ch04_09.htm

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(This allows replacement of system commands with faster, built-in versions.)
Otherwise, the built-in command is always found.</p>
<h4 class="refsect2">Options</h4><dl><dt><tt class="literal">-d</tt></dt><dd>Delete the built-in command <em class="emphasis">name</em></tt>.</p>
</dd>
<dt><tt class="literal">-f</tt></dt><dd>Load new built-in command from <em class="emphasis">library</em></tt>.</p>
</dd>
<dt><tt class="literal">-s</tt></dt><dd>Only


print &#8220;special&#8221; built-ins (those designated
as special by POSIX).</p>
</dd>
</dl>

</td></tr><tr><td valign="top"><a name="case">case</a></a></td>
<td><br><tt class="literal">case</tt> <em class="replaceable"><tt>value</em> <tt class="literal">in</tt><br>&#160;&#160;&#160;&#160;<em class="replaceable">pattern1</em><tt class="literal">)</tt> <em class="replaceable">cmds1</em><tt class="literal">;;</tt><br>&#160;&#160;&#160;&#160;<em class="replaceable">pattern2</em><tt class="literal">)</tt> <em class="replaceable">cmds2</tt></em><tt class="literal">;;</tt><br>&#160;&#160;&#160;&#160;.<br>&#160;&#160;&#160;&#160;.<br>&#160;&#160;&#160;&#160;.<br><tt class="literal">esac</tt>
<p>
<a name="IXT-4-122830" />Execute the first set of commands (<em class="emphasis">cmds1</em>) if <em class="emphasis">value</em> matches
<em class="emphasis">pattern1</em>, execute the second set of commands (<em class="emphasis">cmds2</em>) if 
<em class="emphasis">value</em> matches <em class="emphasis">pattern2</em>, etc.
Be sure the last command in each set ends with <tt class="literal">;;</tt>.
<em class="emphasis">value</em> is typically a positional parameter or other shell variable.
<em class="emphasis">cmds</em> are typically Unix commands, shell programming commands,
or variable assignments.
Patterns can use file-generation metacharacters.
Multiple patterns (separated by <tt class="literal">|</tt>) can be specified on the same line;
in this case, the associated <em class="emphasis">cmds</em> are executed 
whenever <em class="emphasis">value</em> matches any of these patterns.
See the Examples here and under <tt class="literal">eval</tt>.</p>
<h4 class="refsect2">Korn Shell Notes</h4><ul><li><p>The Korn shell allows <em class="emphasis">pattern</em> to be preceded by an optional
open parenthesis, as in <tt class="literal">(</tt><em class="replaceable"><tt>pattern</tt></em><tt class="literal">)</tt>.
It's useful for balancing
parentheses inside a <tt class="literal">$(&#160;)</tt> construct.</p>
</li><li><p>The Korn shell also allows a case to end with <tt class="literal">;&amp;</tt>
instead of <tt class="literal">;;</tt>.
In such cases 


control &#8220;falls through&#8221; to the group of statements for
the next <em class="emphasis">pattern</em>.</p>
</li></ul>

<h4 class="refsect2">Examples</h4><p>Check


first command-line argument and take appropriate action:</p>
<blockquote><pre class="code">case $1 in     # Match the first arg
    no|yes) response=1;;
    -[tT])  table=TRUE;;
    *)      echo "unknown option"; exit 1;;
esac</pre></blockquote>
<p>Read user-supplied lines until user exits:</p>
<blockquote><pre class="code">while :        # Null command; always true
do
   echo "Type . to finish ==&gt; \c"
   read line
   case "$line" in
      .) echo "Message done"
         break ;;
      *) echo "$line" &gt;&gt; $message ;;
   esac
done</pre></blockquote>

</td></tr><tr><td valign="top"><a name="cd">cd</a></a></td><td><tt class="literal">cd</tt> [<em class="replaceable"><tt>dir</em>]<br><tt class="literal">cd</tt> [<tt class="literal">-LP</tt>] [<em class="replaceable">dir</em>]<br><tt class="literal">cd</tt> [<tt class="literal">-LP</tt>] [<tt class="literal">-</tt>]<br><tt class="literal">cd</tt> [<tt class="literal">-LP</tt>] [<em class="replaceable">old new</tt></em>]
<p>
<a name="IXT-4-122831" />
<a name="IXT-4-122832" />With no arguments, change to home directory of user.
Otherwise, change working directory to <em class="emphasis">dir</em>.  If <em class="emphasis">dir</em> is a
relative pathname but is not in the current directory, the CDPATH
variable is searched.  The last three command forms are specific to the 
Korn shell, where <tt class="literal">-</tt> stands for the previous directory.
The fourth syntax modifies the current directory name
by replacing string <em class="emphasis">old</em> with <em class="emphasis">new</em> and
then switches to the resulting directory.</p>
<h4 class="refsect2">Options</h4><dl><dt><tt class="literal">-L</tt></dt><dd>Use the logical path (what the user typed, including any symbolic links)
for <tt class="literal">cd ..</tt>
and the value of PWD.
This is the default.</p>
</dd>
<dt><tt class="literal">-P</tt></dt><dd>Use the actual filesystem physical path
for <tt class="literal">cd ..</tt>
and the value of PWD.</p>
</dd>
</dl>

