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<!--Table of Child-Links-->
<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>

<UL>
<LI><A NAME="tex2html1853"
  HREF="#SECTION001010000000000000000">7.1 Introduction</A>
<LI><A NAME="tex2html1854"
  HREF="#SECTION001020000000000000000">7.2 Looping: the <TT>
<FONT COLOR="#0000ff">while</FONT></TT> and <TT>
<FONT COLOR="#0000ff">until</FONT></TT> Statements</A>
<LI><A NAME="tex2html1855"
  HREF="#SECTION001030000000000000000">7.3 Looping: the <TT>
<FONT COLOR="#0000ff">for</FONT></TT> Statement</A>
<LI><A NAME="tex2html1856"
  HREF="#SECTION001040000000000000000">7.4 <TT>
<FONT COLOR="#0000ff">break</FONT></TT>ing Out of Loops and <TT>
<FONT COLOR="#0000ff">continue</FONT></TT>ing</A>
<LI><A NAME="tex2html1857"
  HREF="#SECTION001050000000000000000">7.5 Looping Over Glob Expressions</A>
<LI><A NAME="tex2html1858"
  HREF="#SECTION001060000000000000000">7.6 The <TT>
<FONT COLOR="#0000ff">case</FONT></TT> Statement</A>
<LI><A NAME="tex2html1859"
  HREF="#SECTION001070000000000000000">7.7 Using Functions: the <TT>
<FONT COLOR="#0000ff">function</FONT></TT> Keyword</A>
<LI><A NAME="tex2html1860"
  HREF="#SECTION001080000000000000000">7.8 Properly Processing Command-Line Args: <TT>
<FONT COLOR="#0000ff">shift</FONT></TT></A>
<LI><A NAME="tex2html1861"
  HREF="#SECTION001090000000000000000">7.9 More on Command-Line Arguments: <TT>
<FONT COLOR="#0000ff">$@</FONT></TT> and <TT>
<FONT COLOR="#0000ff">$0</FONT></TT></A>
<LI><A NAME="tex2html1862"
  HREF="#SECTION0010100000000000000000">7.10 Single Forward Quote Notation</A>
<LI><A NAME="tex2html1863"
  HREF="#SECTION0010110000000000000000">7.11 Double-Quote Notation</A>
<LI><A NAME="tex2html1864"
  HREF="#SECTION0010120000000000000000">7.12 Backward-Quote Substitution</A>
</UL>
<!--End of Table of Child-Links-->
<HR>

<H1><A NAME="SECTION001000000000000000000">
7. Shell Scripting</A>
</H1>

<P>
<A NAME="chap:shellscript"></A>
<P>
This chapter introduces you to the concept of <I>computer programming</I>.
So far, you have entered commands one at a time. Computer programming is merely
the idea of getting a number of commands to be executed, that in combination
do some unique powerful function.

<P>

<H1><A NAME="SECTION001010000000000000000">
7.1 Introduction</A>
</H1>

<P>
To execute a number of commands in sequence, create a file with a <TT>
<FONT COLOR="#0000ff">.sh</FONT></TT>
extension, into which you will enter your commands. The <TT>
<FONT COLOR="#0000ff">.sh</FONT></TT> extension
is not strictly necessary but serves as a reminder that the file contains special
text called a <I>shell script</I>. From now on, the word <I>script</I> will
be used to describe any sequence of commands placed in a text file. Now do a

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>chmod&nbsp;0755&nbsp;myfile.sh</code><br>
</FONT></TD></TR></TABLE><P>
which allows the file to be run in the explained way.

<P>
Edit the file using your favorite text editor. The first line should be as follows with no
whitespace. <FONT COLOR="#ffa500">[Whitespace are tabs and spaces, and in some contexts, newline (end of
line) characters.]</FONT>
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
</FONT></TD></TR></TABLE><P>
The line dictates that the following program is a <I>shell</I> script, meaning
that it accepts the same sort of commands that you have normally been typing
at the prompt. Now enter a number of commands that you would like to be executed.
You can start with

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>echo&nbsp;"Hi&nbsp;there"</code><br>
<code>echo&nbsp;"what&nbsp;is&nbsp;your&nbsp;name?&nbsp;(Type&nbsp;your&nbsp;name&nbsp;here&nbsp;and&nbsp;press&nbsp;Enter)"</code><br>
<code>read&nbsp;NM</code><br>
<code>echo&nbsp;"Hello&nbsp;$NM"</code><br>
</FONT></TD></TR></TABLE><P>

<P>
Now, exit from your editor and type <TT>
<FONT COLOR="#0000ff">./myfile.sh</FONT></TT>. This will <I>execute</I> <FONT COLOR="#ffa500">[Cause the computer to read and act on your list of commands, also called <I>running</I>
the program.
]</FONT> the file. Note that typing <TT>
<FONT COLOR="#0000ff">./myfile.sh</FONT></TT> is no different from typing
any other command at the shell prompt. Your file <TT>
<FONT COLOR="#0000ff">myfile.sh</FONT></TT> has in fact
become a new U<SMALL>NIX</SMALL> command all of its own.

