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<td class="pagetitle">CommandLine::OptionParser</a></td>
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<h1>Welcome to OptionParser</h1>
<ul>
<li>Copyright &#169; 2005 Jim Freeze

</li>
<li>Author: Jim Freeze

</li>
<li><a href="http://www.freeze.org/ruby/optionparser/license.txt">License</a>
</li>
<li><a href="http://rubyforge.org/projects/optionparser/">Project Page</a>
</li>
<li><a href="http://rubyforge.org/frs/?group_id=632&release_id=2345">Download</a>
</li>
</ul>
<tt>OptionParser</tt> is
designed to be a flexible command line parser with a Ruby look and feel to
it. <tt>OptionParser</tt> got
its birth from the need for a parser that is standards compliant, yet
flexible. <tt>OptionParser</tt>
supports the standard command line styles of <tt>Unix</tt>, <tt>Gnu</tt>
and <tt>X Toolkit</tt>, but also lets you break those rules.

<p>
<tt>OptionParser</tt> is
not a port of a traditional command line parser, but it is written to meet
the feature requirements of traditional command line parsers. When using it
as a library, you should notice that it is expressive, supports
Ruby&#8217;s blocks and lambda&#8217;s, and is sprinkled with a little bit
of magic.
</p>
<p>
While the library can be used by itself, it is also designed to work with
the <tt>CommandLine::Application</tt> class. These tools work together to
facilitate the generation of a sophisticated (batch oriented) application
user interface in a matter of minutes.
</p>
<p>
If you need a refresher on the traditional option parsing schemes, see
&quot;Traditional Option Parsing Schemes&quot; below.
</p>
<h1>Jumping Right In</h1>
<h1>OptionParser Usage</h1>
<p>
The OptionParser
library consists of three classes, <tt>Option</tt>, <tt>OptionParser</tt> and
<tt>OptionData</tt>. For each option an <tt>Option</tt> object is created.
When you are ready to prepare for command line parsing, these options are
collected into an array and fed to <tt>OptionParser</tt>.
This <tt>OptionParser</tt>
object controls the type of option scheme that is implemented. When it
comes time to parse a command line, call the method <tt>Option#parse</tt>.
This will parse any array, but parses ARGV by default. The result is an
<tt>OptionData</tt> object. This object can be used from which to extract
values or it can be passed to another class as a fully encapsulated data
object.
</p>
<h2>Getting Started</h2>
<h3>Installing</h3>
<pre>
  gem install -r OptionParser
</pre>
<h3>Loading the library</h3>
<pre>
  require 'rubygems'
  require 'commandline/optionparser'
  include CommandLine
</pre>
<h3>Using Option Parser</h3>
<p>
An option is created with the following syntax:
</p>
<pre>
  opt = Option.new([options], &lt;properties&gt;)
</pre>
<p>
The options can be <tt>:flag</tt> or <tt>:posix</tt>. <tt>:flag</tt> means
that the option is a mode flag and does not take any arguments.
<tt>:posix</tt> means that <tt>Option</tt> will validate the properties to
ensure they are posix compliant.
</p>
<p>
An option object has six properties. Four of these properties define
attributes of the object. The last two define <em>actions</em> that are
taken when a command line is parsed.
</p>
<ol>
<li>:names