<h4 class="refsect2">Example</h4><blockquote><pre class="code">$ <tt class="userinput"><b>pwd</b></tt>
/var/spool/cron
$ <tt class="userinput"><b>cd cron uucp</b></tt>	<i class="lineannotation">cd prints the new directory</i>
/var/spool/uucp</pre></blockquote>

</td></tr><tr><td valign="top"><a name="command">command</a></a></td><td><tt class="literal">command</tt> [<tt class="literal">-pvV</tt>] <em class="replaceable"><tt>name</em> [<em class="replaceable">arg</tt></em> ...]
<p>
<a name="IXT-4-122833" /><tt class="literal">ksh93</tt> only.
Without <tt class="literal">-v</tt> or <tt class="literal">-V</tt>,
execute <em class="emphasis">name</em> with given arguments.
This command bypasses any aliases or functions that may be
defined for <em class="emphasis">name</em>.</p>
<h4 class="refsect2">Options</h4><dl><dt><tt class="literal">-p</tt></dt><dd>Use a predefined, default search path, not the current value of PATH.</p>
</dd>
<dt><tt class="literal">-v</tt></dt><dd>Just like <tt class="literal">whence</tt>.</p>
</dd>
<dt><tt class="literal">-V</tt></dt><dd>Just like <tt class="literal">whence -v</tt>.</p>
</dd>
</dl>

<h4 class="refsect2">Example</h4><p>Create


an alias for <tt class="literal">rm</tt> that will get the system's
version, and run it with the <tt class="literal">-i</tt> option:</p>
<blockquote><pre class="code"><tt class="userinput"><b>alias 'rm=command -p rm -i'</b></tt></pre></blockquote>

</td></tr><tr><td valign="top"><a name="continue">continue</a></a></td><td><tt class="literal">continue</tt> [<tt><em class="replaceable"><tt>n</tt></em></tt>]
<p>
<a name="IXT-4-122834" />Skip remaining commands in a <tt class="literal">for</tt>,
<tt class="literal">while</tt>,
<tt class="literal">select</tt>,
or <tt class="literal">until</tt> loop,
resuming with the next iteration of the loop
(or skipping <em class="emphasis">n</em></tt> loops).</p>
</td></tr><tr><td valign="top"><a name="disown">disown</a></a></td><td><tt class="literal">disown</tt> [<tt><em class="replaceable"><tt>job</tt></em></tt> ...]
<p>
<a name="IXT-4-122835" /><tt class="literal">ksh93</tt> only.
When a login shell exits, do not send a <tt class="literal">SIGHUP</tt>
to the given jobs.
If no jobs are listed, no background jobs
will receive <tt class="literal">SIGHUP</tt>.</p>
</td></tr><tr><td valign="top"><a name="do">do</a></a></td><td><tt class="literal">do</tt>
<p>
<a name="IXT-4-122836" />Reserved word that precedes the command sequence in a
<tt class="literal">for</tt>, <tt class="literal">while</tt>, <tt class="literal">until</tt>, or <tt class="literal">select</tt> statement.</p>
</td></tr><tr><td valign="top"><a name="done">done</a></a></td><td><tt class="literal">done</tt>
<p>
<a name="IXT-4-122837" />Reserved word that ends a 
<tt class="literal">for</tt>, <tt class="literal">while</tt>, <tt class="literal">until</tt>, or <tt class="literal">select</tt> statement.
</p>
</td></tr><tr><td valign="top"><a name="echo">echo</a></a></td><td><tt class="literal">echo</tt> [<tt class="literal">-n</tt>] [<tt><em class="replaceable"><tt>string</tt></em></tt>]
<p>
<a name="IXT-4-122838" />Write <em class="emphasis">string</em></tt> to standard output; if <tt class="literal">-n</tt> is
specified, the output is not terminated by a newline.
If no <em class="emphasis">string</em></tt> is supplied, echo a newline.
In the Korn shell, <tt class="literal">echo</tt> is built-in, and it
emulates the system's real <tt class="literal">echo</tt> command.<a href="#FOOTNOTE-6">[6]</a>
(See also <b class="emphasis-bold"><a href="#echo">echo</a></b> in <a href="ch02_01.htm">Chapter 2</a>.)
<tt class="literal">echo</tt> understands special escape characters,
which must be quoted (or escaped with a \)
to prevent interpretation by the shell:</p><blockquote class="footnote"><a name="FOOTNOTE-6" /><p> [6]But, if a path search finds <tt class="literal">/usr/bin/echo</tt>, the
<tt class="literal">ksh</tt> built-in <tt class="literal">echo</tt> doesn't accept the <tt class="literal">-n</tt> option.
(The situation with <tt class="literal">echo</tt> is a mess; consider using <tt class="literal">printf</tt> instead.)</p>
</blockquote>

