File.read / write / open

# Reads the entire file contents as a string.
content = File.read("file_path")
content = File.read("file_path", encoding: "UTF-8")

# Writes a string to a file (overwrites existing content).
File.write("file_path", content)

# Opens a file and operates on it within a block (automatically closed).
File.open("file_path", "mode") do |f|
  # File operations
end

# Reads the file line by line.
File.foreach("file_path") { |line| process }

# Writes to a file.
File.write("sample.txt", "Hello, Ruby!\nThis is line 1.\n")
puts "Write complete"

# Reads the file contents.
content = File.read("sample.txt")
puts content

# Uses File.open with a block for safe file handling.
File.open("log.txt", "a") do |f|
  f.puts "#{Time.now}: Log entry appended."
end

# Reads and processes the file line by line.
File.open("sample.txt", "r") do |f|
  f.each_line.with_index(1) do |line, number|
    puts "Line #{number}: #{line.chomp}"
  end
end

# Processes line by line with foreach (returns an enumerator without a block).
line_count = File.foreach("sample.txt").count
puts "Line count: #{line_count}"

File operations in Ruby are performed through the File class. For small files, File.read and File.write are the most concise options. Using File.open with a block automatically closes the file when the block finishes, preventing you from forgetting to close it.

If you use File.open without a block, you must call close explicitly. Leaving a file open prevents resources from being released, which can cause problems when handling a large number of files. When reading large files all at once, memory usage increases, so consider using foreach to process the file line by line instead.