ch04_09.htm

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<h4 class="refsect2">Examples</h4><blockquote><pre class="code">trap 'exec 2&gt;&amp;-' 0		<i class="lineannotation">Close standard error when </i>
			<i class="lineannotation">shell script exits (signal 0)</i>
$ <tt class="userinput"><b>exec /bin/csh</b></tt>		<i class="lineannotation">Replace Bourne shell with C shell</i>
$ <tt class="userinput"><b>exec &lt; infile</b></tt>		<i class="lineannotation">Reassign standard input to infile</i></pre></blockquote>

</td></tr><tr><td valign="top"><a name="exit">exit</a></a></td><td><tt class="literal">exit</tt> [<tt><em class="replaceable"><tt>n</tt></em></tt>]
<p>
<a name="IXT-4-122842" />Exit a shell script with status <em class="emphasis">n</em></tt> (e.g., <tt class="literal">exit 1</tt>).
<em class="emphasis">n</em></tt> can be 0 (success) or nonzero (failure).
If <em class="emphasis">n</em></tt> is not given, exit status is that of the most recent command.
<tt class="literal">exit</tt> can be issued at the command line to close a window (log out).
Exit statuses can range in value from 0 to 255.</p>
<h4 class="refsect2">Example</h4><blockquote><pre class="code">if [ $# -eq 0 ]
then
   echo "Usage:  $0 [-c] [-d] file(s)" 1&gt;&amp;2
   exit 1		# Error status
fi</pre></blockquote>

</td></tr><tr><td valign="top"><a name="export">export</a></a></td><td><tt class="literal">export</tt> [<tt><em class="replaceable"><tt>variables</em></tt>]<br><tt class="literal">export</tt> [<tt><em class="replaceable">name</em></tt><tt class="literal">=</tt>[<tt><em class="replaceable">value</tt></em></tt>] ...]<br><tt class="literal">export -p</tt>
<p>
<a name="IXT-4-122843" />Pass (export) the value of one or more shell <em class="emphasis">variables</em></tt>, 
giving global meaning to the variables (which are local by default).
For example, a variable defined in one shell script must be
exported if its value is used in other programs
called by the script.  If no <em class="emphasis">variables</em></tt> are given, 
<tt class="literal">export</tt> lists the variables exported by the current shell.
The
second form is the Korn shell version, which is similar to the first
form except that you can set a variable <em class="emphasis">name</em></tt> to a <em class="emphasis">value</em></tt>
before exporting it.
The third form is specific to <tt class="literal">ksh93</tt>.</p>


<h4 class="refsect2">Option</h4><dl><dt><tt class="literal">-p</tt></dt><dd>Print <tt class="literal">export</tt> before printing the
names and values of exported variables.
This allows saving a list of exported variables for rereading later.</p>
</dd>
</dl>

<h4 class="refsect2">Example</h4><p>In the Bourne shell, you would type:</p>
<blockquote><pre class="code">TERM=vt100
export TERM</pre></blockquote>
<p>In the Korn shell, you could type this instead:</p>
<blockquote><pre class="code">export TERM=vt100</pre></blockquote>

</td></tr><tr><td valign="top"><a name="false">false</a></a></td><td><tt class="literal">false</tt>
<p>
<a name="IXT-4-122844" /><tt class="literal">ksh88</tt> alias for <tt class="literal">let 0</tt>.
Built-in command in <tt class="literal">ksh93</tt> that exits with
a false return value.</p>
</td></tr><tr><td valign="top"><a name="fc">fc</a></a></td><td><tt class="literal">fc</tt> [<em class="replaceable"><tt>options</em>] [<em class="replaceable">first</em> [<em class="replaceable">last</em>]]<br><tt class="literal">fc -e -</tt> [<em class="replaceable">old</em><tt class="literal">=</tt><em class="replaceable">new</em>] [<em class="replaceable">command</tt></em>]
<p>
<a name="IXT-4-122845" /><tt class="literal">ksh88</tt> only.  Display or edit commands in the history list.
(Use only one of <tt class="literal">-l</tt> or <tt class="literal">-e</tt>.)


