Reads and returns a string containing bytes read from the stream,
beginning at the current position;
advances the position by the count of bytes read.

With no arguments given,
reads all remaining bytes in the stream;
returns a new string containing bytes read:

  strio = StringIO.new('Hello') # Five 1-byte characters.
  strio.read                    # => "Hello"
  strio.pos                     # => 5
  strio.read                    # => ""
  StringIO.new('').read         # => ""

With non-negative argument +maxlen+ given,
reads +maxlen+ bytes as available;
returns a new string containing the bytes read, or +nil+ if none:

  strio.rewind
  strio.read(3) # => "Hel"
  strio.read(3) # => "lo"
  strio.read(3) # => nil

  russian = 'Привет' # Six 2-byte characters.
  russian.b
  # => "\xD0\x9F\xD1\x80\xD0\xB8\xD0\xB2\xD0\xB5\xD1\x82"
  strio = StringIO.new(russian)
  strio.read(6) # => "\xD0\x9F\xD1\x80\xD0\xB8"
  strio.read(6) # => "\xD0\xB2\xD0\xB5\xD1\x82"
  strio.read(6) # => nil

  japanese = 'こんにちは'
  japanese.b
  # => "\xE3\x81\x93\xE3\x82\x93\xE3\x81\xAB\xE3\x81\xA1\xE3\x81\xAF"
  strio = StringIO.new(japanese)
  strio.read(9) # => "\xE3\x81\x93\xE3\x82\x93\xE3\x81\xAB"
  strio.read(9) # => "\xE3\x81\xA1\xE3\x81\xAF"
  strio.read(9) # => nil

With argument +max_len+ as +nil+ and string argument +out_string+ given,
reads the remaining bytes in the stream;
clears +out_string+ and writes the bytes into it;
returns +out_string+:

  out_string = 'Will be overwritten'
  strio = StringIO.new('Hello')
  strio.read(nil, out_string) # => "Hello"
  strio.read(nil, out_string) # => ""

With non-negative argument +maxlen+ and string argument +out_string+ given,
reads the +maxlen bytes from the stream, as availble;
clears +out_string+ and writes the bytes into it;
returns +out_string+ if any bytes were read, or +nil+ if none:

  out_string = 'Will be overwritten'
  strio = StringIO.new('Hello')
  strio.read(3, out_string) # => "Hel"
  strio.read(3, out_string) # => "lo"
  strio.read(3, out_string) # => nil

  out_string = 'Will be overwritten'
  strio = StringIO.new(russian)
  strio.read(6, out_string) # => "При"
  strio.read(6, out_string) # => "вет"
  strio.read(6, out_string) # => nil
  strio.rewind
  russian.b
  # => "\xD0\x9F\xD1\x80\xD0\xB8\xD0\xB2\xD0\xB5\xD1\x82"
  strio.read(3)             # => "\xD0\x9F\xD1"
  strio.read(3)             # => "\x80\xD0\xB8"

  out_string = 'Will be overwritten'
  strio = StringIO.new(japanese)
  strio.read(9, out_string) # => "こんに"
  strio.read(9, out_string) # => "ちは"
  strio.read(9, out_string) # => nil
  strio.rewind
  japanese.b
  # => "\xE3\x81\x93\xE3\x82\x93\xE3\x81\xAB\xE3\x81\xA1\xE3\x81\xAF"
  strio.read(4)             # => "\xE3\x81\x93\xE3"
  strio.read(4)             # => "\x82\x93\xE3\x81"

Related: #gets, #readlines.