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permalink #__dir__ ⇒ StringReturns the canonicalized absolute path of the directory of the file from which this method is called. It means symlinks in the path is resolved.
A File is an abstraction of any file object accessible by the program and is closely associated with class IO . File includes the methods of module FileTest as class methods, allowing you to write (for example) File. exist?("foo") .
__FILE__ is simply the name of the current file. realpath(dirname(__FILE__)) gets the name of the directory that the file is in -- in essence, the directory that the app is installed in. And @ is PHP's extremely silly way of suppressing errors.
$0 is the name of the file used to start the program. This check says “If this is the main file being used…” This allows a file to be used as a library, and not to execute code in that context, but if the file is being used as an executable, then execute that code.
It is a reference to the current file name. In the file foo.rb, __FILE__ would be interpreted as "foo.rb".
Edit: Ruby 1.9.2 and 1.9.3 appear to behave a little differently from what Luke Bayes said in his comment. With these files:
# test.rb
puts __FILE__
require './dir2/test.rb'
# dir2/test.rb
puts __FILE__
Running ruby test.rb will output
test.rb
/full/path/to/dir2/test.rb
The value of __FILE__ is a relative path that is created and stored (but never updated) when your file is loaded. This means that if you have any calls to Dir.chdir anywhere else in your application, this path will expand incorrectly.
puts __FILE__
Dir.chdir '../../'
puts __FILE__
One workaround to this problem is to store the expanded value of __FILE__ outside of any application code. As long as your require statements are at the top of your definitions (or at least before any calls to Dir.chdir), this value will continue to be useful after changing directories.
$MY_FILE_PATH = File.expand_path(File.dirname(__FILE__))
# open class and do some stuff that changes directory
puts $MY_FILE_PATH
__FILE__ is the filename with extension of the file containing the code being executed.
In foo.rb, __FILE__ would be "foo.rb".
If foo.rb were in the dir /home/josh then File.dirname(__FILE__) would return /home/josh.
In Ruby, the Windows version anyways, I just checked and __FILE__ does not contain the full path to the file. Instead it contains the path to the file relative to where it's being executed from.
In PHP __FILE__ is the full path (which in my opinion is preferable). This is why, in order to make your paths portable in Ruby, you really need to use this:
File.expand_path(File.dirname(__FILE__) + "relative/path/to/file")
I should note that in Ruby 1.9.1 __FILE__ contains the full path to the file, the above description was for when I used Ruby 1.8.7.
In order to be compatible with both Ruby 1.8.7 and 1.9.1 (not sure about 1.9) you should require files by using the construct I showed above.
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