<P>
Note what the <TT>
<FONT COLOR="#0000ff">read</FONT></TT> command is doing. It creates a pigeonhole called
<TT>
<FONT COLOR="#0000ff">NM</FONT></TT>, and then inserts text read from the keyboard into that pigeonhole.
Thereafter, whenever the shell encounters <TT>
<FONT COLOR="#0000ff">NM</FONT></TT>, its contents are
written out instead of the letters NM (provided you write a <TT>
<FONT COLOR="#0000ff">$</FONT></TT> in front
of it). We say that <TT>
<FONT COLOR="#0000ff">NM</FONT></TT> is a <I>variable</I> because its contents can
vary.

<P>
You can use shell scripts like a calculator. Try
<A NAME="page:arithshellbrace"></A>
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>echo&nbsp;"I&nbsp;will&nbsp;work&nbsp;out&nbsp;X*Y"</code><br>
<code>echo&nbsp;"Enter&nbsp;X"</code><br>
<code>read&nbsp;X</code><br>
<code>echo&nbsp;"Enter&nbsp;Y"</code><br>
<code>read&nbsp;Y</code><br>
<code>echo&nbsp;"X*Y&nbsp;=&nbsp;$X*$Y&nbsp;=&nbsp;$[X*Y]"</code><br>
</FONT></TD></TR></TABLE><P>
The <TT>
<FONT COLOR="#0000ff">[</FONT></TT> and <TT>
<FONT COLOR="#0000ff">]</FONT></TT> mean that everything between must be <I>evaluated</I> <FONT COLOR="#ffa500">[Substituted, worked out, or reduced to some simplified form.
]</FONT> as a <I>numerical expression</I> <FONT COLOR="#ffa500">[Sequence of numbers with <TT>
<FONT COLOR="#0000ff">+</FONT></TT>, <TT>
<FONT COLOR="#0000ff">-</FONT></TT>, <TT>
<FONT COLOR="#0000ff">*</FONT></TT>, etc. between them.
]</FONT>. You can, in fact, do a calculation at any time by typing at the prompt

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>echo&nbsp;$[3*6+2*8+9]</code><br>
</FONT></TD></TR></TABLE><P>
 <FONT COLOR="#ffa500">[Note that the shell that you are using allows such <TT>
<FONT COLOR="#0000ff">[ ]</FONT></TT>
notation. On some U<SMALL>NIX</SMALL> systems you will have to use the <TT>
<FONT COLOR="#0000ff">expr</FONT></TT>
command to get the same effect.]</FONT>
<P>

<H1><A NAME="SECTION001020000000000000000">
7.2 Looping to Repeat Commands: the <TT>
<FONT COLOR="#0000ff">while</FONT></TT> and <TT>
<FONT COLOR="#0000ff">until</FONT></TT> Statements</A>
</H1>

<P>
The shell reads each line in succession from top to bottom: this is called <I>program
flow</I>. Now suppose you would like a command to be executed more than once--you
would like to alter the program flow so that the shell reads particular commands
repeatedly. The <TT>
<FONT COLOR="#0000ff">while</FONT></TT> command executes a sequence of commands many
times. Here is an example (<TT>
<FONT COLOR="#0000ff">-le</FONT></TT> stands for <I>less than or equal</I>):

