xcu_chap02.html

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="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">  2.6.3 </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">  2.6.4 </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="http://www.opengroup.org/onlinepubs/009695399/utilities/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>All changes to variables in an arithmetic expression shall be in effect after the arithmetic expansion, as in the parameter
expansion <tt>"${x=value}"</tt>.</p>

<p>If the shell variable <i>x</i> contains a value that forms a valid integer constant, then the arithmetic expansions
<tt>"$((x))"</tt> and <tt>"$(($x))"</tt> shall return the same value.</p>

<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"><tt><em>The following sections are informative.</em></tt></div>

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

<p><tt>A simple example using arithmetic expansion:</tt></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">  2.6.5 </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 " <i>IFS</i> white space" 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">  2.6.6 </a>Pathname Expansion</h4>

<p>After field splitting, if <a href="http://www.opengroup.org/onlinepubs/009695399/utilities/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>

<h4><a name="tag_02_06_07">  2.6.7 </a>Quote Removal</h4>

<p>The quote characters: <tt>'\'</tt>, <tt>'"</tt>, and <tt>''</tt> (backslash, single-quote, double-quote) that were present in
the original word shall be removed unless they have themselves been quoted.</p>

<h3><a name="tag_02_07">   2.7 </a>Redirection</h3>

<p>Redirection is used to open and close files for the current shell execution environment (see <a href="#tag_02_12">Shell
Execution Environment</a>) or for any command. Redirection operators can be used with numbers representing file descriptors (see
the Base Definitions volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href="http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap03.html#tag_03_165">Section 3.165, File
Descriptor</a>) as described below.</p>

<p>The overall format used for redirection is:</p>

<blockquote>
<pre><b>[</b><i>n</i><b>]</b><i>redir-op word</i>
</pre>
</blockquote>

<p>The number <i>n</i> is an optional decimal number designating the file descriptor number; the application shall ensure it is
delimited from any preceding text and immediately precede the redirection operator <i>redir-op</i>. If <i>n</i> is quoted, the
number shall not be recognized as part of the redirection expression. For example:</p>

<blockquote>
<pre><tt>echo \2&gt;a
</tt>
</pre>
</blockquote>

<p>writes the character 2 into file <b>a</b>. If any part of <i>redir-op</i> is quoted, no redirection expression is recognized.
For example:</p>

<blockquote>
<pre><tt>echo 2\&gt;a
</tt>
</pre>
</blockquote>

<p>writes the characters 2&gt;<i>a</i> to standard output. The optional number, redirection operator, and <i>word</i> shall not
appear in the arguments provided to the command to be executed (if any).</p>

<p>Open files are represented by decimal numbers starting with zero.
The largest possible value is implementation-defined; however,
all implementations shall support at least 0 to 9, inclusive, for use
by the application. These numbers are called "file
descriptors". The values 0, 1, and 2 have special meaning and
conventional uses and are implied by certain redirection operations;
they are referred to as <i>standard input</i>, <i>standard output</i>, and <i>standard error</i>,
respectively. Programs usually
take their input from standard input, and write output on standard
output. Error messages are usually written on standard error.
The redirection operators can be preceded by one or more digits (with
no intervening &lt;blank&gt;s allowed) to designate the file
descriptor number.</p>

<p>If the redirection operator is <tt>"&lt;&lt;"</tt> or <tt>"&lt;&lt;-"</tt>,
the word that follows the redirection operator
shall be subjected to quote removal; it is unspecified whether any of
the other expansions occur. For the other redirection
operators, the word that follows the redirection operator shall be
subjected to tilde expansion, parameter expansion, command
substitution, arithmetic expansion, and quote removal. Pathname
expansion shall not be performed on the word by a non-interactive
shell; an interactive shell may perform it, but shall do so only when
the expansion would result in one word.</p>

<p>If more than one redirection operator is specified with a command, the order of evaluation is from beginning to end.</p>

