What is the difference between require_relative and require in Ruby?

Question

What is the difference between require_relative and require in Ruby?

Answer 1

require_relative is a convenient subset of require

require_relative('path')

equals:

require(File.expand_path('path', File.dirname(__FILE__)))

if __FILE__ is defined, or it raises LoadError otherwise.

This implies that:

• require_relative 'a' and require_relative './a' require relative to the current file (__FILE__).

  This is what you want to use when requiring inside your library, since you don't want the result to depend on the current directory of the caller.

• eval('require_relative("a.rb")') raises LoadError because __FILE__ is not defined inside eval.

  This is why you can't use require_relative in RSpec tests, which get evaled.

The following operations are only possible with require:

• require './a.rb' requires relative to the current directory

• require 'a.rb' uses the search path ($LOAD_PATH) to require. It does not find files relative to current directory or path.

  This is not possible with require_relative because the docs say that path search only happens when "the filename does not resolve to an absolute path" (i.e. starts with / or ./ or ../), which is always the case for File.expand_path.

The following operation is possible with both, but you will want to use require as it is shorter and more efficient:

• require '/a.rb' and require_relative '/a.rb' both require the absolute path.

Reading the source

When the docs are not clear, I recommend that you take a look at the sources (toggle source in the docs). In some cases, it helps to understand what is going on.

require:

VALUE rb_f_require(VALUE obj, VALUE fname) {
  return rb_require_safe(fname, rb_safe_level());
}

require_relative:

VALUE rb_f_require_relative(VALUE obj, VALUE fname) {
    VALUE base = rb_current_realfilepath();
    if (NIL_P(base)) {
        rb_loaderror("cannot infer basepath");
    }
    base = rb_file_dirname(base);
    return rb_require_safe(rb_file_absolute_path(fname, base), rb_safe_level());
}

This allows us to conclude that

require_relative('path')

is the same as:

require(File.expand_path('path', File.dirname(__FILE__)))

because:

rb_file_absolute_path   =~ File.expand_path
rb_file_dirname1        =~ File.dirname
rb_current_realfilepath =~ __FILE__