need some critique.

run_sample.rb

people = People.new.add_dir('./data')
puts 'Output 1:'
people.sort(gender: :asc, last_name: :asc).print
puts
puts 'Output 2:'
people.sort(birthday: :asc).print
puts
puts 'Output 3:'
people.sort(last_name: :desc).print

text_parser.rb

#This program allows you to read from three types of data files that defines user record differently. 
#csv: Abercrombie, Neil, Male, Tan, 2/13/1943
#pipe: Smith | Steve | D | M | Red | 3-3-1985
#space: Kournikova Anna F F 6-3-1975 Red
# the differences are order of attributes(last_name, first_name etc, initial, gender, color, birthday), length of gender string and birthday format.
# the results need to be sortable on any two attributes.
# I wrote the following code. Would like to gather some feedback on if there is a better approach to this problem. Currently the class inheritance is not 
# really helping too much. Is there a better way to deal with different file format and keeping track of their peculiarity? I think I am also generating a parser 
# instance for every record being inserted. Not efficient either. I tried to convert methods of parser to class methods but then I got a little stuck on how to store the parser perculiarity as class_variable or constant. There must be something better.

# Thank you for your comments.

 
require 'date'

class Person
  attr_accessor :first_name, :last_name, :birthday, :fav_color, :gender
  
  def initialize opts={}
    @first_name = opts[:first_name]
    @last_name = opts[:last_name]
    @gender = (opts[:gender]=~/^F/i).nil? ? "Male" : "Female"
    @birthday = opts[:birthday]
    @fav_color = opts[:fav_color]
  end
  
  def print
     "#{@last_name} #{@first_name} #{@gender} #{@birthday.strftime("%-m/%-d/%Y")} #{@fav_color}"
  end
  
end


class People
  attr_accessor :people_list
  
  def initialize
      @people_list = [] 
  end
  
  def add_file *file_paths
    file_paths.each do |file_path|
      File.new(file_path).lines.each do |line|
        parser ||= get_parser_from_string(line)
        @people_list << parser.build_record(line)
      end
    end
    self
  end

  def add_dir dir_path
    add_file(*Dir[File.join(dir_path,"**/*.txt")]) 
    self
  end
  
  def get_parser_from_string string
    parser = case string
      when /\,/
        CSVParser.new
      when /\|/
        PipeParser.new
      when /\s/
        SpaceParser.new
      end
  end
  
  def sort opts={first_name: :asc}
    opts = opts.to_a
    
    if opts.count >= 2
      @people_list.sort! do |a,b| 
        [eval(opts[0][1] == :asc ? 'a' : 'b').send(opts[0][0].to_s),
         eval(opts[1][1] == :asc ? 'a' : 'b').send(opts[1][0].to_s)] <=>
        [eval(opts[0][1] == :asc ? 'b' : 'a').send(opts[0][0].to_s),
         eval(opts[1][1] == :asc ? 'b' : 'a').send(opts[1][0].to_s)]
      end
    elsif opts.count == 1
      @people_list.sort! do |a,b| 
         eval(opts[0][1] == :asc ? 'a' : 'b').send(opts[0][0].to_s) <=> 
         eval(opts[0][1] == :asc ? 'b' : 'a').send(opts[0][0].to_s) 
      end
    end
    self
  end
  
  def print
    @people_list.each{|x| puts x.print}
    nil
  end
  
end

class TextParser

  def build_record(line)
    info = line.split(@delimiter)
    info_hash = Hash[*@headers.map(&:to_sym).zip(info.map(&:strip)).flatten]
    info_hash[:birthday] = Date.strptime(info_hash[:birthday],@date_format) 
    Person.new info_hash
  end

end

class CSVParser < TextParser
  def initialize 
    @delimiter = ','
    @headers =  %w{last_name first_name gender fav_color birthday}
    @date_format = '%m/%d/%Y'
  end
end

class PipeParser < TextParser
  def initialize 
    @delimiter = '|'
    @headers = %w{last_name first_name middle_initial gender fav_color birthday}
    @date_format = '%m-%d-%Y'
  end
end

class SpaceParser < TextParser
  def initialize 
    @delimiter = ' '
    @headers = %w{last_name first_name middle_initial gender birthday fav_color}
    @date_format = '%m-%d-%Y'
  end
end

text_parser_test.rb

require 'test/unit'
require './text_parser.rb'

class PersonTest < Test::Unit::TestCase
  
  def test_initializer
    date = Date.strptime('31/3/1986','%d/%m/%Y')
    person = Person.new(first_name: 'Ben', last_name: 'Zhang', gender: 'Male', birthday: date, fav_color: 'Brown')
    assert_equal person.print, "Zhang Ben Male 3/31/1986 Brown" 
  end
  