<p>A failure to open or create a file shall cause a redirection to fail.</p>

<h4><a name="tag_02_07_01">  2.7.1 </a>Redirecting Input</h4>

<p>Input redirection shall cause the file whose name results from the expansion of <i>word</i> to be opened for reading on the
designated file descriptor, or standard input if the file descriptor is not specified.</p>

<p>The general format for redirecting input is:</p>

<blockquote>
<pre><b>[</b><i>n</i><b>]</b><tt>&lt;</tt><i>word</i>
</pre>
</blockquote>

<p>where the optional <i>n</i> represents the file descriptor number. If the number is omitted, the redirection shall refer to
standard input (file descriptor 0).</p>

<h4><a name="tag_02_07_02">  2.7.2 </a>Redirecting Output</h4>

<p>The two general formats for redirecting output are:</p>

<blockquote>
<pre><b>[</b><i>n</i><b>]</b><tt>&gt;</tt><i>word</i>
<b>[</b><i>n</i><b>]</b><tt>&gt;|</tt><i>word</i>
</pre>
</blockquote>

<p>where the optional <i>n</i> represents the file descriptor number. If the number is omitted, the redirection shall refer to
standard output (file descriptor 1).</p>

<p>Output redirection using the <tt>'&gt;'</tt> format shall fail if the <i>noclobber</i> option is set (see the description of <a href="http://www.opengroup.org/onlinepubs/009695399/utilities/set.html"><i>set</i></a> <b>-C</b>) and the file named by the expansion of <i>word</i> exists and is a regular file. Otherwise,
redirection using the <tt>'&gt;'</tt> or <tt>"&gt;|"</tt> formats shall cause the file whose name results from the expansion of
<i>word</i> to be created and opened for output on the designated file descriptor, or standard output if none is specified. If the
file does not exist, it shall be created; otherwise, it shall be truncated to be an empty file after being opened.</p>

<h4><a name="tag_02_07_03">  2.7.3 </a>Appending Redirected Output</h4>

<p>Appended output redirection shall cause the file whose name results from the expansion of word to be opened for output on the
designated file descriptor. The file is opened as if the <a href="http://www.opengroup.org/onlinepubs/009695399/functions/open.html"><i>open</i>()</a> function as defined in
the System Interfaces volume of IEEE&nbsp;Std&nbsp;1003.1-2001 was called with the O_APPEND flag. If the file does not exist, it
shall be created.</p>

<p>The general format for appending redirected output is as follows:</p>

<blockquote>
<pre><b>[</b><i>n</i><b>]</b><tt>&gt;&gt;</tt><i>word</i>
</pre>
</blockquote>

<p>where the optional <i>n</i> represents the file descriptor number. If the number is omitted, the redirection refers to standard
output (file descriptor 1).</p>

<h4><a name="tag_02_07_04">  2.7.4 </a>Here-Document</h4>

<p>The redirection operators <tt>"&lt;&lt;"</tt> and <tt>"&lt;&lt;-"</tt> both allow redirection of lines contained in a shell
input file, known as a "here-document", to the input of a command.</p>

<p>The here-document shall be treated as a single word that begins
after the next &lt;newline&gt; and continues until there is a
line containing only the delimiter and a &lt;newline&gt;, with no
&lt;blank&gt;s in between. Then the next here-document starts, if
there is one. The format is as follows:</p>

<blockquote>
<pre><b>[</b><i>n</i><b>]</b><tt>&lt;&lt;</tt><i>word
    here-document
delimiter</i>
</pre>
</blockquote>

<p>where the optional <i>n</i> represents the file descriptor number. If the number is omitted, the here-document refers to
standard input (file descriptor 0).</p>

<p>If any character in <i>word</i> is quoted, the delimiter shall be formed by performing quote removal on <i>word</i>, and the
here-document lines shall not be expanded. Otherwise, the delimiter shall be the <i>word</i> itself.</p>

<p>If no characters in <i>word</i> are quoted, all lines of the
here-document shall be expanded for parameter expansion, command
substitution, and arithmetic expansion. In this case, the backslash in
the input behaves as the backslash inside double-quotes (see
<a href="#tag_02_02_03">Double-Quotes</a>). However, the double-quote character ( <tt>' )'</tt> shall not be treated specially
within a here-document, except when the double-quote appears within <tt>"$()"</tt>, <tt>"``"</tt>, or <tt>"${}"</tt>.</p>

