xcu_chap02.html

IEEE 1003.1-2003, Single Unix Specification v3

<div class="box"><em>The following sections are informative.</em></div>

<h5><a name="tag_02_06_02_01"></a>Examples</h5>

<dl compact>
<dt>${<i>parameter</i>:-<i>word</i>}</dt>

<dd><br>
In this example, <a href="../utilities/ls.html"><i>ls</i></a> is executed only if <i>x</i> is null or unset. (The $( <a href=
"../utilities/ls.html"><i>ls</i></a>) command substitution notation is explained in <a href="#tag_02_06_03">Command
Substitution</a> .) 

<blockquote>
<pre>
<tt>${x:-$(ls)}
</tt>
</pre>
</blockquote>
</dd>

<dt>${<i>parameter</i>:=<i>word</i>}</dt>

<dd>
<pre>
<tt>unset X
echo ${X:=abc}
</tt><b>abc</b>
</pre>
</dd>

<dt>${<i>parameter</i>:?<i>word</i>}</dt>

<dd>
<pre>
<tt>unset posix
echo ${posix:?}
</tt><b>sh: posix: parameter null or not set</b>
</pre>
</dd>

<dt>${<i>parameter</i>:+<i>word</i>}</dt>

<dd>
<pre>
<tt>set a b c
echo ${3:+posix}
</tt><b>posix</b>
</pre>
</dd>

<dt>${#<i>parameter</i>}</dt>

<dd>
<pre>
<tt>HOME=/usr/posix
echo ${#HOME}
</tt><b>10</b>
</pre>
</dd>

<dt>${<i>parameter</i>%<i>word</i>}</dt>

<dd>
<pre>
<tt>x=file.c
echo ${x%.c}.o
</tt><b>file.o</b>
</pre>
</dd>

<dt>${<i>parameter</i>%%<i>word</i>}</dt>

<dd>
<pre>
<tt>x=posix/src/std
echo ${x%%/*}
</tt><b>posix</b>
</pre>
</dd>

<dt>${<i>parameter</i>#<i>word</i>}</dt>

<dd>
<pre>
<tt>x=$HOME/src/cmd
echo ${x#$HOME}
</tt><b>/src/cmd</b>
</pre>
</dd>

<dt>${<i>parameter</i>##<i>word</i>}</dt>

<dd>
<pre>
<tt>x=/one/two/three
echo ${x##*/}
</tt><b>three</b>
</pre>
</dd>
</dl>

<p>The double-quoting of patterns is different depending on where the double-quotes are placed:</p>

<dl compact>
<dt><tt>"${x#*}"</tt></dt>

<dd>The asterisk is a pattern character.</dd>

<dt><tt>${x#"*"}</tt></dt>

<dd>The literal asterisk is quoted and not special.</dd>
</dl>

<div class="box"><em>End of informative text.</em></div>

<hr>
<h4><a name="tag_02_06_03"></a>Command Substitution</h4>

<p>Command substitution allows the output of a command to be substituted in place of the command name itself. Command substitution
shall occur when the command is enclosed as follows:</p>

<blockquote>
<pre>
<tt>$(</tt><i>command</i><tt>)
</tt>
</pre>
</blockquote>

<p>or (backquoted version):</p>

<blockquote>
<pre>
<tt>`</tt><i>command</i><tt>`
</tt>
</pre>
</blockquote>

<p>The shell shall expand the command substitution by executing <i>command</i> in a subshell environment (see <a href=
"#tag_02_12">Shell Execution Environment</a> ) and replacing the command substitution (the text of <i>command</i> plus the
enclosing <tt>"$()"</tt> or backquotes) with the standard output of the command, removing sequences of one or more &lt;newline&gt;s
at the end of the substitution. Embedded &lt;newline&gt;s before the end of the output shall not be removed; however, they may be
treated as field delimiters and eliminated during field splitting, depending on the value of <i>IFS</i> and quoting that is in
effect.</p>

<p>Within the backquoted style of command substitution, backslash shall retain its literal meaning, except when followed by:
<tt>'$'</tt> , <tt>'`'</tt> , or <tt>'\'</tt> (dollar sign, backquote, backslash). The search for the matching backquote shall be
satisfied by the first backquote found without a preceding backslash; during this search, if a non-escaped backquote is encountered
within a shell comment, a here-document, an embedded command substitution of the $( <i>command</i>) form, or a quoted string,
undefined results occur. A single-quoted or double-quoted string that begins, but does not end, within the <tt>"`...`"</tt>
sequence produces undefined results.</p>

<p>With the $( <i>command</i>) form, all characters following the open parenthesis to the matching closing parenthesis constitute
the <i>command</i>. Any valid shell script can be used for <i>command</i>, except a script consisting solely of redirections which
produces unspecified results.</p>

<p>The results of command substitution shall not be processed for further tilde expansion, parameter expansion, command
substitution, or arithmetic expansion. If a command substitution occurs inside double-quotes, field splitting and pathname
expansion shall not be performed on the results of the substitution.</p>

<p>Command substitution can be nested. To specify nesting within the backquoted version, the application shall precede the inner
backquotes with backslashes, for example:</p>

