utilconv.html

unix 下的C开发手册,还用详细的例程。

arguments depicted in the
<b>SYNOPSIS</b>
sections are mandatory requirements
placed on portable applications.
The use of conflicting mutually exclusive
arguments produces undefined results,
unless a utility description specifies otherwise.
When an option is shown without the
<b>[&nbsp;]</b>
brackets, it means that option is required for that version of the
<b>SYNOPSIS</b>.
However, it is not required to be the first argument,
as shown in the example above, unless otherwise stated.
<p>
The use of
<i>undefined</i>
for conflicting argument usage
and for repeated usage of the same option
is meant to prevent portable applications
from using conflicting arguments or repeated options,
unless specifically allowed, as is the case with
<i><a href="../xcu/ls.html">ls</a></i>
(which allows simultaneous, repeated use of the
<b>-C</b>,
<b>-l</b>
and
<b>-1</b>
options).
Many historical implementations will tolerate this usage,
choosing either the first or the last applicable argument,
and this tolerance may continue, but portable applications
cannot rely upon it.
(Other implementations may choose to print usage messages instead.)
<p>
The use of
<i>undefined</i>
for conflicting argument usage also allows
an implementation to make reasonable extensions to utilities
where the implementor considers
mutually exclusive options according to the <b>XCU</b> specification to have a
sensible meaning and result.
<p>
<li>
Ellipses (...) are used to denote that one or more occurrences of
an option or operand are allowed.
When an option or an operand
followed by ellipses is enclosed in brackets, zero or more
options or operands can be specified.
The forms:
<pre><code>

utility_name -f <i>option_argument</i>...<b>[</b><i>operand</i>...<b>]</b>

utility_name <b>[</b>-g <i>option_argument</i><b>]</b>...<b>[</b><i>operand</i>...<b>]</b>
</code>
</pre>
indicate that multiple occurrences of the option and its
option-argument preceding the ellipses
are valid, with semantics as indicated in the
<b>OPTIONS</b>
section of the utility.
(See also Guideline 11 in
<xref href=usg><a href="#tag_009_002">
Utility Syntax Guidelines
</a></xref>.)
In the first example, each option-argument requires a preceding
<b>-f</b>
and at least one
<b>-f</b>&nbsp;<i>option_argument</i>
must be given.
<p>
The <b>XCU</b> specification does not define the result of a utility when an
option-argument or operand is not followed by ellipses and
the application specifies more than one of that option-argument or operand.
This allows an
implementation to define valid (although non-standard) behaviour
for the utility when more than one such option or operand are specified.
<p>
<li>
When the synopsis line is too long to be printed on a single line in
the <b>XCU</b> specification, the indented lines following the initial line are
continuation lines.
An actual use of the command would appear on a single logical line.
<p>
</ol>
<h3><a name = "tag_009_002"><a name="usg">Utility Syntax Guidelines</a>&nbsp;</a></h3>
<xref type="2" name="usg"></xref>
The following guidelines are established for the naming of
utilities and for the specification of options,
option-arguments and operands.
The
<i><a href="../xsh/getopt.html">getopt()</a></i>
function in the <b>XSH</b> specification assists utilities
in handling options and operands that conform to these guidelines.
<p>
Operands and option-arguments can contain characters not specified in
the portable character set.
<p>
The guidelines are intended to provide guidance to the
authors of future utilities, such as those written specific
to a local system or that are to be components of
a larger application.
Some of the standard utilities do not
conform to all of these guidelines; in those cases, the
<b>OPTIONS</b>
sections describe the deviations.
<dl compact>

<dt><b>Guideline&nbsp;&nbsp;1:</b><dd>Utility names should be between two and nine characters, inclusive.

<dt><b>Guideline&nbsp;&nbsp;2:</b><dd>Utility names should include lower-case letters
(the
<b>lower</b>
character classification)
and digits only
from the portable character set.

Guidelines 1 and 2 are offered as guidance for locales
using Latin alphabets.
No recommendations are made by this specification set
concerning utility naming in other locales.

In the <b>XCU</b> specification, <a href="../xcu/chap2.html#tag_001_009_001"><b>Simple Commands</b>&nbsp;</a>,
it is further stated that a command used in
the XSI Shell Command Language cannot be named with a trailing colon.

<dt><b>Guideline&nbsp;&nbsp;3:</b><dd>Each option name should be a single alphanumeric character
(the
<b>alnum</b>
character classification)
from the portable character set.

Multi-digit options are not allowed.
Instances of historical utilities that used them
have been marked <b>LEGACY</b> in the <b>XCU</b> specification, with the numbers
being changed from option names to option-arguments.