</li>
<li>:arg_arity

</li>
<li>:opt_description

</li>
<li>:arg_description

</li>
<li>:opt_found

</li>
<li>:opt_not_found

</li>
</ol>
<p>
It is not necessary to set values for all of these properties. Some are set
automatically, as we&#8217;ll see below.
</p>
<h3>Posix</h3>
<p>
The default <tt>Option</tt> object is non-posix.
</p>
<pre>
    op1  = OptionParser.new(:posix, opts)
    op2  = OptionParser.new(opts)
    op1.posix  #=&gt; true
    op2.posix  #=&gt; false
</pre>
<h3>Mode-Flag</h3>
<p>
To create a mode flag, that is, an option that is either true or false
depending if it is seen on the command line or not, we could write:
</p>
<pre>
  opt_debug = Option.new(
    :names           =&gt; %w(--debug -d),       # the flag has two names
    :arg_arity       =&gt; [0,0],                # this says take no arugments
    :opt_description =&gt; &quot;Sets debug to true&quot;,
    :arg_description =&gt; &quot;&quot;,
    :opt_found       =&gt; true,                 # true if seen on command line
    :opt_not_found   =&gt; false                 # false if not seen on command line
  )
</pre>
<p>
Now, this is a lot of work just for a common mode-flag. However, there is a
shorter way:
</p>
<pre>
  opt = Option.new(:flag, :names =&gt; %w(--debug -d))
</pre>
<p>
When <tt>Option</tt> sees the :flag option, it makes some assignments
behind the scenes and what you are left with is:
</p>
<pre>
    :names           =&gt; [&quot;--debug&quot;, &quot;-d&quot;]
    :arg_arity       =&gt; [0, 0]
    :opt_description =&gt; &quot;Sets debug to true.&quot;  # debug is taken from the first name
    :arg_description =&gt; &quot;&quot;
    :opt_found       =&gt; true
    :opt_not_found   =&gt; false
</pre>
<p>
For a common option like a mode-flag, <tt>Option</tt> will use the first
option &#8216;word&#8217; it finds in the :names list and use that in the
automatic option text. Of course, if you don&#8217;t want any text, just
set the option description to an empty string:
</p>
<pre>
  :opt_description =&gt; &quot;&quot;.
</pre>
<h3>Option Arguments</h3>
<p>
If an option is not a mode flag, then it takes arguments. Most option
parsers only permit a single argument per option flag. If your application
needs multiple arguments, the standard method is just to repeat the option
multiple times, once for each required argument. For example, if I need to
pass two files to an application I would need something like:
</p>
<pre>
  myapp -f file1 -f file2
</pre>
<p>
But, it would be cleaner if the command line could be expressed as:
</p>
<pre>
  myapp -f file1 file2
</pre>
<p>
Well, no longer do you have to suffer with thirty-year old option parser
technology. <tt>OptionParser</tt>
permits multiple arguments per option flag and the number of arguments can
be defined to be variable.
</p>
<p>
To define an option that takes 1 or more arguments, the following can be
done:
</p>
<pre>
  opt = Option.new(:names =&gt; &quot;--file&quot;, :arg_arity =&gt; [1,-1])
</pre>
<p>
Let&#8217;s say the option required at least two arguments, but not more
than five. This is defined with:
</p>
<pre>
  opt = Option.new(:names =&gt; &quot;--file&quot;, :arg_arity =&gt; [2,5])
  OptionParser.new(opt).parse

  % myapp --file file1                    # exception raised
  % myapp --file file1 file2              # ok
  % myapp --file file1 file2 file3        # ok
  % myapp --file f1 f2 f3 f4 f5 f6        # f6 remains on the command line
</pre>
<p>
This ability is handy on occassions where an option argument is
&#8216;optional&#8217;.
</p>
<pre>
  myapp --custom                 # no args, uses $HOME/.myapprc
  myapp --custom my_custom_file  # uses my_custom_file
</pre>
<p>
This type of option is defined by:
</p>
<pre>
  opt = Option.new(:names =&gt; &quot;--custom&quot;, :arg_arity =&gt; [0,1])
</pre>
<p>
If the <tt>:arg_arity</tt> is not satisfied, an exception is raised.
</p>
<h3>Actions</h3>
<p>
The option properties <tt>:opt_found</tt> and <tt>:opt_not_found</tt> are
the source of the value returned for an option when it is parsed. These
properties can be either an object or a proc/lambda. If they are an object,
then the stored object is simply returned. If they are lambdas, then the
stored value is the return value of the proc/lambda. So, the following will
have the same result:
</p>
<pre>
  opt_debug = Option.new(:flag
    :names           =&gt; %w(--debug -d),
    :opt_found       =&gt; true,
    :opt_not_found   =&gt; false
  )

  opt_debug = Option.new(:flag
    :names           =&gt; %w(--debug -d),
    :opt_found       =&gt; lambda { true },
    :opt_not_found   =&gt; lambda { false }
  )
</pre>
<p>
Notice that there is no need to set an instance variable to a default
value. Normally one does:
</p>
<pre>
  @debug = false
  # option setup
  ... parse the commandline
  @debug = true if parse_results[&quot;--debug&quot;]
</pre>
<p>
But with <tt>OptionParser</tt>, one
has the capability of doing the following:
</p>
<pre>
  opt_debug = Option.new(:flag, :names =&gt; %w(--debug -d))
  ... parse the commandline
  @debug = option_data[:debug]  # value is set without need for default

