Ruby - File I/O

Ruby provides a whole set of I/O-related methods implemented in the Kernel module. All the I/O methods are derived from the class IO.

The class IO provides all the basic methods, such as read, write, gets, puts, readline, getc, and printf.

This chapter will cover all the basic I/O functions available in Ruby. For more functions, please refer to Ruby Class IO.

The puts Statement

In the previous chapters, you have assigned values to variables and then printed the output using puts statement.

The puts statement instructs the program to display the value stored in the variable. This will add a new line at the end of each line it writes.



val1 = "This is variable one"
val2 = "This is variable two"
puts val1
puts val2

This will produce the following result −

This is variable one
This is variable two

The gets Statement

The gets statement can be used to take any input from the user from standard screen called STDIN.


The following code shows you how to use the gets statement. This code will prompt the user to enter a value, which will be stored in a variable val and finally will be printed on STDOUT.


puts "Enter a value :"
val = gets
puts val

This will produce the following result −

Enter a value :
This is entered value
This is entered value

The putc Statement

Unlike the puts statement, which outputs the entire string onto the screen, the putc statement can be used to output one character at a time.


The output of the following code is just the character H −


str = "Hello Ruby!"
putc str

This will produce the following result −


The print Statement

The print statement is similar to the puts statement. The only difference is that the puts statement goes to the next line after printing the contents, whereas with the print statement the cursor is positioned on the same line.



print "Hello World"
print "Good Morning"

This will produce the following result −

Hello WorldGood Morning

Opening and Closing Files

Until now, you have been reading and writing to the standard input and output. Now, we will see how to play with actual data files.

The Method

You can create a File object using method for reading, writing, or both, according to the mode string. Finally, you can use File.close method to close that file.


aFile ="filename", "mode")
   # ... process the file

The Method

You can use method to create a new file object and assign that file object to a file. However, there is one difference in between and methods. The difference is that the method can be associated with a block, whereas you cannot do the same using the method."filename", "mode") do |aFile|
   # ... process the file

Reading and Writing Files

The same methods that we've been using for 'simple' I/O are available for all file objects. So, gets reads a line from standard input, and aFile.gets reads a line from the file object aFile.

However, I/O objects provides additional set of access methods to make our lives easier.

The sysread Method

You can use the method sysread to read the contents of a file. You can open the file in any of the modes when using the method sysread. For example −

Following is the input text file −

This is a simple text file for testing purpose.

Now let's try to read this file −


aFile ="input.txt", "r")
if aFile
   content = aFile.sysread(20)
   puts content
   puts "Unable to open file!"

This statement will output the first 20 characters of the file. The file pointer will now be placed at the 21st character in the file.

The syswrite Method

You can use the method syswrite to write the contents into a file. You need to open the file in write mode when using the method syswrite. For example −


aFile ="input.txt", "r+")
if aFile
   puts "Unable to open file!"

This statement will write "ABCDEF" into the file.

The each_byte Method

This method belongs to the class File. The method each_byte is always associated with a block. Consider the following code sample −


aFile ="input.txt", "r+")
if aFile
   aFile.each_byte {|ch| putc ch; putc ?. }
   puts "Unable to open file!"

Characters are passed one by one to the variable ch and then displayed on the screen as follows −

s. .a. .s.i.m.p.l.e. .t.e.x.t. .f.i.l.e. .f.o.r. .t.e.s.t.i.n.g. .p.u.r.p.o.s.e...

The IO.readlines Method

The class File is a subclass of the class IO. The class IO also has some methods, which can be used to manipulate files.

One of the IO class methods is IO.readlines. This method returns the contents of the file line by line. The following code displays the use of the method IO.readlines


arr = IO.readlines("input.txt")
puts arr[0]
puts arr[1]

In this code, the variable arr is an array. Each line of the file input.txt will be an element in the array arr. Therefore, arr[0] will contain the first line, whereas arr[1] will contain the second line of the file.

