Kernel#open

open(path, mode="r") -> file or io

The open method is versatile, opening files for reading/writing and also creating IO pipes for command execution. It’s a core method for file and process interaction.

Opening Files

# Read mode (default)
file = open("data.txt")
content = file.read
file.close

# Block form (auto-closes)
open("data.txt") { |f| f.read }

File Modes

# Various modes
open("file.txt", "r")   # Read
open("file.txt", "w")   # Write (truncates)
open("file.txt", "a")   # Append
open("file.txt", "r+")  # Read/write
open("file.txt", "w+")  # Read/write (truncates)
open("file.txt", "a+")  # Read/append

Command Execution with Pipes

# Read output from command
output = open("|ls -la") { |p| p.read }
puts output

# Write to command
open("|tr a-z A-Z", "w") { |p| p.puts "hello world" }

Practical Examples

Reading Files

# Line by line
open("file.txt") do |f|
  f.each_line { |line| puts line }
end

# All content
content = open("file.txt", &:read)

Writing Files

# Write data
open("output.txt", "w") do |f|
  f.puts "Hello"
  f.puts "World"
end

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

Processing CSV

# Simple CSV reading
open("data.csv", "r") do |f|
  f.each_line do |line|
    fields = line.chomp.split(",")
    # Process fields
  end
end

Network-like Operations

# Reading from URL (with open-uri)
require 'open-uri'
open("https://example.com") { |f| puts f.read }

Binary Files

# Read binary data
open("image.png", "rb") do |f|
  data = f.read
  # Process binary
end

# Write binary
open("output.bin", "wb") do |f|
  f.write(bytes)
end

With Encoding

# Specify encoding
open("file.txt", "r:UTF-8") { |f| f.read }
open("file.txt", "r:UTF-16LE:UTF-8") { |f| f.read }

Error Handling

begin
  file = open("missing.txt")
  # Process
rescue Errno::ENOENT => e
  puts "File not found: #{e.message}"
ensure
  file.close if file
end

The open method is fundamental to Ruby’s IO system, supporting everything from simple file operations to complex pipe-based inter-process communication.

See Also

• Kernel#gets
• Kernel#readlines
• Kernel#load