  # or

  opt_debug = Option.new(:flag
    :names           =&gt; %w(--debug -d),
    :opt_found       =&gt; lambda { @debug = true },
    :opt_not_found   =&gt; lambda { @debug = false }
  )
  # do nothing, variable already set.
</pre>
<p>
I find this much easier to manage than having to worry about setting
default behaviour. Now that we know how to create options, let&#8217;s move
on to the commandline parser.
</p>
<h2>OptionParser</h2>
<p>
Once the options are defined, we load them into an <tt>OptionParser</tt> and
parse the command line. The syntax for creating an <tt>OptionParser</tt>
object is:
</p>
<pre>
  OptionParser.new(prop_flags, option)
  OptionParser.new(prop_flags, [options])
  OptionParser.new(option)
  OptionParser.new([options])
</pre>
<p>
where the possible property flags are:
</p>
<pre>
  :posix
  :unknown_options_action =&gt; :collect | :ignore | :raise
</pre>
<p>
If you want to parse posix, you must specify so. <tt>OptionParser</tt> will
not assume posix mode just because all of the options are posix options.
This allows you to use posix only options but not require the strict
parsing rules.
</p>
<p>
Below are a few examples of creating an <tt>OptionParser</tt>
object:
</p>
<pre>
  opt = Option.new(:flag, :names =&gt; %w(-h))
  op1 = OptionParser.new(:posix, opt)
  op2 = OptionParser.new(opt)
</pre>
<p>
or
</p>
<pre>
  opts = []
  opts &lt;&lt; Option.new(:flag, :names =&gt; %w(--help h))
  opts &lt;&lt; Option.new(:flag, :names =&gt; %w(--debug d))
</pre>
<p>
Options may be added to an <tt>OptionParser</tt> by
three different methods:
</p>
<pre>
  # Options added as arguments during OptionParser construction
  op = OptionParser.new(opt1, opt2)
  op = OptionParser.new([opt1, opt2])
</pre>
<p>
or
</p>
<pre>
  # Options added in a block constructor
  op = OptionParser.new { |o| o &lt;&lt; opts }
</pre>
<p>
or
</p>
<pre>
  # Options added to an existing OptionParser object
  op  = OptionParser.new
  op &lt;&lt; opts
</pre>
<h3>Parsing the Command Line</h3>
<p>
Parsing the command line is as simple as calling <tt>#parse</tt>:
</p>
<pre>
  option_data = op.parse
</pre>
<h3>Printing an Option Summary</h3>
<p>
A <tt>OptionParser</tt> with
a complete set of options added to it defines the human interface that your
application presents to a user. Therefore, the parser should be able to
provide a nicely formatted summary for the user.
</p>
<p>
An example is shown below with its corresponding output:
</p>
<pre>
  require 'rubygems'
  require 'commandline/optionparser'
  include CommandLine
  puts OptionParser.new { |o|
    o &lt;&lt; Option.new(:flag, :names =&gt; %w[--debug -d])
    o &lt;&lt; Option.new(:flag, :names =&gt; %w[--help  -h],
              :opt_description =&gt; &quot;Prints this page.&quot;)
    o &lt;&lt; Option.new(:names =&gt; %w[--ouput -o],
              :opt_description =&gt; &quot;Defines the output file.&quot;,
              :arg_description =&gt; &quot;output_file&quot;)
    o &lt;&lt; Option.new(:names =&gt; %w[--a-long-opt --with-many-names -a -A],
              :arg_arity =&gt; [2,-1],
              :opt_description =&gt; &quot;Your really long description here.&quot;,
              :arg_description =&gt; &quot;file1 file2 [file3 ...]&quot;)
  }.to_s
</pre>
<p>
Generates the output:
</p>
<pre>
  OPTIONS

      --debug,-d
          Sets debug to true.

      --help,-h
          Prints this page.

      --ouput,-o output_file
          Defines the output file.