end

class PeopleTest < Test::Unit::TestCase
  
  def test_initializer
    people = People.new
    assert_equal people.people_list, []
  end
  
  def test_add_one_file
    people = People.new
    people.add_file('data/pipe.txt')
    assert_equal people.people_list.count, 3
  end
  
  def test_add_multiple_files 
    people = People.new
    people.add_file('data/csv.txt','data/pipe.txt','data/space.txt')
    assert_equal people.people_list.count, 9
  end
  
  def test_add_directory
    people = People.new
    people.add_dir('./data')
    assert_equal people.people_list.count, 9
  end
  
  def test_get_parser_from_string
    people = People.new
    assert_equal people.get_parser_from_string('asdf,asdf,dsf\n').class, CSVParser
    assert_equal people.get_parser_from_string('asdf | asdf | dsf\n').class, PipeParser
    assert_equal people.get_parser_from_string('asdf asdf dsf\n').class, SpaceParser
    
  end
  
  def test_sort_by_one_attribute
    people = People.new.add_dir('./data')
    
    people.sort(gender: :asc)
    assert_equal people.people_list.first.gender, "Female"
    
    people.sort(gender: :desc)
    assert_equal people.people_list.first.gender, "Male"
    
    people.sort(first_name: :asc)
    assert_equal people.people_list.first.first_name, "Anna"
    
    people.sort(gender: :desc)
    assert_equal people.people_list.first.first_name, "Timothy"
    
  end
  
  def test_sorting_by_two_attributes
    people = People.new.add_dir('./data')
    
    people.sort(gender: :asc,first_name: :asc)
    assert_equal people.people_list.first.gender, "Female"
    assert_equal people.people_list.first.first_name, "Anna"
  
    people.sort(gender: :asc,first_name: :desc)
    assert_equal people.people_list.first.gender, "Female"
    assert_equal people.people_list.first.first_name, "Sue"
  
    people.sort(gender: :desc,first_name: :asc)
    assert_equal people.people_list.first.gender, "Male"
    assert_equal people.people_list.first.first_name, "Francis"
  
    people.sort(gender: :desc,first_name: :desc)
    assert_equal people.people_list.first.gender, "Male"
    assert_equal people.people_list.first.first_name, "Timothy"
  end
end

class ParserTest < Test::Unit::TestCase
  
  def test_csv_parser
    person = CSVParser.new.build_record("Kelly, Sue, Female, Pink, 7/12/1959")
    assert_equal person.print, "Kelly Sue Female 7/12/1959 Pink"
  end
  
  def test_pipe_parser
    person = PipeParser.new.build_record("Smith | Steve | D | M | Red | 3-3-1985")
    assert_equal person.print, "Smith Steve Male 3/3/1985 Red"
  end
  
  def test_space_parser
    person = SpaceParser.new.build_record("Seles Monica H F 12-2-1973 Black")
    assert_equal person.print, "Seles Monica Female 12/2/1973 Black"
  end

end

A few notes:

• I generally use parenthesis pretty extensively, so:

def add_file(*file_paths)

assert_equal(person.print, "Smith Steve Male 3/3/1985 Red")

It goes a long way towards clarifying what's getting called where.

• Try not to get meta. Your sort method looks really confusing and I'd imagine it can be structured a lot more cleanly without eval and save. More lines is much better than longer, more confusing lines.
• It looks like your classes might be a bit off. I think the People class might be better called something like PeopleManager.
• I generally shy away from accessing @ variables directly in the code. Instead use accessors. For example, @people_list would probably be better handled with attr_accessor or a private method:

def people_list
  @people_list
end

It doesn't make a big deal right now, but in the long run it makes your code more flexible, which is a quality most decent interviewers will be looking for.

Also, your tests don't really look very well structured to me. I like to see them nested inside of context blocks and focused on testing a specific method. Carbon Five's guide is really good:

http://blog.carbonfive.com/2010/10/21/rspec-best-practices/

It's for rspec, but I doubt it would be difficult to port it to Test::Unit.

The print method on Person doesn't actually print. It returns a string representation of that person. I'd probably rename it to to_s.

The print method on People might be better off being changed to to_s as well, removing the puts inside and just returning a string representation of the group.

TextParser#build_record is a little confusing to me. Line 97 in particular looks like it should be expanded across 3-4 lines. Again, clarity is more valuable than brevity. It's fun to come up with one line solutions to problems, but ultimately expanding it is a lot easier to maintain.

Whitespace: you use 2, 3, and 4 spaces to indent methods in different places, and some methods have an empty line before the end. Tighten that up.

I'd move the +get_parser_from_string+ into the TextParser class, so that People doesn't care about delimiters at all.