Programming Tutorials

CGI class in Ruby

By: Jeeva in Ruby Tutorials on 2009-03-03  

CGI is a large class, providing several categories of methods, many of which are mixed in from other modules. Some of the documentation is in this class, some in the modules CGI::QueryExtension and CGI::HtmlExtension. CGI::Cookie handles cookies.

For queries, CGI provides methods to get at environmental variables, parameters, cookies, and multipart request data. For responses, CGI provides methods for writing output and generating HTML.

Read on for more details. Examples are provided at the bottom.


The CGI class dynamically mixes in parameter and cookie-parsing functionality, environmental variable access, and support for parsing multipart requests (including uploaded files) from the CGI::QueryExtension module.

Environmental Variables

The standard CGI environmental variables are available as read-only attributes of a CGI object. The following is a list of these variables:

  AUTH_TYPE               HTTP_HOST          REMOTE_IDENT
  HTTP_FROM               REMOTE_HOST

For each of these variables, there is a corresponding attribute with the same name, except all lower case and without a preceding HTTP_. content_length and server_port are integers; the rest are strings.


The method params() returns a hash of all parameters in the request as name/value-list pairs, where the value-list is an Array of one or more values. The CGI object itself also behaves as a hash of parameter names to values, but only returns a single value (as a String) for each parameter name.

For instance, suppose the request contains the parameter "favourite_colours" with the multiple values "blue" and "green". The following behaviour would occur:

  cgi.params["favourite_colours"]  # => ["blue", "green"]
  cgi["favourite_colours"]         # => "blue"

If a parameter does not exist, the former method will return an empty array, the latter an empty string. The simplest way to test for existence of a parameter is by the has_key? method.


HTTP Cookies are automatically parsed from the request. They are available from the cookies() accessor, which returns a hash from cookie name to CGI::Cookie object.

Multipart requests

If a request's method is POST and its content type is multipart/form-data, then it may contain uploaded files. These are stored by the QueryExtension module in the parameters of the request. The parameter name is the name attribute of the file input field, as usual. However, the value is not a string, but an IO object, either an IOString for small files, or a Tempfile for larger ones. This object also has the additional singleton methods:

local_path(): the path of the uploaded file on the local filesystem
original_filename(): the name of the file on the client computer
content_type(): the content type of the file


The CGI class provides methods for sending header and content output to the HTTP client, and mixes in methods for programmatic HTML generation from CGI::HtmlExtension and CGI::TagMaker modules. The precise version of HTML to use for HTML generation is specified at object creation time.

Writing output

The simplest way to send output to the HTTP client is using the out() method. This takes the HTTP headers as a hash parameter, and the body content via a block. The headers can be generated as a string using the header() method. The output stream can be written directly to using the print() method.

Generating HTML

Each HTML element has a corresponding method for generating that element as a String. The name of this method is the same as that of the element, all lowercase. The attributes of the element are passed in as a hash, and the body as a no-argument block that evaluates to a String. The HTML generation module knows which elements are always empty, and silently drops any passed-in body. It also knows which elements require matching closing tags and which don't. However, it does not know what attributes are legal for which elements.

There are also some additional HTML generation methods mixed in from the CGI::HtmlExtension module. These include individual methods for the different types of form inputs, and methods for elements that commonly take particular attributes where the attributes can be directly specified as arguments, rather than via a hash.

Examples of use

Get form values

  require "cgi"
  cgi =
  value = cgi['field_name']   # <== value string for 'field_name'
    # if not 'field_name' included, then return "".
  fields = cgi.keys            # <== array of field names

  # returns true if form has 'field_name'

CAUTION! cgi['field_name"] returned an Array with the old cgi.rb(included in ruby 1.6)

Get form values as hash

  require "cgi"
  cgi =
  params = cgi.params

cgi.params is a hash.

  cgi.params['new_field_name'] = ["value"]  # add new param
  cgi.params['field_name'] = ["new_value"]  # change value
  cgi.params.delete('field_name')           # delete param
  cgi.params.clear                          # delete all params

Save form values to file

  require "pstore"
  db ="query.db")
  db.transaction do
    db["params"] = cgi.params

Restore form values from file

  require "pstore"
  db ="query.db")
  db.transaction do
    cgi.params = db["params"]

Get multipart form values

  require "cgi"
  cgi =
  value = cgi['field_name']   # <== value string for 'field_name'                  # <== body of value
  value.local_path            # <== path to local file of value
  value.original_filename     # <== original filename of value
  value.content_type          # <== content_type of value

and value has StringIO or Tempfile class methods.

Get cookie values

  require "cgi"
  cgi =
  values = cgi.cookies['name']  # <== array of 'name'
    # if not 'name' included, then return [].
  names = cgi.cookies.keys      # <== array of cookie names

and cgi.cookies is a hash.

Get cookie objects

  require "cgi"
  cgi =
  for name, cookie in cgi.cookies
    cookie.expires = + 30
  cgi.out("cookie" => cgi.cookies) {"string"}

  cgi.cookies # { "name1" => cookie1, "name2" => cookie2, ... }

  require "cgi"
  cgi =
  cgi.cookies['name'].expires = + 30
  cgi.out("cookie" => cgi.cookies['name']) {"string"}

Print http header and html string to $DEFAULT_OUTPUT ($>)

  require "cgi"
  cgi ="html3")  # add HTML generation methods
  cgi.out() do
    cgi.html() do
      cgi.head{ cgi.title{"TITLE"} } +
      cgi.body() do
        cgi.form() do
          cgi.textarea("get_text") +
        end +
        cgi.pre() do
            "params: " + cgi.params.inspect + "\n" +
            "cookies: " + cgi.cookies.inspect + "\n" +
            ENV.collect() do |key, value|
              key + " --> " + value + "\n"

  # add HTML generation methods"html3")    # html3.2"html4")    # html4.01 (Strict)"html4Tr")  # html4.01 Transitional"html4Fr")  # html4.01 Frameset

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