      --a-long-opt,--with-many-names,-a,-A file1 file2 [file3 ...]
          Your really long description here.
</pre>
<h2>Option Data</h2>
<p>
The <tt>OptionData</tt> is the return value of <tt>OptionParser#parse</tt>.
The parsing results for each option are accessed with the bracket notation
#[].
</p>
<pre>
  opt = Option.new(:posix,
                   :names =&gt; %w(-r),
                   :opt_found =&gt; OptionParser::GET_ARGS)
  od = OptionParser.new(:posix, opt).parse([&quot;-rubygems&quot;])
  od[&quot;-r&quot;] #=&gt; &quot;ubygems&quot;

  od = OptionParser.new(:posix, opt).parse([&quot;-r&quot;, &quot;ubygems&quot;])
  od[&quot;-r&quot;] #=&gt; &quot;ubygems&quot;
</pre>
<p>
<tt>OptionData</tt> behaves similar to a hash object in that the parsed
option data is accessed with #[] where the key is the first item in the
:names array of each option. An option cannot access its parsed values
using just any of its names.
</p>
<pre>
  od = OptionParser.new { |o|
    o &lt;&lt; Option.new(:flag, :names =&gt; %w(--valid --notvalid))
    o &lt;&lt; Option.new(:flag, :names =&gt; %w(--first --second))
  }.parse(%w(--notvalid --second))
  od[&quot;--valid&quot;]    #=&gt; true
  od[&quot;--first&quot;]    #=&gt; true
  od[&quot;--notvalid&quot;] #=&gt; CommandLine::OptionData::UnknownOptionError
  od[&quot;--second&quot;]   #=&gt; CommandLine::OptionData::UnknownOptionError
</pre>
<h3>Built-in Data Handlers</h3>
<p>
OptionParser has
built-in data handlers for handling common scenarios. These lambdas can
save a lot of typing.
</p>
<h3>GET_ARG_ARRAY</h3>
<p>
This is useful for options that take a variable number of arguments. It
returns all the arguments in an array.
</p>
<pre>
  # GET_ARG_ARRAY returns all arguments in an array, even if no
  # arguments are present. This is not to be confused with the option
  # occuring multiple times on the command line.
  opt = Option.new(:names          =&gt; %w(--file),
                   :argument_arity =&gt; [0,-1],
                   :opt_found      =&gt; OptionParser::GET_ARG_ARRAY)
                   #:opt_found      =&gt; :collect)  # would this be better?
  od  = OptionParser.new(opt).parse(%w(--file))
  od[&quot;--file&quot;]    #=&gt; []
  od  = OptionParser.new(opt).parse(%w(--file=file))
  od[&quot;--file&quot;]    #=&gt; [&quot;file&quot;]
  od  = OptionParser.new(opt).parse(%w(--file=file1 --file file2))
  od[&quot;--file&quot;]    #=&gt; [&quot;file2&quot;]
  od  = OptionParser.new(opt).parse(%w(--file=file1 file2))
  od[&quot;--file&quot;]    #=&gt; [&quot;file1&quot;, &quot;file2&quot;]
  od  = OptionParser.new(opt).parse(%w(--file file1 file2))
  od[&quot;--file&quot;]    #=&gt; [&quot;file1&quot;, &quot;file2&quot;]
</pre>
<h3>GET_ARGS</h3>
<p>
This is a &#8216;smart&#8217; option getter. If no arguments are found, it
returns true. If a single argument is found, it returns that argument. If
more than one argument is found, it returns an array of those arguments.
</p>
<pre>
  opt = Option.new(:names          =&gt; %w(--file),
                   :argument_arity =&gt; [0,-1],
                   :opt_found      =&gt; OptionParser::GET_ARGS)
                   #:opt_found      =&gt; :smart_collect)  # would this be better?
  od  = OptionParser.new(opt).parse(%w(--file))
  od[&quot;--file&quot;]    #=&gt; true
  od  = OptionParser.new(opt).parse(%w(--file=file))
  od[&quot;--file&quot;]    #=&gt; &quot;file&quot;
  od  = OptionParser.new(opt).parse(%w(--file=file1 --file file2))
  od[&quot;--file&quot;]    #=&gt; &quot;file2&quot;
  od  = OptionParser.new(opt).parse(%w(--file=file1 file2))
  od[&quot;--file&quot;]    #=&gt; [&quot;file1&quot;, &quot;file2&quot;]
  od  = OptionParser.new(opt).parse(%w(--file file1 file2))
  od[&quot;--file&quot;]    #=&gt; [&quot;file1&quot;, &quot;file2&quot;]
</pre>
<p>
And, for those oxymoronic non-optional options:
</p>
<pre>
  opt = Option.new(:names =&gt; %w(--not-really-an-option),
    :opt_not_found =&gt; OptionParser::OPT_NOT_FOUND_BUT_REQUIRED
  )
  OptionParser.new(opt).parse([])   #=&gt; OptionParser::MissingRequiredOptionError
</pre>
<h3><tt>OptionData</tt></h3>
<p>
We have just shown that after parsing a command line, the result of each
option is found from OptionData. The values that remain on the command line
are assigned to <tt>args</tt>. Other attributes of <tt>OptionData</tt> are:
</p>
<pre>
  od.argv             # the original command line
  od.unknown_options  # If OptionParser was told to :collect unknown options
  od.args             # arguments not claimed by any option
  od.not_parsed       # arguments following a '--' on the command line
  od.cmd              # not yet implemented - but a cvs like command
</pre>
<hr size="2"></hr><h1>Traditional Option Parsing Schemes</h1>
<p>
This section is a brief overview of traditional command line parsing.
</p>
<p>
Command line options traditionally occur in three flavors:
</p>
<ul>
<li><em>Unix</em> (or POSIX.2)

