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How to Work with Files in Ruby

· 2 min read · Updated March 19, 2026 · beginner
ruby file io cookbook

Ruby provides robust tools for working with files. Here are practical recipes for everyday file operations.

Reading Files

Problem

You need to read file contents efficiently.

Solution

# Read entire file into string
content = File.read("config.txt")

# Read into array of lines
lines = File.readlines("log.txt")

# Process line by line (memory efficient)
File.foreach("large_log.txt") do |line|
  puts line if line.include?("ERROR")
end

# With block ensures file closes automatically
File.open("input.txt", "r") do |file|
  file.each_line { |line| process(line) }
end

Writing Files

Problem

You need to write content to a file.

Solution

# Overwrite file
File.write("output.txt", "Hello, world!")

# Append to file
File.open("log.txt", "a") do |file|
  file.puts "#{Time.now} - Log entry"
end

File Information

Problem

You need to check if a file exists or get its metadata.

Solution

File.exist?("config.yml")    # => true/false
File.file?("data.txt")       # => true if regular file
File.directory?("logs/")     # => true if directory
File.size("data.bin")        # => size in bytes
File.mtime("script.rb")      # => modification time

Paths

Problem

You need to manipulate file paths safely across platforms.

Solution

require "pathname"

base = Pathname.new("/home/user/project")
config = base + "config" + "settings.yml"

config.dirname   # directory path
config.basename  # filename
config.extname   # extension like .yml
config.exist?    # true/false

Summary

Use File.read and File.write for simple tasks. Process large files line by line with File.foreach. Pathname handles complex path operations reliably.

See Also