Explanation of if __FILE__ == $0

Ruby
1

So I’ve tracked this down:
FILE is the name of the current source file
$0 at the top level is the name of the top-level program being executed

But I’m still confused as to what this if statement is accomplishing?
It’s obviously useful for something since it’s used quite often. Does
this pertain to when a script is called from another script?

Thanks,
PJ

2

Hi –

On Sat, 18 Mar 2006, PJ Hyett wrote:

So I’ve tracked this down:
FILE is the name of the current source file
$0 at the top level is the name of the top-level program being executed

But I’m still confused as to what this if statement is accomplishing?
It’s obviously useful for something since it’s used quite often. Does
this pertain to when a script is called from another script?

Yes; if FILE (the name of the current file) is the same as $0 (the
file where execution started), then you’re in the startup file.
You’ll often see this used in, say, a library file that usually gets
loaded from another file (so FILE == $0 is false), but can also be
run standalone for the sake of running tests.

David

–
David A. Black ([email protected])
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! Ruby for Rails

3

On Mar 17, 2006, at 5:49 PM, PJ Hyett wrote:

PJ
It is used when you want to run something when the file is executed
directly. Some put tests in their code like this, I think this is bad
form. Tests belong in a separate file.

File a.rb

class Foo
end

if FILE == $PROGRAM_NAME # [1]
puts “Testing out class Foo”

…

end

File b.rb

require ‘a’
puts “The if block in a.rb is not executed”

END

$ruby a.rb
Testing out class Foo
$ruby b.rb
The if block in a.rb is not executed

[1] $PROGRAM_NAME is the less cryptic name for $0

– Daniel

4

On Mar 17, 2006, at 10:58 AM, Daniel H. wrote:

if FILE == $PROGRAM_NAME # [1]
[1] $PROGRAM_NAME is the less cryptic name for $0

Most definitely. I would like to see this changed in existing code.

Being a long time Ruby user, I always used $0 until
recently, Then I read some code by a new ruby user (whom I introduced
to ruby).
I never bothered to see if an alternative to $0 existed.

You can learn from anyone. Even the nubies. :slight_smile:

Jim F.

5

I agree with Jim F. - I think

if FILE == $PROGRAM_NAME

is slightly more readable than

if FILE == $0

Elegance is what keeps me attached to ruby.

6

Thanks guys, that’s helpful.

-PJ