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>N=1</code><br>
<code>while&nbsp;test&nbsp;"$N"&nbsp;-le&nbsp;"10"</code><br>
<code>do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"Number&nbsp;$N"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;N=$[N+1]</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>
The <TT>
<FONT COLOR="#0000ff">N=1</FONT></TT> creates a variable called <TT>
<FONT COLOR="#0000ff">N</FONT></TT> and places the number
<TT>
<FONT COLOR="#0000ff">1</FONT></TT> into it. The <TT>
<FONT COLOR="#0000ff">while</FONT></TT> command executes all the commands between
the <TT>
<FONT COLOR="#0000ff">do</FONT></TT> and the <TT>
<FONT COLOR="#0000ff">done</FONT></TT> repetitively until the <TT>
<FONT COLOR="#0000ff">test</FONT></TT>
condition is no longer true (i.e., until <TT>
<FONT COLOR="#0000ff">N</FONT></TT> is greater than <TT>
<FONT COLOR="#0000ff">10</FONT></TT>).
The <TT>
<FONT COLOR="#0000ff">-le</FONT></TT> stands for <I>less than or equal to</I>. See <TT>
<FONT COLOR="#0000ff">test</FONT></TT>(1) (that is, run <TT>
<FONT COLOR="#0000ff">man 1 test</FONT></TT>)
to learn about the other types of tests you can do on variables. Also be aware
of how <TT>
<FONT COLOR="#0000ff">N</FONT></TT> is replaced with a new value that becomes <TT>
<FONT COLOR="#0000ff">1</FONT></TT> greater
with each repetition of the <TT>
<FONT COLOR="#0000ff">while</FONT></TT> loop.

<P>
You should note here that each line is a distinct command--the commands are
<I>newline-separated</I>. You can also have more than one command on a line
by separating them with a semicolon as follows:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>N=1&nbsp;&#059;&nbsp;while&nbsp;test&nbsp;"$N"&nbsp;-le&nbsp;"10"&#059;&nbsp;do&nbsp;echo&nbsp;"Number&nbsp;$N"&#059;&nbsp;N=$[N+1]&nbsp;&#059;&nbsp;done</code><br>
</FONT></TD></TR></TABLE><P>
(Try counting down from 10 with <TT>
<FONT COLOR="#0000ff">-ge</FONT></TT> (<I>greater than or equal</I>).)
It is easy to see that shell scripts are extremely powerful, because any kind
of command can be executed with conditions and loops.

<P>
The <TT>
<FONT COLOR="#0000ff">until</FONT></TT> statement is identical to <TT>
<FONT COLOR="#0000ff">while</FONT></TT> except that the
reverse logic is applied. The same functionality can be achieved with
<TT>
<FONT COLOR="#0000ff">-gt</FONT></TT> (<I>greater than</I>):

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>N=1&nbsp;&#059;&nbsp;until&nbsp;test&nbsp;"$N"&nbsp;-gt&nbsp;"10"&#059;&nbsp;do&nbsp;echo&nbsp;"Number&nbsp;$N"&#059;&nbsp;N=$[N+1]&nbsp;&#059;&nbsp;done</code><br>
</FONT></TD></TR></TABLE><P>

<P>

<H1><A NAME="SECTION001030000000000000000">
7.3 Looping to Repeat Commands: the <TT>
<FONT COLOR="#0000ff">for</FONT></TT> Statement</A>
</H1>

<P>
The <TT>
<FONT COLOR="#0000ff">for</FONT></TT> command also allows execution of commands multiple times. It
works like this:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>for&nbsp;i&nbsp;in&nbsp;cows&nbsp;sheep&nbsp;chickens&nbsp;pigs</code><br>
<code>do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"$i&nbsp;is&nbsp;a&nbsp;farm&nbsp;animal"</code><br>
<code>done</code><br>
<code>echo&nbsp;-e&nbsp;"but\nGNUs&nbsp;are&nbsp;not&nbsp;farm&nbsp;animals"</code><br>
</FONT></TD></TR></TABLE><P>

<P>
The <TT>
<FONT COLOR="#0000ff">for</FONT></TT> command takes each string after the <TT>
<FONT COLOR="#0000ff">in</FONT></TT>, and executes
the lines between <TT>
<FONT COLOR="#0000ff">do</FONT></TT> and <TT>
<FONT COLOR="#0000ff">done</FONT></TT> with <TT>
<FONT COLOR="#0000ff">i</FONT></TT> substituted
for that string. The strings can be anything (even numbers) but are often file names.

<P>
The <TT>
<FONT COLOR="#0000ff">if</FONT></TT> command executes a number of commands if a condition is met
(<TT>
<FONT COLOR="#0000ff">-gt</FONT></TT> stands for <I>greater than</I>, <TT>
<FONT COLOR="#0000ff">-lt</FONT></TT> stands for <I>less
than</I>). The <TT>
<FONT COLOR="#0000ff">if</FONT></TT> command executes all the lines between the <TT>
<FONT COLOR="#0000ff">if</FONT></TT>
and the <TT>
<FONT COLOR="#0000ff">fi</FONT></TT> (``if'' spelled backwards).

