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📁 File & IO — File reading, writing, and directory operations

Ruby file I/O uses blocks that auto-close handles (RAII pattern). Three-argument open prevents injection attacks.

📋 Methods & Functions

Name	Signature	Description
File.read	File.read(path) → String	Read entire file as string
File.readlines	File.readlines(path) → Array	Read file as array of lines
File.write	File.write(path, content) → Integer	Write string to file
File.open	File.open(path, mode) {\|f\| block}	Open file with block (auto-close)
File.exist?	File.exist?(path) → bool	True if path exists
File.directory?	File.directory?(path) → bool	True if path is directory
File.size	File.size(path) → Integer	Return file size in bytes
File.delete	File.delete(path) → Integer	Delete file
File.rename	File.rename(old, new) → 0	Rename / move file
File.basename	File.basename(path, ext) → String	Return filename part
File.dirname	File.dirname(path) → String	Return directory part
File.extname	File.extname(path) → String	Return extension (.txt etc.)
File.join	File.join(*parts) → String	Join path components
File.expand_path	File.expand_path(path) → String	Return absolute path
Dir.mkdir	Dir.mkdir(path) → 0	Create directory
Dir.glob	Dir.glob(pattern) → Array	Return files matching pattern
Dir.pwd	Dir.pwd → String	Return current working directory
IO.foreach	IO.foreach(path) {\|line\| block}	Iterate file lines without loading all
IO.readlines	IO.readlines(path) → Array	Read all lines
IO.write	IO.write(path, str) → Integer	Write string to file
puts/print	puts(obj) / print(obj)	Write to stdout with/without newline
gets	gets → String	Read one line from stdin
STDIN/STDOUT	IO objects	Standard input/output streams
require	require 'csv'	Load standard or gem library
StringIO	require 'stringio'; StringIO.new(str)	In-memory IO stream

💡 Example Program

Run with ruby script.rb.

require "tmpdir"

# Write a file
File.write("students.txt", <<~DATA)
  Alice 95
  Bob   87
  Carol 92
  Dave  78
DATA

# Read entire file
puts File.read("students.txt")

# Iterate lines without loading all into memory
IO.foreach("students.txt") do |line|
  name, score = line.split
  puts "#{name} => #{score.to_i >= 90 ? "A" : "B"}" if name
end

# Open with block (auto-close)
File.open("students.txt", "a") { |f| f.puts "Eve   98" }

# readlines + processing
top = File.readlines("students.txt")
       .map { |l| l.split }
       .select { |parts| parts.size == 2 && parts[1].to_i >= 90 }
       .map { |name, score| "#{name}(#{score})" }
puts "Top students: #{top.join(", ")}"

# File metadata
puts "Size: #{File.size("students.txt")} bytes"
puts "Basename: #{File.basename("students.txt", ".txt")}"
puts "Absolute: #{File.expand_path("students.txt")}"

# Dir.glob
puts Dir.glob("*.txt").inspect

File.delete("students.txt")
puts "Done."
# ruby file_demo.rb

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