class CGI


The Common Gateway Interface (CGI) is a simple protocol for passing an HTTP request from a web server to a standalone program, and returning the output to the web browser. Basically, a CGI program is called with the parameters of the request passed in either in the environment (GET) or via $stdin (POST), and everything it prints to $stdout is returned to the client.

This file holds the CGI class. This class provides functionality for retrieving HTTP request parameters, managing cookies, and generating HTML output.

The file CGI::Session provides session management functionality; see that class for more details.

See for more information on the CGI protocol.


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. See CGI::Cookie for specific information on handling cookies, and cgi/session.rb (CGI::Session) for information on sessions.

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

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:


the path of the uploaded file on the local filesystem


the name of the file on the client computer


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 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