<p>If the redirection symbol is <tt>"&lt;&lt;-"</tt>, all leading &lt;tab&gt;s shall be stripped from input lines and the line
containing the trailing delimiter. If more than one <tt>"&lt;&lt;"</tt> or <tt>"&lt;&lt;-"</tt> operator is specified on a line,
the here-document associated with the first operator shall be supplied first by the application and shall be read first by the
shell.</p>

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

<h5><a name="tag_02_07_04_01"></a>Examples</h5>

<p>An example of a here-document follows:</p>

<blockquote>
<pre><tt>cat &lt;&lt;eof1; cat &lt;&lt;eof2
Hi,
eof1
Helene.
eof2
</tt>
</pre>
</blockquote>

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

<hr>
<h4><a name="tag_02_07_05">  2.7.5 </a>Duplicating an Input File Descriptor</h4>

<p>The redirection operator:</p>

<blockquote>
<pre><b>[</b><i>n</i><b>]</b><tt>&lt;&amp;</tt><i>word</i>
</pre>
</blockquote>

<p>shall duplicate one input file descriptor from another, or shall close one. If <i>word</i> evaluates to one or more digits, the
file descriptor denoted by <i>n</i>, or standard input if <i>n</i> is not specified, shall be made to be a copy of the file
descriptor denoted by <i>word</i>; if the digits in <i>word</i> do not represent a file descriptor already open for input, a
redirection error shall result; see <a href="#tag_02_08_01">Consequences of Shell Errors</a>. If <i>word</i> evaluates to
<tt>'-'</tt>, file descriptor <i>n</i>, or standard input if <i>n</i> is not specified, shall be closed. Attempts to close a file
descriptor that is not open shall not constitute an error. If <i>word</i> evaluates to something else, the behavior is
unspecified.</p>

<h4><a name="tag_02_07_06">  2.7.6 </a>Duplicating an Output File Descriptor</h4>

<p>The redirection operator:</p>

<blockquote>
<pre><b>[</b><i>n</i><b>]</b><tt>&gt;&amp;</tt><i>word</i>
</pre>
</blockquote>

<p>shall duplicate one output file descriptor from another, or shall close one. If <i>word</i> evaluates to one or more digits, the
file descriptor denoted by <i>n</i>, or standard output if <i>n</i> is not specified, shall be made to be a copy of the file
descriptor denoted by <i>word</i>; if the digits in <i>word</i> do not represent a file descriptor already open for output, a
redirection error shall result; see <a href="#tag_02_08_01">Consequences of Shell Errors</a>. If <i>word</i> evaluates to
<tt>'-'</tt>, file descriptor <i>n</i>, or standard output if <i>n</i> is not specified, is closed. Attempts to close a file
descriptor that is not open shall not constitute an error. If <i>word</i> evaluates to something else, the behavior is
unspecified.</p>

<h4><a name="tag_02_07_07">  2.7.7 </a>Open File Descriptors for Reading and Writing</h4>

<p>The redirection operator:</p>

<blockquote>
<pre><b>[</b><i>n</i><b>]</b><tt>&lt;&gt;</tt><i>word</i>
</pre>
</blockquote>

<p>shall cause the file whose name is the expansion of <i>word</i> to be opened for both reading and writing on the file descriptor
denoted by <i>n</i>, or standard input if <i>n</i> is not specified. If the file does not exist, it shall be created.</p>

<h3><a name="tag_02_08">   2.8 </a>Exit Status and Errors</h3>

<h4><a name="tag_02_08_01">  2.8.1 </a>Consequences of Shell Errors</h4>

<p>For a non-interactive shell, an error condition encountered by a special built-in (see <a href="#tag_02_14">Special Built-In
Utilities</a>) or other type of utility shall cause the shell to write a diagnostic message to standard error and exit as shown in
the following table:</p>

