encoding-line-break-test.rb

#!/usr/bin/ruby -w

FILE_NAME = "x"

printf "%-20s %p\n%-20s %p\n",
  "Default external", Encoding.default_external,
  "Default internal", Encoding.default_internal

# p File.instance_methods.grep(/enc|opt/)

def show(line)
  printf "%-20s %p\n", line.encoding, line
end

# prepare
File.open(FILE_NAME, "w", encoding: "BINARY") {|io| io.write("1\n2\r3\rn4\n") }

puts

File.open FILE_NAME, universal_newline: true do |io|
  printf "%-20s %p\n%-20s %p\n",
    "external", io.external_encoding,
    "internal", io.internal_encoding

  io.each_line do |line|
    show line
  end
end

puts

File.open FILE_NAME, internal_encoding: Encoding.default_external, universal_newline: true do |io|
  printf "%-20s %p\n%-20s %p\n",
    "external", io.external_encoding,
    "internal", io.internal_encoding

  io.each_line do |line|
    show line
  end
end

puts

File.open FILE_NAME, internal_encoding: "ISO-8859-1", universal_newline: true do |io|
  printf "%-20s %p\n%-20s %p\n",
    "external", io.external_encoding,
    "internal", io.internal_encoding

  io.each_line do |line|
    show line
  end
end

output.txt

$ ./encoding-line-break-test.rb
Default external     #<Encoding:UTF-8>
Default internal     nil

external             #<Encoding:UTF-8>
internal             nil
UTF-8                "1\n"
UTF-8                "2\r3\rn4\n"

external             #<Encoding:UTF-8>
internal             nil
UTF-8                "1\n"
UTF-8                "2\r3\rn4\n"

external             #<Encoding:UTF-8>
internal             #<Encoding:ISO-8859-1>
ISO-8859-1           "1\n"
ISO-8859-1           "2\n"
ISO-8859-1           "3\n"
ISO-8859-1           "n4\n"