<dt><b>Guideline&nbsp;&nbsp;4:</b><dd>All options should be preceded by the "-" delimiter character.

<dt><b>Guideline&nbsp;&nbsp;5:</b><dd>Options without option-arguments should be
accepted when grouped behind one "-" delimiter.

<dt><b>Guideline&nbsp;&nbsp;6:</b><dd>Each option and option-argument should be a separate argument,
except as noted in
<xref href=utilarg><a href="#tag_009_001">
Utility Argument Syntax
</a></xref>,
item (2).

<dt><b>Guideline&nbsp;&nbsp;7:</b><dd>Option-arguments should not be optional.

<dt><b>Guideline&nbsp;&nbsp;8:</b><dd>When multiple option-arguments are specified to follow a single option,
they should be presented as a single argument, using commas
within that argument or
blank characters
within that argument to separate them.

It is up to the utility to parse a comma-separated list itself
because
<i><a href="../xsh/getopt.html">getopt()</a></i>
just returns a single string.
This situation was retained so that certain historical
utilities would not violate the guidelines.
Applications preparing for international use should be aware
of an occasional problem with comma-separated lists:
in some locales, the comma is used as the radix character.
Thus, if an application is preparing operands for a utility
that expects a comma-separated list, it should avoid generating
non-integer values through one of the means that is influenced
by setting the
<i>LC_NUMERIC</i>
variable (such as
<i><a href="../xcu/awk.html">awk</a></i>,
<i><a href="../xcu/bc.html">bc</a></i>,
<i><a href="../xcu/printf.html">printf</a></i>
or
<i><a href="../xsh/printf.html">printf()</a></i>).

<dt><b>Guideline&nbsp;&nbsp;9:</b><dd>All options should precede operands on the command line.

<dt><b>Guideline&nbsp;10:</b><dd>The argument
--
should be accepted as a delimiter indicating the end of options.
Any following arguments should be treated as operands, even if they
begin with the "-" character.
The
--
argument should not be used as an option or as an operand.

Applications calling any utility with a first operand starting with
-
should usually specify
--,
as indicated by Guideline 10, to mark the end of the options.
This is true even if the
<b>SYNOPSIS</b>
in the <b>XCU</b> specification does not specify any options;
implementations may provide options as extensions to the <b>XCU</b> specification.
The standard utilities that do not support Guideline 10 indicate that fact
in the
<b>OPTIONS</b>
section of the utility description.

<dt><b>Guideline&nbsp;11:</b><dd>The order of different options relative to one another should not matter,
unless the options are documented as
mutually exclusive and such an option is documented to
override any incompatible options preceding it.
If an option that has option-arguments is repeated,
the option and option-argument combinations should be
interpreted in the order specified on the command line.

The order of repeated options that also have option-arguments may be
significant; therefore, such options are required to be
interpreted in the order that they are specified.
The
<i><a href="../xcu/make.html">make</a></i>
utility is an instance of a historical utility that
uses repeated options in which the order is significant.
Multiple files are specified by giving multiple instances of the
<b>-f</b>
option, for example:
<pre><code>

make -f common_header -f specific_rules target
</code>
</pre>

<dt><b>Guideline&nbsp;12:</b><dd>The order of operands may matter and
position-related interpretations should
be determined on a utility-specific basis.

<dt><b>Guideline&nbsp;13:</b><dd>For utilities that use operands to represent files
to be opened for either reading or writing, the "-"
operand should be used only to mean standard input
(or standard output when it is clear from context
that an output file is being specified).

Guideline 13 does not imply that all of the standard utilities
automatically accept the operand "-"
to mean standard input or output, nor does it specify
the actions of the utility upon encountering multiple "-" operands.
It simply says that, by default, "-"
operands are not used for other purposes
in the file reading or writing (but not when using
<i><a href="../xsh/stat.html">stat()</a></i>,
<i><a href="../xsh/unlink.html">unlink()</a></i>,
<i><a href="../xcu/touch.html">touch</a></i>,
and so forth) utilities.
All information concerning actual treatment of the "-"
operand is found in the individual utility sections.

</dl>
<p>
The utilities in the <b>XCU</b> specification that
claim conformance to these guidelines
were written as if the term
<i>should</i>
imposed a specific requirement on their interface
and applications and users can rely on the behaviour stated here;
the Guidelines are rules for the standard utilities that
claim conformance to them.
On some systems, the utilities will
accept usage in violation of these guidelines for backward
compatibility as well as accepting the required form.
<p>
It is recommended that all future
utilities and applications use these guidelines to enhance
user portability.
The fact that some historical utilities could not be changed
(to avoid breaking existing applications) should not deter this
future goal.

</blockquote>
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