<center>
<table align="center" border="1" cellpadding="3">
<tbody><tr valign="top">
<th align="center">
<p class="tent"><b>Error</b></p>
</th>
<th align="center">
<p class="tent"><b>Special Built-In</b></p>
</th>
<th align="center">
<p class="tent"><b>Other Utilities</b></p>
</th>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Shell language syntax error</p>
</td>
<td align="left">
<p class="tent">Shall exit</p>
</td>
<td align="left">
<p class="tent">Shall exit</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Utility syntax error (option or operand error)</p>
</td>
<td align="left">
<p class="tent">Shall exit</p>
</td>
<td align="left">
<p class="tent">Shall not exit</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Redirection error</p>
</td>
<td align="left">
<p class="tent">Shall exit</p>
</td>
<td align="left">
<p class="tent">Shall not exit</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Variable assignment error</p>
</td>
<td align="left">
<p class="tent">Shall exit</p>
</td>
<td align="left">
<p class="tent">Shall not exit</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Expansion error</p>
</td>
<td align="left">
<p class="tent">Shall exit</p>
</td>
<td align="left">
<p class="tent">Shall exit</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Command not found</p>
</td>
<td align="left">
<p class="tent">N/A</p>
</td>
<td align="left">
<p class="tent">May exit</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Dot script not found</p>
</td>
<td align="left">
<p class="tent">Shall exit</p>
</td>
<td align="left">
<p class="tent">N/A</p>
</td>
</tr>
</tbody></table>
</center>

<p>An expansion error is one that occurs when the shell expansions defined in <a href="#tag_02_06">Word Expansions</a> are carried
out (for example, <tt>"${x!y}"</tt>, because <tt>'!'</tt> is not a valid operator); an implementation may treat these as syntax
errors if it is able to detect them during tokenization, rather than during expansion.</p>

<p>If any of the errors shown as "shall exit" or "(may) exit" occur in a subshell, the subshell shall (respectively may) exit
with a non-zero status, but the script containing the subshell shall not exit because of the error.</p>

<p>In all of the cases shown in the table, an interactive shell shall write a diagnostic message to standard error without
exiting.</p>

<h4><a name="tag_02_08_02">  2.8.2 </a>Exit Status for Commands</h4>

<p>Each command has an exit status that can influence the behavior of other shell commands. The exit status of commands that are
not utilities is documented in this section. The exit status of the standard utilities is documented in their respective
sections.</p>

<p>If a command is not found, the exit status shall be 127. If the
command name is found, but it is not an executable utility, the
exit status shall be 126. Applications that invoke utilities without
using the shell should use these exit status values to report
similar errors.</p>

<p>If a command fails during word expansion or redirection, its exit status shall be greater than zero.</p>

<p>Internally, for purposes of deciding whether a command exits with a non-zero exit status, the shell shall recognize the entire
status value retrieved for the command by the equivalent of the <a href="http://www.opengroup.org/onlinepubs/009695399/functions/wait.html"><i>wait</i>()</a> function
WEXITSTATUS macro (as defined in the System Interfaces volume of IEEE&nbsp;Std&nbsp;1003.1-2001). When reporting the exit status
with the special parameter <tt>'?'</tt>, the shell shall report the full eight bits of exit status available. The exit status of a
command that terminated because it received a signal shall be reported as greater than 128.</p>

<h3><a name="tag_02_09">   2.9 </a>Shell Commands</h3>

<p>This section describes the basic structure of shell commands. The following command descriptions each describe a format of the
command that is only used to aid the reader in recognizing the command type, and does not formally represent the syntax. Each
description discusses the semantics of the command; for a formal definition of the command language, consult <a href="#tag_02_10">Shell Grammar</a>.</p>

<p>A <i>command</i> is one of the following:</p>

<ul>
<li>
<p>Simple command (see <a href="#tag_02_09_01">Simple Commands</a>)</p>
</li>

<li>
<p>Pipeline (see <a href="#tag_02_09_02">Pipelines</a>)</p>
</li>

<li>
<p>List compound-list (see <a href="#tag_02_09_03">Lists</a>)</p>
</li>

<li>
<p>Compound command (see <a href="#tag_02_09_04">Compound Commands</a>)</p>
</li>

