How To Read and Write Files in Ruby (With Examples)

Ruby provides simple, elegant, and powerful ways to read from and write to files. The language emphasizes readability and developer happiness, making file I/O operations straightforward. This guide covers the most common and idiomatic approaches with practical examples.

Basic Concepts

In Ruby, file operations are typically performed using the File class or the shorter File.open method. Most file operations use ブロック to ensure files are properly closed after use, preventing resource leaks.

Opening a File

The recommended way is to use a block form:

ruby
File.open("filename.txt", "r") do |file|
  # work with the file
終わり
# file is automatically closed here

Without a block, you must manually close the file:

ruby
file = File.open("filename.txt", "r")
# ... operations ...
file.close

File Modes

The second argument to File.open is the mode string:

Add “b” for binary mode on Windows (e.g., “rb”, “wb”).

Reading Files in Ruby

1. Read Entire File as a String

ruby
content = File.read("example.txt")
puts content

Or using File.open:

ruby
content = File.open("example.txt", "r") { |f| f.read }

2. Read Entire File into an Array of Lines

ruby
lines = File.readlines("example.txt")
lines.each { |line| puts line }

# Or more idiomatically:
File.readlines("example.txt").each do |line|
  puts "Line: #{line.chomp}"
end

3. Read Line by Line (Most Memory-Efficient)

Ideal for large files:

ruby
File.open("large_file.txt", "r") do |file|
  file.each_line do |line|
    puts line.chomp.upcase
  end
end

Or using the shorter foreach:

ruby
File.foreach("large_file.txt") do |line|
  process(line.chomp)
end

4. Read Specific Number of Bytes or Lines

ruby
File.open("data.bin", "rb") do |f|
  header = f.read(16)      # read first 16 bytes
  puts header
end

# Read first 5 lines
first_five = File.foreach("log.txt").first(5)

5. Read with Encoding Specified

ruby
content = File.read("utf8_file.txt", encoding: "UTF-8")
# or
File.open("file.txt", "r:UTF-8") do |f|
  f.each_line { |line| puts line }
end

Writing Files in Ruby

1. Write a String to a File (Overwrites)

ruby
File.write("output.txt", "Hello, Ruby!\nThis is a new file.")

This is the simplest and most common way.

2. Write Using File.open (Block Form)

ruby
File.open("greeting.txt", "w") do |file|
  file.puts "Hello World"
  file.puts "Welcome to Ruby file I/O"
  file.write "This line is written without newline"
  file << " << This uses the shovel operator\n"
end

Methods available inside the block:

• puts – adds newline
• print – no newline
• write – no newline (alias for <<)
• << – shovel operator

3. Append to a  File

ruby
File.open("log.txt", "a") do |file|
  file.puts "#{Time.now}: Application started"
end

# Or shorter:
File.write("log.txt", "New log entry\n", mode: "a")

4. Write Array of Lines

ruby
lines = ["First line", "Second line", "Third line"]

File.open("lines.txt", "w") do |file|
  lines.each { |line| file.puts line }
end

# Or even shorter:
File.write("lines.txt", lines.join("\n") + "\n")

Practical Examples

Example 1: Copy a File

ruby
def copy_file(source, destination)
  File.write(destination, File.read(source))
end

copy_file("source.txt", "backup.txt")

Or in binary mode:

ruby
File.write("backup.bin", File.read("original.bin", mode: "rb"), mode: "wb")

Example 2: Simple CSV Writer

ruby
data = [
  ["Name", "Age", "City"],
  ["Alice", 30, "New York"],
  ["Bob", 25, "London"]
]

File.open("people.csv", "w") do |file|
  data.each do |row|
    file.puts row.join(",")
  end
end

Example 3: Simple CSV Reader

ruby
File.foreach("people.csv") do |line|
  name, age, city = line.chomp.split(",")
  puts "#{name} is #{age} years old and lives in #{city}"
end

Example 4: Count Lines in a File

ruby
line_count = File.foreach("large.txt").inject(0) { |count, _| count + 1 }
puts "Total lines: #{line_count}"

# Or more simply (loads all lines into memory):
puts File.readlines("file.txt").size

Example 5: Safe File Writing (Atomic Write)

To avoid corrupted files on crash, write to temporary file first:

ruby
require 'tempfile'

Tempfile.create('safe_write') do |temp_file|
  temp_file.puts "Important data"
  temp_file.puts "More important data"
  temp_file.flush

  # Now atomically replace the target file
  File.rename(temp_file.path, "important.txt")
end

Working with Paths

用途 File.expand_path, File.join、 そして Pathname for robust path handling:

ruby
require 'pathname'

path = Pathname.new("/tmp") / "myapp" / "log.txt"
File.write(path, "Log entry")

# Or using File.join
full_path = File.join(Dir.home, "Documents", "notes.txt")

Error Handling

Always handle potential errors:

ruby
begin
  content = File.read("nonexistent.txt")
rescue Errno::ENOENT
  puts "File not found!"
rescue Errno::EACCES
  puts "Permission denied!"
rescue => e
  puts "Error: #{e.message}"
end

Or more idiomatically:

ruby
if File.exist?("config.txt")
  config = File.read("config.txt")
else
  puts "Config file missing, using defaults"
end

Best Practices to Read and Write Files in Ruby

1. Always use block form when possible – ensures file closure.
2. 用途 File.read/File.write for simple whole-file operations.
3. 用途 foreach for large files to avoid loading everything into memory.
4. Specify encoding when working with non-ASCII text.
5. Check file existence before reading if needed.
6. Use appropriate modes (especially binary mode on Windows).
7. Handle exceptions gracefully in production code.

結論

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