Ruby: The Basics of Reading a File

This howto covers the basics of reading a text file in Ruby. As with most scripting languages there is more than one way to do this, but hopefully the most straight forward methods are covered here. Below are three different examples of reading a text file. Each example reads a file and prints the file along with the line numbers for that file.

readfile.rb

   1:#!/usr/bin/ruby -w
   2:
   3:# Created by Michael Williams 12/19/2005
   4:# Licensed under Create Commons Attribution License
   5:
   6:# Example 1 - Read File and close
   7:counter = 1
   8:file = File.new("readfile.rb", "r")
   9:while (line = file.gets)
  10:    puts "#{counter}: #{line}"
  11:    counter = counter + 1
  12:end
  13:file.close
  14:
  15:# Example 2 - Pass file to block
  16:File.open("readfile.rb", "r") do |infile|
  17:    while (line = infile.gets)
  18:        puts "#{counter}: #{line}"
  19:        counter = counter + 1
  20:    end
  21:end
  22:
  23:# Example 3 - Read File with Exception Handling
  24:counter = 1
  25:begin
  26:    file = File.new("readfile.rb", "r")
  27:    while (line = file.gets)
  28:        puts "#{counter}: #{line}"
  29:        counter = counter + 1
  30:    end
  31:    file.close
  32:rescue => err
  33:    puts "Exception: #{err}"
  34:    err
  35:end
  36:

Download source for: readfile.rb

Different Ways to Read a File

The first example on lines 4-11 shows a method for reading the file that would seem pretty typical. On line 6, a file object is created with the new method and the file is open for reading. The file name and read/write mode is specified. Your file mode options are as follows:

Mode	Description
r	Read-only, starts at beginning of file (default mode).
r+	Read-write, starts at beginning of file.
w	Write-only, truncates existing file to zero length or creates a new file for writing.
w+	Read-write, truncates existing file to zero length or creates a new file for reading and writing.
a	Write-only, starts at end of file if file exists, otherwise creates a new file for writing.
a+	Read-write, starts at end of file if file exists, otherwise creates a new file for reading and writing.

The while loop is defined on line 7. Notice that the gets method is used to read each line. Ruby has a readline method as well, but it raises an exception when the end of the file is reached. The gets method returns nil when the end of the file is reached. Thus it is preferable as no error is generated.

The second example reads the file by passing it to a block. Notice that on line 14 the open method is used instead of new. This passes the file right to the block without having to create an object. The advantage of this approach is that you don't have to close the file with the close method.

The third example shows you how to gracefully catch an exception and exit the application if something goes wrong. This is similar to a die statement in Perl for a try/catch block in Java. The code is surrounded by a begin and end block. A rescue statement follows the code and will catch any exception thrown and print the error message. Of course there is more to Ruby exceptions, but that is not covered in this how to.

^{Content last updated 10/2012}