<li>
<p>Function definition (see <a href="#tag_02_09_05">Function Definition Command</a>)</p>
</li>
</ul>

<p>Unless otherwise stated, the exit status of a command shall be that
of the last simple command executed by the command. There
shall be no limit on the size of any shell command other than that
imposed by the underlying system (memory constraints, {ARG_MAX},
and so on).</p>

<h4><a name="tag_02_09_01">  2.9.1 </a>Simple Commands</h4>

<p>A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by
words and redirections, terminated by a control operator.</p>

<p>When a given simple command is required to be executed (that is, when any conditional construct such as an AND-OR list or a
<b>case</b> statement has not bypassed the simple command), the following expansions, assignments, and redirections shall all be
performed from the beginning of the command text to the end:</p>

<ol>
<li>
<p>The words that are recognized as variable assignments or redirections according to <a href="#tag_02_10_02">Shell Grammar
Rules</a> are saved for processing in steps 3 and 4.</p>
</li>

<li>
<p>The words that are not variable assignments or redirections shall be expanded. If any fields remain following their expansion,
the first field shall be considered the command name and remaining fields are the arguments for the command.</p>
</li>

<li>
<p>Redirections shall be performed as described in <a href="#tag_02_07">Redirection</a>.</p>
</li>

<li>
<p>Each variable assignment shall be expanded for tilde expansion, parameter expansion, command substitution, arithmetic expansion,
and quote removal prior to assigning the value.</p>
</li>
</ol>

<p>In the preceding list, the order of steps 3 and 4 may be reversed for the processing of special built-in utilities; see <a href="#tag_02_14">Special Built-In Utilities</a>.</p>

<p>If no command name results, variable assignments shall affect the
current execution environment. Otherwise, the variable
assignments shall be exported for the execution environment of the
command and shall not affect the current execution environment
(except for special built-ins). If any of the variable assignments
attempt to assign a value to a read-only variable, a variable
assignment error shall occur. See <a href="#tag_02_08_01">Consequences of Shell Errors</a> for the consequences of these
errors.</p>

<p>If there is no command name, any redirections shall be performed in
a subshell environment; it is unspecified whether this
subshell environment is the same one as that used for a command
substitution within the command. (To affect the current execution
environment, see the <a href="http://www.opengroup.org/onlinepubs/009695399/utilities/exec.html#tag_04_46"><i>exec</i>()</a>
special built-in.) If any of the redirections performed in the
current shell execution environment fail, the command shall immediately
fail with an exit status greater than zero, and the shell
shall write an error message indicating the failure. See <a href="#tag_02_08_01">Consequences of Shell Errors</a> for the
consequences of these failures on interactive and non-interactive shells.</p>

<p>If there is a command name, execution shall continue as described in <a href="#tag_02_09_01_01">Command Search and Execution</a>
. If there is no command name, but the command contained a command
substitution, the command shall complete with the exit status of
the last command substitution performed. Otherwise, the command shall
complete with a zero exit status.</p>

<h5><a name="tag_02_09_01_01"></a>Command Search and Execution</h5>

<p>If a simple command results in a command name and an optional list of arguments, the following actions shall be performed:</p>

<ol>
<li>
<p>If the command name does not contain any slashes, the first successful step in the following sequence shall occur:</p>

<ol type="a">
<li>
<p>If the command name matches the name of a special built-in utility, that special built-in utility shall be invoked.</p>
</li>

<li>
<p>If the command name matches the name of a function known to this shell, the function shall be invoked as described in <a href="#tag_02_09_05">Function Definition Command</a>. If the implementation has provided a standard utility in the form of a function,
it shall not be recognized at this point. It shall be invoked in conjunction with the path search in step 1d.</p>
</li>

<li>
<p>If the command name matches the name of a utility listed in the following table, that utility shall be invoked.</p>