<em class="emphasis">first</em> and <em class="emphasis">last</em> are numbers or
strings specifying the range of commands to display or edit.
If <em class="emphasis">last</em> is omitted, <tt class="literal">fc</tt> applies to a single command (specified
by <em class="emphasis">first</em>).  If both <em class="emphasis">first</em> and <em class="emphasis">last</em> are omitted,
<tt class="literal">fc</tt> edits the previous command or lists the last 16.
The second form of <tt class="literal">fc</tt> takes a history <em class="emphasis">command</em>,
replaces <em class="emphasis">old</em> string with <em class="emphasis">new</em> string,
and executes the modified command.  If no strings are
specified, <em class="emphasis">command</em> is just reexecuted.  If no <em class="emphasis">command</em>
is given either, the previous command is reexecuted.
<em class="emphasis">command</em> is a number or string like <em class="emphasis">first</em>.
See the examples in <a href="ch04_05.htm#unut-ch-4-sect-5">Section 4.5</a>.</p>
<h4 class="refsect2">Options</h4>
<dl><dt><tt class="literal">-e</tt> [<tt><em class="replaceable">editor</em></tt>]</dt><dd>Invoke <em class="emphasis">editor</em></tt> to edit the specified history commands.
The default <em class="emphasis">editor</em></tt> is set by the shell variable FCEDIT.
If that variable is not
set, the default is <tt class="literal">/bin/ed</tt>.</p>
</dd>
<dt><tt class="literal">-e</tt> <tt class="literal">-</tt></dt><dd>Execute (or redo) a history command; refer to second syntax line above.</p>
</dd>
<dt><tt class="literal">-l</tt></dt><dd>List the specified command or range of commands, or list the last 16.</p>
</dd>
<dt><tt class="literal">-n</tt></dt><dd>Suppress command numbering from the <tt class="literal">-l</tt> listing.</p>
</dd>
<dt><tt class="literal">-r</tt></dt><dd>Reverse the order of the <tt class="literal">-l</tt> listing.</p>
</dd>
</dl>

</td></tr><tr><td valign="top"><a name="fc">fc</a></a></td><td><tt class="literal">fc</tt>
<p><tt class="literal">ksh93</tt> alias for <tt class="literal">hist</tt>.
<a name="IXT-4-122846" /></p>
</td></tr><tr><td valign="top"><a name="fg">fg</a></a></td><td><tt class="literal">fg</tt> [<tt><em class="replaceable"><tt>jobIDs</tt></em></tt>]
<p>
<a name="IXT-4-122847" />Bring current job or <em class="emphasis">jobIDs</em></tt> to the foreground.  
See <a href="ch04_06.htm#unut-ch-4-sect-6">Section 4.6</a></p>
</td></tr><tr><td valign="top"><a name="fi">fi</a></a></td><td><tt class="literal">fi</tt>
<p>
<a name="IXT-4-122848" />Reserved word that ends an <tt class="literal">if</tt> statement.
(Don't forget to use it!)</p>
</td></tr><tr><td valign="top"><a name="for">for</a></a></td><td><tt class="literal">for x</tt> [<tt class="literal">in</tt> <tt><em class="replaceable"><tt>list</em></tt>]<br><tt class="literal">do </tt><br>&#160;&#160;&#160;&#160;<tt><em class="replaceable">commands</tt></em></tt> <br><tt class="literal">done</tt>
<p>
<a name="IXT-4-122849" />For variable <em class="emphasis">x</em></tt> (in optional <em class="emphasis">list</em></tt> of values) do <em class="emphasis">commands</em></tt>.
If <tt class="literal">in</tt> <em class="emphasis">list</em></tt> is omitted,
<tt class="literal">"$@"</tt> (the positional parameters) is assumed.</p>
<h4 class="refsect2">Examples</h4><p>Paginate files specified on the command line; save each result:</p>
<blockquote><pre class="code">for file; do
     pr $file &gt; $file.tmp
done</pre></blockquote>
<p>Search chapters for a list of words (like <tt class="literal">fgrep -f</tt>):</p>
<blockquote><pre class="code">for item in `cat program_list`
do
     echo "Checking chapters for"
     echo "references to program $item..."
     grep -c "$item.[co]" chap*
done</pre></blockquote>