<dl><dt><tt class="literal">\a</tt></dt><dd>Alert (ASCII BEL).
(Not in <tt class="literal">/bin/sh</tt>'s <tt class="literal">echo</tt>.)</p>
</dd>
<dt><tt class="literal">\b</tt></dt><dd>Backspace.</p>
</dd>
<dt><tt class="literal">\c</tt></dt><dd>Suppress the terminating newline (same as <tt class="literal">-n</tt>).</p>
</dd>
<dt><tt class="literal">\f</tt></dt><dd>Formfeed.</p>
</dd>
<dt><tt class="literal">\n</tt></dt><dd>Newline.</p>
</dd>
<dt><tt class="literal">\r</tt></dt><dd>Carriage return.</p>
</dd>
<dt><tt class="literal">\t</tt></dt><dd>Tab character.</p>
</dd>
<dt><tt class="literal">\v</tt></dt><dd>Vertical-tab character.</p>
</dd>
<dt><tt class="literal">\\</tt></dt><dd>Backslash.</p>
</dd>
<dt><tt class="literal">\0</tt><tt><em class="replaceable">nnn</em></tt></dt><dd>ASCII character represented by octal number <em class="emphasis">nnn</em></tt>, where <em class="emphasis">nnn</em></tt>
is one, two, or three digits and is preceded by a 0.</p>
</dd>
</dl>
<h4 class="refsect2">Examples</h4><blockquote><pre class="code">$ <tt class="userinput"><b>echo "testing printer" | lp</b></tt>
$ <tt class="userinput"><b>echo "Warning: ringing bell \a"</b></tt></pre></blockquote>


</td></tr><tr><td valign="top"><a name="esac">esac</a></a></td><td><tt class="literal">esac</tt>
<p>
<a name="IXT-4-122839" />Reserved word that ends a <tt class="literal">case</tt> statement.
Omitting <tt class="literal">esac</tt> is a common programming error.</p>
</td></tr><tr><td valign="top"><a name="eval">eval</a></a></td><td><tt class="literal">eval</tt> <tt><em class="replaceable"><tt>args</tt></em></tt>
<p>
<a name="IXT-4-122840" />Typically, <tt class="literal">eval</tt> is used in shell scripts,
and <em class="emphasis">args</em></tt> is a line of code that contains shell variables. 
<tt class="literal">eval</tt> forces variable expansion to happen first
and then runs the resulting command.
This &#8220;double-scanning&#8221; is useful any time shell variables 
contain input/output redirection symbols, aliases, or other shell variables.
(For example, redirection normally happens before variable 
expansion, so a variable containing redirection symbols must be
expanded first using <tt class="literal">eval</tt>; otherwise, the redirection symbols 
remain uninterpreted.)  See the C shell <tt class="literal">eval</tt>
(<a href="ch05_01.htm">Chapter 5</a>) for another example.</p>
<h4 class="refsect2">Example</h4><p>This fragment of a Bourne shell script shows how <tt class="literal">eval</tt>
constructs a command that is


interpreted in the right order:</p>
<blockquote><pre class="code">for option
do
   case "$option" in   <i class="lineannotation">Define where output goes</i>
      save) out=' &gt; $newfile' ;;
      show) out=' | more' ;;
   esac
done

eval sort $file $out</pre></blockquote>

</td></tr>

<tr><td valign="top"><a name="exec">exec</a></a></td><td><p><tt class="literal">exec</tt> [<tt><em class="replaceable"><tt>command</em></tt> <tt><em class="replaceable">args</em></tt> ...]</p><p><tt class="literal">exec</tt> [<tt class="literal">-a</tt> <tt><em class="replaceable">name</em></tt>] [<tt class="literal">-c</tt>] [<tt><em class="replaceable">command</em></tt> <tt><em class="replaceable">args</tt></em></tt> ... ]</p>
<p>
<a name="IXT-4-122841" />Execute <em class="emphasis">command</em></tt> in place of the current process
(instead of creating a new process).  <tt class="literal">exec</tt> is also useful for
opening, closing, or copying file descriptors.
The second form is for <tt class="literal">ksh93</tt> only.</p>
<h4 class="refsect2">Options</h4><dl><dt><tt class="literal">-a</tt></dt><dd>Use <em class="emphasis">name</em></tt> for the value
of <tt class="literal">argv[0]</tt>.</p>
</dd>
<dt><tt class="literal">-c</tt></dt><dd>Clear the environment before executing the program.</p>
</dd>
</dl>