<center>
<table align="center" cellpadding="3">
<tbody><tr valign="top">
<td align="left">
<p class="tent"><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/alias.html"><i>alias</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/bg.html"><i>bg</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/cd.html"><i>cd</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/command.html"><i>command</i></a><br>
&nbsp;</p>
</td>
<td align="left">
<p class="tent"><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/false.html"><i>false</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/fc.html"><i>fc</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/fg.html"><i>fg</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/getopts.html"><i>getopts</i></a><br>
&nbsp;</p>
</td>
<td align="left">
<p class="tent"><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/jobs.html"><i>jobs</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/kill.html"><i>kill</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/newgrp.html"><i>newgrp</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/pwd.html"><i>pwd</i></a><br>
&nbsp;</p>
</td>
<td align="left">
<p class="tent"><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/read.html"><i>read</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/true.html"><i>true</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/umask.html"><i>umask</i></a><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/unalias.html"><i>unalias</i></a><br>
&nbsp;</p>
</td>
<td align="left">
<p class="tent"><br>
<a href="http://www.opengroup.org/onlinepubs/009695399/utilities/wait.html"><i>wait</i></a><br>
&nbsp;</p>
</td>
</tr>
</tbody></table>
</center>
</li>

<li>
<p>Otherwise, the command shall be searched for using the <i>PATH</i> environment variable as described in the Base Definitions
volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href="http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html">Chapter 8, Environment Variables</a>:</p>

<ol type="i">
<li>
<p>If the search is successful:</p>

<ol type="a">
<li>
<p>If the system has implemented the utility as a regular built-in or as a shell function, it shall be invoked at this point in the
path search.</p>
</li>

<li>
<p>Otherwise, the shell executes the utility in a separate utility environment (see <a href="#tag_02_12">Shell Execution
Environment</a>) with actions equivalent to calling the <a href="http://www.opengroup.org/onlinepubs/009695399/functions/execve.html"><i>execve</i>()</a> function as defined
in the System Interfaces volume of IEEE&nbsp;Std&nbsp;1003.1-2001 with the <i>path</i> argument set to the pathname resulting from
the search, <i>arg</i>0 set to the command name, and the remaining arguments set to the operands, if any.</p>

<p>If the <a href="http://www.opengroup.org/onlinepubs/009695399/functions/execve.html"><i>execve</i>()</a>
function fails due to an error equivalent to the [ENOEXEC] error
defined in the System Interfaces volume of
IEEE&nbsp;Std&nbsp;1003.1-2001, the shell shall execute a command
equivalent to having a
shell invoked with the pathname resulting from the search as its first
operand, with any remaining arguments passed to the new
shell, except that the value of <tt>"$0"</tt> in the new shell may be
set to the command name. If the executable file is not a text
file, the shell may bypass this command execution. In this case, it
shall write an error message, and shall return an exit status
of 126.</p>
</li>
</ol>

<p>Once a utility has been searched for and found (either as a result of this specific search or as part of an unspecified shell
start-up activity), an implementation may remember its location and need not search for the utility again unless the <i>PATH</i>
variable has been the subject of an assignment. If the remembered location fails for a subsequent invocation, the shell shall
repeat the search to find the new location for the utility, if any.</p>
</li>

<li>
<p>If the search is unsuccessful, the command shall fail with an exit status of 127 and the shell shall write an error message.</p>
</li>
</ol>
</li>
</ol>
</li>

<li>
<p>If the command name contains at least one slash, the shell shall execute the utility in a separate utility environment with
actions equivalent to calling the <a href="http://www.opengroup.org/onlinepubs/009695399/functions/execve.html"><i>execve</i>()</a> function defined in the System Interfaces
volume of IEEE&nbsp;Std&nbsp;1003.1-2001 with the <i>path</i> and <i>arg</i>0 arguments set to the command name, and the remaining
arguments set to the operands, if any.</p>

<p>If the <a href="http://www.opengroup.org/onlinepubs/009695399/functions/execve.html"><i>execve</i>()</a> function fails due to an error equivalent to the [ENOEXEC] error,
the shell shall execute a command equivalent to having a shell invoked with the command name as its first operand, with any
remaining arguments passed to the new shell. If the executable file is not a text file, the shell may bypass this command
execution. In this case, it shall write an error message and shall return an exit status of 126.</p>
</li>
</ol>