</li>
<li><em>Gnu</em>

</li>
<li><em>X Toolkit</em>

</li>
</ul>
<p>
Below is a summary of these schemes. <em>(Note: I did not invent these
traditional parsing conventions. Most of the information contained below
was pulled from internet resources and I have quoted these resources where
possible.)</em>
</p>
<h2>Unix Style (POSIX)</h2>
<p>
The Unix style command line options are a single character preceded by a
single dash (hyphen character). In general, lowercase options are preferred
with their uppercase counterparts being the special case variant.
</p>
<h3>Mode Flag</h3>
<p>
If an option does not take an argument, then it is a mode-flag.
</p>
<h3>Optional Separation Between the Option Flag and Its Argument</h3>
<p>
If the option takes an argument, the argument follows it with optional
white space separating the two. For example, the following forms are both
valid:
</p>
<pre>
  sort -k 5
  sort -k5
</pre>
<h3>Grouping</h3>
<p>
A mode-flag can be grouped together with other mode-flags behind a single
dash. For example:
</p>
<pre>
  tar -c -v -f
</pre>
<p>
is equivalent to:
</p>
<pre>
  tar -cvf
</pre>
<p>
If grouping is done, the last option in a group can be an option that takes
an argument. For example
</p>
<pre>
  sort -r -n -k 5
</pre>
<p>
can be written as
</p>
<pre>
  sort -rnk 5
</pre>
<p>
but not
</p>
<pre>
  sort -rkn 5
</pre>
<p>
because the &#8216;5&#8217; argument belongs to the &#8216;k&#8217; option
flag.
</p>
<h3>Option Parsing Termination</h3>
<p>
It is convention that a double hyphen is a signal to stop option
interpretation and to read the remaining statements on the command line
literally. So, a command such as:
</p>
<pre>
 app -- -x -y -z
</pre>
<p>
will not &#8216;see&#8217; the three mode-flags. Instead, they will be
treated as arguments to the application:
</p>
<pre>
 #args = [&quot;-x&quot;, &quot;-y&quot;, &quot;-z&quot;]
</pre>
<h3>POSIX Summary</h3>
<ol>
<li>An option is a hyphen followed by a single alphanumeric character.

</li>
<li>An option may require an argument which must follow the option with an
optional space in between.

<pre>
  -r ubygems
  -rubygems
  -r=ubygems   # not ok. '=' is Gnu style
</pre>
</li>
<li>Options that do not require arguments can be grouped after a hyphen.

</li>
<li>Options can appear in any order.

</li>
<li>Options can appear multiple times.

</li>
<li>Options precede other nonoption arguments. TODO: Test for this

</li>
<li>The &#8212; argument terminates options.

</li>
<li>The - option is used to represent the standard input stream.

