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<h4 class="subsection">25.1.1 Program Argument Syntax Conventions</h4>

<p><a name="index-program-argument-syntax-3016"></a><a name="index-syntax_002c-for-program-arguments-3017"></a><a name="index-command-argument-syntax-3018"></a>
POSIX recommends these conventions for command line arguments. 
<code>getopt</code> (see <a href="Getopt.html#Getopt">Getopt</a>) and <code>argp_parse</code> (see <a href="Argp.html#Argp">Argp</a>) make
it easy to implement them.

     <ul>
<li>Arguments are options if they begin with a hyphen delimiter (&lsquo;<samp><span class="samp">-</span></samp>&rsquo;).

     <li>Multiple options may follow a hyphen delimiter in a single token if
the options do not take arguments.  Thus, &lsquo;<samp><span class="samp">-abc</span></samp>&rsquo; is equivalent to
&lsquo;<samp><span class="samp">-a -b -c</span></samp>&rsquo;.

     <li>Option names are single alphanumeric characters (as for <code>isalnum</code>;
see <a href="Classification-of-Characters.html#Classification-of-Characters">Classification of Characters</a>).

     <li>Certain options require an argument.  For example, the &lsquo;<samp><span class="samp">-o</span></samp>&rsquo; command
of the <code>ld</code> command requires an argument&mdash;an output file name.

     <li>An option and its argument may or may not appear as separate tokens.  (In
other words, the whitespace separating them is optional.)  Thus,
&lsquo;<samp><span class="samp">-o&nbsp;foo</span></samp>&rsquo;<!-- /@w --> and &lsquo;<samp><span class="samp">-ofoo</span></samp>&rsquo; are equivalent.

     <li>Options typically precede other non-option arguments.

     <p>The implementations of <code>getopt</code> and <code>argp_parse</code> in the GNU C
library normally make it appear as if all the option arguments were
specified before all the non-option arguments for the purposes of
parsing, even if the user of your program intermixed option and
non-option arguments.  They do this by reordering the elements of the
<var>argv</var> array.  This behavior is nonstandard; if you want to suppress
it, define the <code>_POSIX_OPTION_ORDER</code> environment variable. 
See <a href="Standard-Environment.html#Standard-Environment">Standard Environment</a>.

     <li>The argument &lsquo;<samp><span class="samp">--</span></samp>&rsquo; terminates all options; any following arguments
are treated as non-option arguments, even if they begin with a hyphen.

     <li>A token consisting of a single hyphen character is interpreted as an
ordinary non-option argument.  By convention, it is used to specify
input from or output to the standard input and output streams.

     <li>Options may be supplied in any order, or appear multiple times.  The
interpretation is left up to the particular application program. 
</ul>

   <p><a name="index-long_002dnamed-options-3019"></a>GNU adds <dfn>long options</dfn> to these conventions.  Long options consist
of &lsquo;<samp><span class="samp">--</span></samp>&rsquo; followed by a name made of alphanumeric characters and
dashes.  Option names are typically one to three words long, with
hyphens to separate words.  Users can abbreviate the option names as
long as the abbreviations are unique.

   <p>To specify an argument for a long option, write
&lsquo;<samp><span class="samp">--</span><var>name</var><span class="samp">=</span><var>value</var></samp>&rsquo;.  This syntax enables a long option to
accept an argument that is itself optional.

   <p>Eventually, the GNU system will provide completion for long option names
in the shell.

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