<blockquote>
<pre>
<tt>\`</tt><i>command</i><tt>\`
</tt>
</pre>
</blockquote>

<p>If the command substitution consists of a single subshell, such as:</p>

<blockquote>
<pre>
<tt>$( (</tt><i>command</i><tt>) )
</tt>
</pre>
</blockquote>

<p>a conforming application shall separate the <tt>"$("</tt> and <tt>'('</tt> into two tokens (that is, separate them with white
space). This is required to avoid any ambiguities with arithmetic expansion.</p>

<h4><a name="tag_02_06_04"></a>Arithmetic Expansion</h4>

<p>Arithmetic expansion provides a mechanism for evaluating an arithmetic expression and substituting its value. The format for
arithmetic expansion shall be as follows:</p>

<blockquote>
<pre>
<tt>$((</tt><i>expression</i><tt>))
</tt>
</pre>
</blockquote>

<p>The expression shall be treated as if it were in double-quotes, except that a double-quote inside the expression is not treated
specially. The shell shall expand all tokens in the expression for parameter expansion, command substitution, and quote
removal.</p>

<p>Next, the shell shall treat this as an arithmetic expression and substitute the value of the expression. The arithmetic
expression shall be processed according to the rules given in <a href="xcu_chap01.html#tag_01_07_02_01"><i>Arithmetic Precision and
Operations</i></a> , with the following exceptions:</p>

<ul>
<li>
<p>Only signed long integer arithmetic is required.</p>
</li>

<li>
<p>Only the decimal-constant, octal-constant, and hexadecimal-constant constants specified in the ISO&nbsp;C standard, Section
6.4.4.1 are required to be recognized as constants.</p>
</li>

<li>
<p>The <i>sizeof</i>() operator and the prefix and postfix <tt>"++"</tt> and <tt>"--"</tt> operators are not required.</p>
</li>

<li>
<p>Selection, iteration, and jump statements are not supported.</p>
</li>
</ul>

<p>As an extension, the shell may recognize arithmetic expressions beyond those listed. The shell may use a signed integer type
with a rank larger than the rank of <b>signed long</b>. The shell may use a real-floating type instead of <b>signed long</b> as
long as it does not affect the results in cases where there is no overflow. If the expression is invalid, the expansion fails and
the shell shall write a message to standard error indicating the failure.</p>

<hr>
<div class="box"><em>The following sections are informative.</em></div>

<h5><a name="tag_02_06_04_01"></a>Examples</h5>

<p>A simple example using arithmetic expansion:</p>

<blockquote>
<pre>
<tt># repeat a command 100 times
x=100
while [ $x -gt 0 ]
do
   </tt> <i>command</i> <tt>   x=$(($x-1))
done
</tt>
</pre>
</blockquote>

<div class="box"><em>End of informative text.</em></div>

<hr>
<h4><a name="tag_02_06_05"></a>Field Splitting</h4>

<p>After parameter expansion ( <a href="#tag_02_06_02">Parameter Expansion</a> ), command substitution ( <a href=
"#tag_02_06_03">Command Substitution</a> ), and arithmetic expansion ( <a href="#tag_02_06_04">Arithmetic Expansion</a> ), the
shell shall scan the results of expansions and substitutions that did not occur in double-quotes for field splitting and multiple
fields can result.</p>

<p>The shell shall treat each character of the <i>IFS</i> as a delimiter and use the delimiters to split the results of parameter
expansion and command substitution into fields.</p>

<ol>
<li>
<p>If the value of <i>IFS</i> is a &lt;space&gt;, &lt;tab&gt;, and &lt;newline&gt;, or if it is unset, any sequence of
&lt;space&gt;s, &lt;tab&gt;s, or &lt;newline&gt;s at the beginning or end of the input shall be ignored and any sequence of those
characters within the input shall delimit a field. For example, the input:</p>

<blockquote>
<pre>
<tt>&lt;newline&gt;&lt;space&gt;&lt;tab&gt;foo&lt;tab&gt;&lt;tab&gt;bar&lt;space&gt;
</tt>
</pre>
</blockquote>

<p>yields two fields, <b>foo</b> and <b>bar</b>.</p>
</li>

<li>
<p>If the value of <i>IFS</i> is null, no field splitting shall be performed.</p>
</li>

<li>
<p>Otherwise, the following rules shall be applied in sequence. The term &quot; <i>IFS</i> white space&quot; is used to mean any sequence
(zero or more instances) of white space characters that are in the <i>IFS</i> value (for example, if <i>IFS</i> contains
&lt;space&gt;/ &lt;comma&gt;/ &lt;tab&gt;, any sequence of &lt;space&gt;s and &lt;tab&gt;s is considered <i>IFS</i> white
space).</p>

<ol type="a">
<li>
<p><i>IFS</i> white space shall be ignored at the beginning and end of the input.</p>
</li>

<li>
<p>Each occurrence in the input of an <i>IFS</i> character that is not <i>IFS</i> white space, along with any adjacent <i>IFS</i>
white space, shall delimit a field, as described previously.</p>
</li>

<li>
<p>Non-zero-length <i>IFS</i> white space shall delimit a field.</p>
</li>
</ol>
</li>
</ol>

<h4><a name="tag_02_06_06"></a>Pathname Expansion</h4>

<p>After field splitting, if <a href="set.html"><i>set</i></a> <b>-f</b> is not in effect, each field in the resulting command line
shall be expanded using the algorithm described in <a href="#tag_02_13">Pattern Matching Notation</a> , qualified by the rules in
<a href="#tag_02_13_03">Patterns Used for Filename Expansion</a> .</p>