Mc-Ac Mc-Ac - 6 months ago 116
Ruby Question

What does __FILE__ == $PROGRAM_NAME mean in ruby?

I have stumbled across this sintax 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
simbolize then? Also, why is this if statement necessary, since the program works with or without it?

Answer

__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!