</li>
</ol>
<h3>References</h3>
<p>
<a
href="http://www.mkssoftware.com/docs/man1/getopts.1.asp">www.mkssoftware.com/docs/man1/getopts.1.asp</a>
</p>
<h2>Gnu Style</h2>
<p>
The Gnu style command line options provide support for option words (or
keywords), yet still maintain compatibility with the Unix style options.
The options in this style are sometimes referred to as
<em>long_options</em> and the Unix style options as <em>short_options</em>.
The compatibility is maintained by preceding the <em>long_options</em> with
two dashes. The option word must be two or more characters.
</p>
<h3>Separation Between the Option Flag and Its Argument</h3>
<p>
Gnu style options cannot be grouped. For options that have an argument, the
argument follows the option with either whitespace or an &#8217;=&#8217;.
For example, the following are equivalent:
</p>
<pre>
  app --with-optimizer yes
  app --with-optimizer=yes
</pre>
<h3>Option Parsing Termination</h3>
<p>
Similar to the <em>Unix</em> style double-hyphen &#8217;- -&#8217;, the
<em>Gnu</em> style has a triple-hyphen &#8217;- - -&#8217; to signal that
option parsing be halted and to treat the remaining text as arguments (that
is, read literally from the command line)
</p>
<pre>
 app --- -x -y -z
 args = [&quot;-x&quot;, &quot;-y&quot;, &quot;-z&quot;]
</pre>
<h3>Mixing <em>Gnu</em> and <em>Unix</em> Styles</h3>
<p>
The <em>Gnu</em> and the <em>Unix</em> option types can be mixed on the
same commandline. The following are equivalent:
</p>
<pre>
  app -a -b --with-c
  app -ab --with-c
  app -ba --with-c
  app --with-c -ab
</pre>
<h2>X Toolkit Style</h2>
<p>
The X Toolkit style uses the single hyphen followed by a keyword option.
This style is not compatible with the <em>Unix</em> or the <em>Gnu</em>
option types. In most situations this is OK since these options will be
filtered from the command line before passing them to an application.
</p>
<h3>&#8217;-&#8217; and STDIN</h3>
<p>
It is convention that a bare hypen indicates to read from stdin.
</p>
<h2>The OptionParser Style</h2>
<p>
The CommandLine::OptionParser does not
care what style you use. It is designed for maximum flexiblity so it may be
used within any organiziation to meet their standards.
</p>
<h3>Multiple Option Names</h3>
<p> OptionParser does
not place restrictions on the number of options. The only restriction is
that an option name begin with a hyphen &#8217;-&#8217;. A definitely
conjured example of this freedom is:
</p>
<pre>
  :names =&gt; %w(
    --file --File --f --F -file -File -f -F
  )
</pre>
<h3>Prefix Matching</h3>
<p>
Although not encouraged, some prefer the ability to truncate option words
to their first unique match. For example, an application that support this
style and accepts the following two option words:
</p>
<pre>
 [&quot;--foos&quot;, &quot;--fbars&quot;]
</pre>
<p>
will accept any of the following as valid options
</p>
<pre>
  app --fo
  app --foo
  app --foos
</pre>
<p>
for the &quot;&#8212;foos&quot; option flag since it can be determined that
&quot;&#8212;fo&quot; will only match &quot;&#8212;foos&quot; and not
&quot;&#8212;fbars&quot;.
</p>
<h3>Repeated Arguments</h3>
<p>
A common question is how an option parser should respond when an option is
specified on the command line multiple times. This is true for mode flags,
but especially true for options that require an argument, For example, what
should happen when the following is given:
</p>
<pre>
  app -f file1 -f file2
</pre>
<p>
Should the parser flag this as an error or should it accept both arguments.
</p>
<p> OptionParser gives
you the choice of whether it raises an exception when an option is seen
more than once, or it just passes the data onto the user.
</p>
<p>
How the data is handled is up to the user, but it typically boils down to
either Append, Replace or Raise. This is described in more detail in the
usage section.
</p>
<h2>CVS Mode</h2>
<p>
CVS is a common application with a unique command line structure. The cvs
application commandline can be given options, but requires a command. This
command can also be given options. This means that there are two sets of
options, one set for the cvs application and one set for the cvs-command.
Some example formats are:
</p>
<pre>
  cvs [cvs-options]
  cvs [cvs-options] command [command-options-and-arguments]

  cvs -r update
  cvs -r update .
  cvs edit -p file
</pre>
<p>
To handle this, the first unclaimed argument is treated as a command and
the options and option-arguments that follow belong to that command. More
on how this is handled in the usage section.
</p>
<h2>Option Grouping</h2>
<p>
A conflict can occur where a grouping of single letter Unix options has the
value as a word option preceded by a single dash. For this reason, it is
customary to use the double-dash notation for word options. Unless
double-dashes are enforced for word options, OptionParser will
check for possible name conflicts and raise an exception if it finds one.
</p>

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