<h4><a name="tag_02_09_02">  2.9.2 </a>Pipelines</h4>

<p>A <i>pipeline</i> is a sequence of one or more commands separated by the control operator <tt>'|'</tt>. The standard output of
all but the last command shall be connected to the standard input of the next command.</p>

<p>The format for a pipeline is:</p>

<blockquote>
<pre><b>[</b><tt>!</tt><b>]</b> <i>command1</i> <b>[</b> <tt>|</tt> <i>command2</i> <tt>...</tt><b>]</b>
</pre>
</blockquote>

<p>The standard output of <i>command1</i> shall be connected to the standard input of <i>command2</i>. The standard input, standard
output, or both of a command shall be considered to be assigned by the pipeline before any redirection specified by redirection
operators that are part of the command (see <a href="#tag_02_07">Redirection</a>).</p>

<p>If the pipeline is not in the background (see <a href="#tag_02_09_03_02">Asynchronous Lists</a>), the shell shall wait for the
last command specified in the pipeline to complete, and may also wait for all commands to complete.</p>

<h5><a name="tag_02_09_02_01"></a>Exit Status</h5>

<p>If the reserved word <b>!</b> does not precede the pipeline, the
exit status shall be the exit status of the last command
specified in the pipeline. Otherwise, the exit status shall be the
logical NOT of the exit status of the last command. That is, if
the last command returns zero, the exit status shall be 1; if the last
command returns greater than zero, the exit status shall be
zero.</p>

<h4><a name="tag_02_09_03">  2.9.3 </a>Lists</h4>

<p>An <i>AND-OR list</i> is a sequence of one or more pipelines separated by the operators <tt>"&amp;&amp;"</tt> and <tt>"||"</tt>
.</p>

<p>A <i>list</i> is a sequence of one or more AND-OR lists separated by the operators <tt>';'</tt> and <tt>'&amp;'</tt> and
optionally terminated by <tt>';'</tt>, <tt>'&amp;'</tt>, or &lt;newline&gt;.</p>

<p>The operators <tt>"&amp;&amp;"</tt> and <tt>"||"</tt> shall have equal precedence and shall be evaluated with left
associativity. For example, both of the following commands write solely <b>bar</b> to standard output:</p>

<blockquote>
<pre><tt>false &amp;&amp; echo foo || echo bar
true || echo foo &amp;&amp; echo bar
</tt>
</pre>
</blockquote>

<p>A <tt>';'</tt> or &lt;newline&gt; terminator shall cause the preceding AND-OR list to be executed sequentially; an
<tt>'&amp;'</tt> shall cause asynchronous execution of the preceding AND-OR list.</p>

<p>The term "compound-list" is derived from the grammar in <a href="#tag_02_10">Shell Grammar</a>; it is equivalent to a
sequence of <i>lists</i>, separated by &lt;newline&gt;s, that can be preceded or followed by an arbitrary number of
&lt;newline&gt;s.</p>

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

<h5><a name="tag_02_09_03_01"></a>Examples</h5>

<p>The following is an example that illustrates &lt;newline&gt;s in compound-lists:</p>

<blockquote>
<pre><tt>while
    # a couple of &lt;newline&gt;s
<br>
    # a list
    date &amp;&amp; who || ls; cat file
    # a couple of &lt;newline&gt;s
<br>
    # another list
    wc file &gt; output &amp; true
<br>
do
    # 2 lists
    ls
    cat file
done
</tt>
</pre>
</blockquote>

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

<hr>
<h5><a name="tag_02_09_03_02"></a>Asynchronous Lists</h5>

<p>If a command is terminated by the control operator ampersand ( <tt>'&amp;'</tt> ), the shell shall execute the command
asynchronously in a subshell. This means that the shell shall not wait for the command to finish before executing the next
command.</p>

