The IO class is the basis for all input and output in Ruby. An I/O stream may be duplexed (that is, bidirectional), and so may use more than one native operating system stream.

Many of the examples in this section use the File class, the only standard subclass of IO. The two classes are closely associated. Like the File class, the Socket library subclasses from IO (such as TCPSocket or UDPSocket).

The Kernel#open method can create an IO (or File) object for these types of arguments:

  • A plain string represents a filename suitable for the underlying operating system.

  • A string starting with "|" indicates a subprocess. The remainder of the string following the "|" is invoked as a process with appropriate input/output channels connected to it.

  • A string equal to "|-" will create another Ruby instance as a subprocess.

The IO may be opened with different file modes (read-only, write-only) and encodings for proper conversion. See for these options. See Kernel#open for details of the various command formats described above.

IO.popen, the Open3 library, or Process#spawn may also be used to communicate with subprocesses through an IO.

Ruby will convert pathnames between different operating system conventions if possible. For instance, on a Windows system the filename "/gumby/ruby/test.rb" will be opened as "\gumby\ruby\test.rb". When specifying a Windows-style filename in a Ruby string, remember to escape the backslashes:


Our examples here will use the Unix-style forward slashes; File::ALT_SEPARATOR can be used to get the platform-specific separator character.

The global constant ARGF (also accessible as $<) provides an IO-like stream which allows access to all files mentioned on the command line (or STDIN if no files are mentioned). ARGF#path and its alias ARGF#filename are provided to access the name of the file currently being read.

IO Reference


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").

In the description of File methods, permission bits are a platform-specific set of bits that indicate permissions of a file. On Unix-based systems, permissions are viewed as a set of three octets, for the owner, the group, and the rest of the world. For each of these entities, permissions may be set to read, write, or execute the file:

The permission bits 0644 (in octal) would thus be interpreted as read/write for owner, and read-only for group and other. Higher-order bits may also be used to indicate the type of file (plain, directory, pipe, socket, and so on) and various other special features. If the permissions are for a directory, the meaning of the execute bit changes; when set the directory can be searched.

On non-Posix operating systems, there may be only the ability to make a file read-only or read-write. In this case, the remaining permission bits will be synthesized to resemble typical values. For instance, on Windows NT the default permission bits are 0644, which means read/write for owner, read-only for all others. The only change that can be made is to make the file read-only, which is reported as 0444.

Various constants for the methods in File can be found in File::Constants.

File Reference


Part of standard library. You need to require 'stringio' before using.

Pseudo I/O on String object, with interface corresponding to IO.

Commonly used to simulate $stdio or $stderr

require 'stringio'

# Writing stream emulation
io =
io.puts "Hello World"
io.string #=> "Hello World\n"

# Reading stream emulation
io = "first\nsecond\nlast\n"
io.getc #=> "f"
io.gets #=> "irst\n" #=> "second\nlast\n"

StringIO Reference

io/* libraries

This family of libraries extends IO class with several methods.

require 'io/console'

Extend IO with class and instance methods for working with a terminal console.

io/console Reference

require 'io/nonblock'

Extends IO with #nonblock, #nonblock= and #nonblock? instance methods for using IO object in nonblocking mode.

io/nonblock Reference

require 'io/wait'

Extends IO for methods to wait until it is readable or writable, without blocking.

io/wait Reference