that association.
For your convenience, Perl sets up a few special filehandles that are already open when you run. These include STDIN , STDOUT , STDERR , and ARGV . Since those are pre-opened, you can use them right away without having to go to the trouble of opening them yourself:

 print STDERR "This is a debugging message.\n";
 print STDOUT "Please enter something: ";
 $response = <STDIN> // die "how come no input?";
 print STDOUT "Thank you!\n";
 while (<ARGV>) { ... }

STDOUT and STDERR are output handles, and STDIN and ARGV are input handles. They are in all capital letters because they are reserved words in Perl, much like the

1. @ARGV array and
2. %ENV hash

are.
Their external associations were set up by your shell.
You will need to open every other filehandle on your own. Although there are many variants, the most common way to call Perl's open() function is with three arguments and one return value:

OK = open(HANDLE, MODE, PATHNAME)

The ability to control or interact with another process is called interprocess communication (IPC). It can take many different forms,

1. from reading or writing information from or to another process,
2. through to terminating or restarting processes,
3. all the way to exchanging large volumes of information between two or more processes.

There are many different ways of doing this by using internal methods, such as open, and external methods that make use of the networking techniques. Network communication is expensive resource-wise, and is probably overkill for many solutions that simply require a conduit for exchanging small pieces of discrete data. Most of the solutions center around the use of pipes, which, as the name suggests, provide the necessary conduit for data exchange. Unix users will be familiar with the use of pipes, and Windows users will probably have used them without realizing.
In this module, we will look at all the different facilities built into Perl for interprocess communication. This will include process creation, control, and communication to allow you to interact with other processes. We�ll also look at the signal system, supported under most platforms as a way of signaling a specific state to a process, and then look at some IPC-specific techniques.