What is the difference between require_relative and require in Ruby?

Just look at the docs:

require_relative complements the builtin method require by allowing you to load a file that is relative to the file containing the require_relative statement. For example, if you have unit test classes in the "test" directory, and data for them under the test "test/data" directory, then you might use a line like this in a test case: require_relative "data/customer_data_1"

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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__

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From Ruby API:

require_relative complements the builtin method require by allowing you to load a file that is relative to the file containing the require_relative statement. When you use require to load a file, you are usually accessing functionality that has been properly installed, and made accessible, in your system. require does not offer a good solution for loading files within the project’s code. This may be useful during a development phase, for accessing test data, or even for accessing files that are "locked" away inside a project, not intended for outside use. For example, if you have unit test classes in the "test" directory, and data for them under the test "test/data" directory, then you might use a line like this in a test case: require_relative "data/customer_data_1" Since neither "test" nor "test/data" are likely to be in Ruby’s library path (and for good reason), a normal require won’t find them. require_relative is a good solution for this particular problem. You may include or omit the extension (.rb or .so) of the file you are loading. path must respond to to_str.

You can find the documentation at http://extensions.rubyforge.org/rdoc/classes/Kernel.html

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Summary

Use require for installed gems

Use require_relative for local files

require uses your $LOAD_PATH to find the files.
require_relative uses the current location of the file using the statement

require

Require relies on you having installed (e.g. gem install [package]) a package somewhere on your system for that functionality.

When using require you can use the "./" format for a file in the current directory, e.g. require "./my_file" but that is not a common or recommended practice and you should use require_relative instead.

require_relative

This simply means include the file 'relative to the location of the file with the require_relative statement'. I generally recommend that files should be "within" the current directory tree as opposed to "up", e.g. don't use

require_relative '../../../filename'

(up 3 directory levels) within the file system because that tends to create unnecessary and brittle dependencies. However in some cases if you are already 'deep' within a directory tree then "up and down" another directory tree branch may be necessary. More simply perhaps, don't use require_relative for files outside of this repository (assuming you are using git which is largely a de-facto standard at this point, late 2018).

Note that require_relative uses the current directory of the file with the require_relative statement (so not necessarily your current directory that you are using the command from). This keeps the require_relative path "stable" as it always be relative to the file requiring it in the same way.

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The top answers are correct, but deeply technical. For those newer to Ruby:

require_relative will most likely be used to bring in code from another file that you wrote.

for example, what if you have data in ~/my-project/data.rb and you want to include that in ~/my-project/solution.rb? in solution.rb you would add require_relative 'data'.

it is important to note these files do not need to be in the same directory. require_relative '../../folder1/folder2/data' is also valid.

require will most likely be used to bring in code from a library someone else wrote.

for example, what if you want to use one of the helper functions provided in the active_support library? you'll need to install the gem with gem install activesupport and then in the file require 'active_support'.

require 'active_support/all'
"FooBar".underscore

Said differently--

require_relative requires a file specifically pointed to relative to the file that calls it.

require requires a file included in the $LOAD_PATH.

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I just saw the RSpec's code has some comment on require_relative being O(1) constant and require being O(N) linear. So probably the difference is that require_relative is the preferred one than require.

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I want to add that when using Windows you can use require './1.rb' if the script is run local or from a mapped network drive but when run from an UNC \\servername\sharename\folder path you need to use require_relative './1.rb'.

I don't mingle in the discussion which to use for other reasons.

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absolute path

require './app/example_file.rb'

shortened name

require_relative 'example_file'

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