The IO.foreach Method

This method also returns output line by line. The difference between the method foreach and the method readlines is that the method foreach is associated with a block. However, unlike the method readlines, the method foreach does not return an array. For example −


IO.foreach("input.txt"){|block| puts block}

This code will pass the contents of the file test line by line to the variable block, and then the output will be displayed on the screen.

Renaming and Deleting Files

You can rename and delete files programmatically with Ruby with the rename and delete methods.

Following is the example to rename an existing file test1.txt


# Rename a file from test1.txt to test2.txt
File.rename( "test1.txt", "test2.txt" )

Following is the example to delete an existing file test2.txt


# Delete file test2.txt

File Modes and Ownership

Use the chmod method with a mask to change the mode or permissions/access list of a file −

Following is the example to change mode of an existing file test.txt to a mask value −


file = "test.txt", "w" )
file.chmod( 0755 )

File Inquiries

The following command tests whether a file exists before opening it −

#!/usr/bin/ruby"file.rb") if File::exists?( "file.rb" )

The following command inquire whether the file is really a file −


# This returns either true or false
File.file?( "text.txt" ) 

The following command finds out if the given file name is a directory −


# a directory
File::directory?( "/usr/local/bin" ) # => true

# a file
File::directory?( "file.rb" ) # => false

The following command finds whether the file is readable, writable or executable −


File.readable?( "test.txt" )   # => true
File.writable?( "test.txt" )   # => true
File.executable?( "test.txt" ) # => false

The following command finds whether the file has zero size or not −

#!/usr/bin/ruby "test.txt" )      # => true

The following command returns size of the file −


File.size?( "text.txt" )     # => 1002

The following command can be used to find out a type of file −


File::ftype( "test.txt" )     # => file

The ftype method identifies the type of the file by returning one of the following − file, directory, characterSpecial, blockSpecial, fifo, link, socket, or unknown.

The following command can be used to find when a file was created, modified, or last accessed −


File::ctime( "test.txt" ) # => Fri May 09 10:06:37 -0700 2008
File::mtime( "text.txt" ) # => Fri May 09 10:44:44 -0700 2008
File::atime( "text.txt" ) # => Fri May 09 10:45:01 -0700 2008

Directories in Ruby

All files are contained within various directories, and Ruby has no problem handling these too. Whereas the File class handles files, directories are handled with the Dir class.

Navigating Through Directories

To change directory within a Ruby program, use Dir.chdir as follows. This example changes the current directory to /usr/bin.


You can find out what the current directory is with Dir.pwd

puts Dir.pwd # This will return something like /usr/bin

You can get a list of the files and directories within a specific directory using Dir.entries

puts Dir.entries("/usr/bin").join(' ')

Dir.entries returns an array with all the entries within the specified directory. Dir.foreach provides the same feature −

Dir.foreach("/usr/bin") do |entry|
   puts entry

An even more concise way of getting directory listings is by using Dir's class array method −


Creating a Directory

The Dir.mkdir can be used to create directories −


You can also set permissions on a new directory (not one that already exists) with mkdir −

NOTE − The mask 755 sets permissions owner, group, world [anyone] to rwxr-xr-x where r = read, w = write, and x = execute.

Dir.mkdir( "mynewdir", 755 )

Deleting a Directory

The Dir.delete can be used to delete a directory. The Dir.unlink and Dir.rmdir performs exactly the same function and are provided for convenience.


Creating Files & Temporary Directories

Temporary files are those that might be created briefly during a program's execution but aren't a permanent store of information.

Dir.tmpdir provides the path to the temporary directory on the current system, although the method is not available by default. To make Dir.tmpdir available it's necessary to use require 'tmpdir'.

You can use Dir.tmpdir with File.join to create a platform-independent temporary file −

require 'tmpdir'
   tempfilename = File.join(Dir.tmpdir, "tingtong")
   tempfile =, "w")
   tempfile.puts "This is a temporary file"

This code creates a temporary file, writes data to it, and deletes it. Ruby's standard library also includes a library called Tempfile that can create temporary files for you −

require 'tempfile'
   f ='tingtong')
   f.puts "Hello"
   puts f.path

Built-in Functions

Here are the ruby built-in functions to process files and directories −

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