<p>Extract a one-word title from each file and use as new filename:</p>
<blockquote><pre class="code">for file
do
     name=`sed -n 's/NAME: //p' $file`
     mv $file $name
done</pre></blockquote>

</td></tr><tr><td valign="top"><a name="for">for</a></a></td><td><tt class="literal">for ((</tt><tt><em class="replaceable"><tt>init</em></tt><tt class="literal">;</tt> <tt><em class="replaceable">cond</em></tt><tt class="literal">;</tt> <tt><em class="replaceable">incr</em></tt><tt class="literal">))</tt><br><tt class="literal">do</tt><br>&#160;&#160;&#160;&#160;<tt><em class="replaceable">commands</tt></em></tt> <br><tt class="literal">done</tt>
<p>
<a name="IXT-4-122850" /><tt class="literal">ksh93</tt> only.
Arithmetic <tt class="literal">for</tt> loop, similar to C's.
Evaluate <em class="emphasis">init</em></tt>.
While <em class="emphasis">cond</em></tt> is true, execute the body of the loop.
Evaluate <em class="emphasis">incr</em></tt> before re-testing <em class="emphasis">cond</em></tt>.
Any one of the expressions may be omitted; a missing <em class="emphasis">cond</em></tt>
is treated as being true.</p>
<h4 class="refsect2">Examples</h4><p>Search for a phrase in each odd chapter:</p>
<blockquote><pre class="code">for ((x=1; x &lt;= 20; x += 2))
do
	grep $1 chap$x
done</pre></blockquote>

</td></tr><tr><td valign="top"><a name="function">function</a></a></td><td><tt class="literal">function</tt> <tt><em class="replaceable"><tt>name</em></tt> <tt class="literal">{</tt> <tt><em class="replaceable">commands</tt></em></tt><tt class="literal">; }</tt>
<p>
<a name="IXT-4-122851" />Korn shell only.
Define <em class="emphasis">name</em></tt> as a shell function.
See the description of semantic issues in the <em class="emphasis">name</em></tt>
<tt class="literal">()</tt> entry earlier.</p>
<h4 class="refsect2">Example</h4><p>Define a function to count files.</p>
<blockquote><pre class="code">$ <tt class="userinput"><b>function fcount {</b></tt>
&gt;	<tt class="userinput"><b>ls | wc -l</b></tt>
&gt; <tt class="userinput"><b>}</b></tt></pre></blockquote>

</td></tr><tr><td valign="top"><a name="functions">functions</a></a></td><td><tt class="literal">functions</tt>


<p>
<a name="IXT-4-122852" />Korn shell alias for <tt class="literal">typeset -f</tt>.
(Note the &#8220;s&#8221; in the name; <tt class="literal">function</tt>
is a Korn shell keyword.)
See <b class="emphasis-bold"><a href="#typeset">typeset</a></b> later in this listing.</p>
</td></tr>

