How to Read File in Ruby: Simple Syntax and Examples

In Ruby, you can read a file using File.read('filename') to get the entire content as a string, or use File.open('filename', 'r') with a block to read line by line. These methods let you easily access file contents for processing.

Syntax

The basic ways to read a file in Ruby are:

• File.read('filename'): Reads the whole file content as a single string.
• File.open('filename', 'r') do |file| ... end: Opens the file for reading and allows processing line by line inside the block.

Use 'r' mode to open the file for reading.

ruby

content = File.read('example.txt')

File.open('example.txt', 'r') do |file|
  file.each_line do |line|
    puts line
  end
end

Example

This example shows how to read the entire file content at once and then how to read it line by line.

ruby

File.write('example.txt', "Hello\nWorld\nRuby")

# Read whole file content
content = File.read('example.txt')
puts "Whole content:\n#{content}"

# Read file line by line
puts "\nReading line by line:" 
File.open('example.txt', 'r') do |file|
  file.each_line do |line|
    puts line.chomp
  end
end

Output

Whole content:
Hello
World
Ruby

Reading line by line:
Hello
World
Ruby

Common Pitfalls

Common mistakes when reading files in Ruby include:

• Not closing the file after opening it without a block, which can cause resource leaks.
• Using File.read on very large files, which can consume too much memory.
• Forgetting to handle exceptions if the file does not exist.

Using a block with File.open automatically closes the file after reading.

ruby

begin
  # Wrong: opens file but does not close it
  file = File.open('missing.txt', 'r')
  puts file.read
rescue Errno::ENOENT
  puts 'File not found!'
ensure
  file.close if file
end

# Right: using block ensures file is closed
begin
  File.open('missing.txt', 'r') do |file|
    puts file.read
  end
rescue Errno::ENOENT
  puts 'File not found!'
end

Output

File not found!

Quick Reference

Here is a quick summary of common file reading methods in Ruby:

Method	Description
File.read('filename')	Reads entire file content as a string
File.open('filename', 'r') { |f| ... }	Opens file for reading with automatic close
file.each_line { |line| ... }	Iterates over each line in the file
File.readlines('filename')	Returns an array of all lines in the file

Key Takeaways

Use File.read('filename') to quickly get all file content as a string.

Use File.open with a block to safely read files line by line and auto-close.

Always handle exceptions for missing files to avoid crashes.

Avoid reading very large files all at once to prevent memory issues.

File.open with a block is the safest and most common pattern for reading files.