Program Input and Output

Reading from Standard Input

• The perl expression <STDIN>, aside from being irritating to write in native HTML format, allows you to read from the pipe named STDIN.
• <STDIN> is the standard source for user input data for a program on any computer you are likely to see.
• Evaluating <STDIN> in a scalar context gives the next complete line of user input, or undef if the input pipe has terminated.
• Example:
  

  # Load the next line the user types in.  Block til they hit enter.
  $userInput =  <STDIN>;
  

• Evaluating <STDIN> in an array context loads the rest of the user inputs (the perl program blocks until the input pipe is terminated) into the specified array.

• Example:
  

  # Load everything the user types in.  Block til they hit CTRL-D.
  @userInput =  <STDIN>;
  

• There is a secret perl variable named "$_". This variable is shorthand for the last variable to have been operated on, and is the default argument for many perl operators.

• Example:
  

  # Echo everything user types back to screen in an especially cryptic fashion
  # Look Ma!  No Arguments.
  while (  <STDIN> ) {
      chomp;
      print;
  }