<tr><td valign="top"><a name="getconf">getconf</a></a></td><td><p><tt class="literal">getconf</tt> [<tt><em class="replaceable"><tt>name</em></tt> [<tt><em class="replaceable">path</tt></em></tt>]]</p>
<p>
<a name="IXT-4-122853" /><tt class="literal">ksh93</tt> only.
Retrieve the values for parameters that can vary across
systems.  <em class="emphasis">name</em></tt> is the parameter to retrieve;
<em class="emphasis">path</em></tt> is a filename to test for parameters that
can vary on different filesystem types.</p>
<p>The parameters are defined by the POSIX 1003.1 and 1003.2 standards.
See the entry for <b class="emphasis-bold"><a href="#getconf">getconf</a></b> in <a href="ch02_01.htm">Chapter 2</a>.</p>
<h4 class="refsect2">Example</h4><p>Print the maximum value that can be held 
in a C <tt class="literal">int</tt>.</p>
<blockquote><pre class="code">$ <tt class="userinput"><b>getconf INT_MAX</b></tt>
2147483647</pre></blockquote>

</td></tr><tr><td valign="top"><a name="getopts">getopts</a></a></td><td><tt class="literal">getopts</tt> [<tt class="literal">-a</tt> <tt><em class="replaceable"><tt>name</em></tt>] <tt><em class="replaceable">string</em></tt> <tt><em class="replaceable">name</em></tt> [<tt><em class="replaceable">args</tt></em></tt>]
<p>
<a name="IXT-4-122854" />Process command-line arguments (or <em class="emphasis">args</em></tt>,
if specified) and check
for legal options.  <tt class="literal">getopts</tt>
is used in shell script loops and is intended to ensure standard syntax for
command-line options.  Standard syntax dictates that command-line
options begin with a <tt class="literal">+</tt> or a <tt class="literal">-</tt>.  Options can be
stacked; i.e., consecutive letters can follow a single <tt class="literal">-</tt>.  End
processing of options by specifying <tt class="literal">--</tt> on
the command line.  
<em class="emphasis">string</em></tt> contains the option letters
to be recognized by <tt class="literal">getopts</tt> when running
the shell script.  Valid options are processed
in turn and stored in the shell variable <em class="emphasis">name</em></tt>.  
If an option is followed by a
colon, the option must be followed by one or more arguments.  
(Multiple arguments must be given to the command as one shell <em class="emphasis">word</em></tt>.
This is done by quoting the arguments or separating them with commas.
The application must be written to expect multiple arguments in this
format.)
<tt class="literal">getopts</tt> uses the shell variables OPTARG and OPTIND.
<tt class="literal">getopts</tt> is available to non-Bourne shell users as
<tt class="literal">/usr/bin/getopts</tt>.</p>
<h4 class="refsect2">Option</h4><dl><dt><tt class="literal">-a</tt></dt><dd>Use <em class="emphasis">name</em></tt> in error messages about invalid
options. <tt class="literal">ksh93</tt> only.</p>
</dd>
</dl>

</td></tr><tr><td valign="top"><a name="hash">hash</a></a></td><td><tt class="literal">hash</tt> [<tt class="literal">-r</tt>] [<em class="replaceable"><tt>commands</tt></em>]
<p>
<a name="IXT-4-122855" />Bourne shell version.
As the shell finds commands along the search path ($PATH),
it remembers the found location in an internal hash table.
The next time you enter a command, the shell uses the value stored
in its hash table.</p>
<p>With no arguments, <tt class="literal">hash</tt> lists the current hashed commands.
The display shows <em class="emphasis">hits</em> (the number of times the command
is called by the shell) and <em class="emphasis">cost</em> (the level of work needed
to find the command).
Commands that were found in a relative directory have an
asterisk (<tt class="literal">*</tt>) added in the <em class="emphasis">hits</em> column.</p>
<p>With <em class="emphasis">commands</em>, the shell will add those commands to the
hash table.</p>
<p><tt class="literal">-r</tt> removes
commands from the hash list, either all of them or just the specified
<em class="emphasis">commands</em>.
The hash table is also cleared when PATH is assigned.
Use <tt class="literal">PATH=$PATH</tt> to clear the hash table without
affecting your search path.
This is most useful if you have installed a new version of a command
in a directory that is earlier in $PATH than the current
version of the command.</p>