What Does 'If _File_ == $0' Mean in Ruby

Meaning of __FILE__ = $0 as given on ruby-lang.org

Suppose your file is foo.rb, and defines a Foo class. The file can be used in one of two ways.

The first, which you're already familiar with, is to include it in another file or in IRB. It'll go something like:

# in otherfile.rb
require 'foo'
foo = Foo.new

The if __FILE__ == $0 line is for the second use case, where you make the file executable and call it directly from the shell. It'll go something like

# in the shell
./foo.rb
# alternatively: ruby foo.rb

if __FILE__ == $0 problem with main

Where is the file that defines Camping or 'Menu'? You will need to 'require' those files:

require 'camping'
require 'menus' # etc

class Main
#.. 

Read more about require here.

What does __FILE__ == $PROGRAM_NAME mean in ruby?

__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!

Ruby 'script = $0'

$0 is one of Ruby's global variables. From here:

$0 -- Contains the name of the script being executed. May be assignable.

Can I write Ruby code that is executed only when my script is run, but not when it is required?

This is usually done with

if __FILE__ == $0
  Foo.run
end

but I prefer

if File.identical?(__FILE__, $0)
  Foo.run
end

because programs like ruby-prof can make $0 not equal __FILE__ even when you use --replace-progname.

$0 refers to the name of the program ($PROGRAM_NAME), while __FILE__ is the name of the current source file.

Checking if a Ruby program was executed or imported via require

Change foo.rb to read:

if __FILE__ == $0
  puts "Hello"
end

That checks __FILE__ - the name of the current ruby file - against $0 - the name of the script which is running.

Require file without executing code?

Is it possible to load a file without having it to run the code?

No, everything in a ruby file is executable code, including class and method definitions (you can see this when you try to define a method inside an if-statement for example, which works just fine). So if you wouldn't execute anything in the file, nothing would be defined.

You can however tell ruby that certain code shall only execute if the file is run directly - not if it is required. For this simply put the code in question inside an if __FILE__ == $0 block. So for your example, this would work:

file.rb

def method
  puts "This won't be outputted."
end
if __FILE__ == $0
  puts "This will not be outputted."
end

main.rb

require "./file"

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