Is the relative path to the file from the current execution directory
Assume we have this directory structure: /home/stackoverflow/script.rb
script.rb contains:

puts __FILE__

If you are inside /home/stackoverflow and execute the script like ruby script.rb then __FILE__ will output script.rb If you are inside /home then it will output stackoverflow/script.rb

Very useful to get the path of the script in versions prior to 2.0 where __dir__ doesn't exist.

Note __FILE__ is not equal to __dir__

__dir__ is not a constant but a function
__dir__ is equal to File.dirname(File.realpath(__FILE__))

Contains the name of the script being executed.
Is the same as __FILE__ if you are executing that script.

The process number of the Ruby running this script

Contains the subpattern from the corresponding set of parentheses in the last successful pattern matched, not counting patterns matched in nested blocks that have been exited already, or nil if the last pattern match failed. These variables are all read-only.

Command line arguments given for the script. The options for Ruby interpreter are already removed.

The standard input. The default value for $stdin

The standard output. The default value for $stdout

The standard error output. The default value for $stderr

The current standard error output.

The current standard output

The current standard input

The hash-like object contains current environment variables. Setting a value in ENV changes the environment for child processes.