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Explanation of if __FILE__ == $0

pjhyett

3/17/2006 4:49:00 PM

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
3 Answers

Daniel Harple

3/17/2006 4:58:00 PM

0

On Mar 17, 2006, at 5:49 PM, 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?
>
> Thanks,
> 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

Jim Freeze

3/17/2006 5:05:00 PM

0

On Mar 17, 2006, at 10:58 AM, Daniel Harple 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. :)

Jim Freeze

pjhyett

3/17/2006 6:05:00 PM

0

Thanks guys, that's helpful.

-PJ

On 3/17/06, Jim Freeze <jimfreeze@gmail.com> wrote:
> On Mar 17, 2006, at 10:58 AM, Daniel Harple 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. :)
>
> Jim Freeze
>
>
>
>
>