rubyvariablesspecial-variables

What does __FILE__ == $PROGRAM_NAME mean in ruby?

I have stumbled across this syntax while reviewing a code in Ruby. The code is:

if __FILE__ == $PROGRAM_NAME
  #some code...
end

I suppose __FILE__ is a variable that gets me the name of the file I am in? But what does $PROGRAM_NAME symbolize, then? Also, why is this if statement necessary, since the program works with or without it?

Solution

  • __FILE__ always returns the path of the source file. It's not a variable so you can't assign value to it. Whether it returns a relative path or an absolute one depends on how you run the script.

    $PROGRAM_NAME or $0 by default returns the command that boots the program (minus the path of ruby interpreter). For example, you have a script file test.rb like this:

    #!/usr/bin/env ruby
puts __FILE__
puts $PROGRAM_NAME

    If you run this script with ruby test.rb, it prints 

    test.rb
test.rb

    If you run the script with ruby /path/to/test.rb, it prints 

    /path/to/test.rb
/path/to/test.rb

    If you give the script an execution permission and run it with ./test.rb, it prints

    ./test.rb
./test.rb

    Unlike __FILE__, $PROGRAM_NAME and $0 are real global variables, and you can change their values. $PROGRAM_NAME and $0 are aliases to each other, so you change the value of either one, the value of the other will change accordingly. For example, you have a test2.rb like this:

    #!/usr/bin/env ruby
$0 = 'Hello, world!'
puts $0
puts $PROGRAM_NAME

    it prints

    Hello, world!
Hello, world!