<p>The format for running a command in the background is:</p>

<blockquote>
<pre><i>command1</i> <tt>&amp;</tt> <b>[</b><i>command2</i> <tt>&amp; ...</tt> <b>]</b>
</pre>
</blockquote>

<p>The standard input for an asynchronous list, before any explicit redirections are performed, shall be considered to be assigned
to a file that has the same properties as <b>/dev/null</b>. If it is an interactive shell, this need not happen. In all cases,
explicit redirection of standard input shall override this activity.</p>

<p>When an element of an asynchronous list (the portion of the list ended by an ampersand, such as <i>command1</i>,
above) is
started by the shell, the process ID of the last command in the
asynchronous list element shall become known in the current shell
execution environment; see <a href="#tag_02_12">Shell Execution Environment</a>. This process ID shall remain known until:</p>

<ol>
<li>
<p>The command terminates and the application waits for the process ID.</p>
</li>

<li>
<p>Another asynchronous list invoked before <tt>"$!"</tt> (corresponding to the previous asynchronous list) is expanded in the
current execution environment.</p>
</li>
</ol>

<p>The implementation need not retain more than the {CHILD_MAX} most recent entries in its list of known process IDs in the current
shell execution environment.</p>

<h5><a name="tag_02_09_03_03"></a>Exit Status</h5>

<p>The exit status of an asynchronous list shall be zero.</p>

<h5><a name="tag_02_09_03_04"></a>Sequential Lists</h5>

<p>Commands that are separated by a semicolon ( <tt>';'</tt> ) shall be executed sequentially.</p>

<p>The format for executing commands sequentially shall be:</p>

<blockquote>
<pre><i>command1</i> <b>[</b><tt>;</tt> <i>command2</i><b>]</b> <tt>...
</tt>
</pre>
</blockquote>

<p>Each command shall be expanded and executed in the order specified.</p>

<h5><a name="tag_02_09_03_05"></a>Exit Status</h5>

<p>The exit status of a sequential list shall be the exit status of the last command in the list.</p>

<h5><a name="tag_02_09_03_06"></a>AND Lists</h5>

<p>The control operator <tt>"&amp;&amp;"</tt> denotes an AND list. The format shall be:</p>

<blockquote>
<pre><i>command1</i> <b>[</b> <tt>&amp;&amp;</tt> <i>command2</i><b>]</b> <tt>...
</tt>
</pre>
</blockquote>

<p>First <i>command1</i> shall be executed. If its exit status is zero, <i>command2</i> shall be executed, and so on, until a
command has a non-zero exit status or there are no more commands left to execute. The commands are expanded only if they are
executed.</p>

<h5><a name="tag_02_09_03_07"></a>Exit Status</h5>

<p>The exit status of an AND list shall be the exit status of the last command that is executed in the list.</p>

<h5><a name="tag_02_09_03_08"></a>OR Lists</h5>

<p>The control operator <tt>"||"</tt> denotes an OR List. The format shall be:</p>

<blockquote>
<pre><i>command1</i> <b>[</b> <tt>||</tt> <i>command2</i><b>]</b> <tt>...
</tt>
</pre>
</blockquote>

<p>First, <i>command1</i> shall be executed. If its exit status is non-zero, <i>command2</i> shall be executed, and so on, until a
command has a zero exit status or there are no more commands left to execute.</p>

<h5><a name="tag_02_09_03_09"></a>Exit Status</h5>

<p>The exit status of an OR list shall be the exit status of the last command that is executed in the list.</p>

<h4><a name="tag_02_09_04">  2.9.4 </a>Compound Commands</h4>

<p>The shell has several programming constructs that are "compound
commands", which provide control flow for commands. Each of
these compound commands has a reserved word or control operator at the
beginning, and a corresponding terminator reserved word or
operator at the end. In addition, each can be followed by redirections
on the same line as the terminator. Each redirection shall
apply to all the commands within the compound command that do not
explicitly override that redirection.</p>

<h5><a name="tag_02_09_04_01"></a>Grouping Commands</h5>

<p>The format for grouping commands is as follows:</p>

<dl compact="compact">