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>X=10</code><br>
<code>Y=5</code><br>
<code>if&nbsp;test&nbsp;"$X"&nbsp;-gt&nbsp;"$Y"&nbsp;&#059;&nbsp;then</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"$X&nbsp;is&nbsp;greater&nbsp;than&nbsp;$Y"</code><br>
<code>fi</code><br>
</FONT></TD></TR></TABLE><P>

<P>
The <TT>
<FONT COLOR="#0000ff">if</FONT></TT> command in its full form can contain as much as:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>X=10</code><br>
<code>Y=5</code><br>
<code>if&nbsp;test&nbsp;"$X"&nbsp;-gt&nbsp;"$Y"&nbsp;&#059;&nbsp;then</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"$X&nbsp;is&nbsp;greater&nbsp;than&nbsp;$Y"</code><br>
<code>elif&nbsp;test&nbsp;"$X"&nbsp;-lt&nbsp;"$Y"&nbsp;&#059;&nbsp;then</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"$X&nbsp;is&nbsp;less&nbsp;than&nbsp;$Y"</code><br>
<code>else</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"$X&nbsp;is&nbsp;equal&nbsp;to&nbsp;$Y"</code><br>
<code>fi</code><br>
</FONT></TD></TR></TABLE><P>

<P>
Now let us create a script that interprets its arguments. Create a new script
called <TT>
<FONT COLOR="#0000ff">backup-lots.sh</FONT></TT>, containing:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>for&nbsp;i&nbsp;in&nbsp;0&nbsp;1&nbsp;2&nbsp;3&nbsp;4&nbsp;5&nbsp;6&nbsp;7&nbsp;8&nbsp;9&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;cp&nbsp;$1&nbsp;$1.BAK-$i</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>

<P>
Now create a file <TT>
<FONT COLOR="#0000ff">important_data</FONT></TT> with anything in it and then run
<TT>
<FONT COLOR="#0000ff">./backup-lots.sh&nbsp;important_data</FONT></TT>, which will copy the file 10 times
with 10 different extensions. As you can see, the variable <TT>
<FONT COLOR="#0000ff">$1</FONT></TT> has
a special meaning--it is the first argument on the command-line. Now let's
get a little bit more sophisticated (<TT>
<FONT COLOR="#0000ff">-e</FONT></TT> test whether the file <I>exists</I>):

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>10</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>if&nbsp;test&nbsp;"$1"&nbsp;=&nbsp;""&nbsp;&#059;&nbsp;then</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"Usage:&nbsp;backup-lots.sh&nbsp;&#060;filename&#062;"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit</code><br>
<code>fi</code><br>
<code>for&nbsp;i&nbsp;in&nbsp;0&nbsp;1&nbsp;2&nbsp;3&nbsp;4&nbsp;5&nbsp;6&nbsp;7&nbsp;8&nbsp;9&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;NEW_FILE=$1.BAK-$i</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if&nbsp;test&nbsp;-e&nbsp;$NEW_FILE&nbsp;&#059;&nbsp;then</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"backup-lots.sh:&nbsp;**warning**&nbsp;$NEW_FILE"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;already&nbsp;exists&nbsp;-&nbsp;skipping"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;cp&nbsp;$1&nbsp;$NEW_FILE</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;fi</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>

<P>

<H1><A NAME="SECTION001040000000000000000">
7.4 <TT>
<FONT COLOR="#0000ff">break</FONT></TT>ing Out of Loops and <TT>
<FONT COLOR="#0000ff">continue</FONT></TT>ing</A>
</H1>

<P>
A loop that requires premature termination can
include the <TT>
<FONT COLOR="#0000ff">break</FONT></TT> statement within it:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>10</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>for&nbsp;i&nbsp;in&nbsp;0&nbsp;1&nbsp;2&nbsp;3&nbsp;4&nbsp;5&nbsp;6&nbsp;7&nbsp;8&nbsp;9&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;NEW_FILE=$1.BAK-$i</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if&nbsp;test&nbsp;-e&nbsp;$NEW_FILE&nbsp;&#059;&nbsp;then</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"backup-lots.sh:&nbsp;**error**&nbsp;$NEW_FILE"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;already&nbsp;exists&nbsp;-&nbsp;exitting"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;break</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;cp&nbsp;$1&nbsp;$NEW_FILE</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;fi</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>
which causes program execution to continue on the line after the <TT>
<FONT COLOR="#0000ff">done</FONT></TT>.
If two loops are nested within each other, then the command <TT>
<FONT COLOR="#0000ff">break 2</FONT></TT>
causes program execution to break out of <I>both</I> loops; and so
on for values above <TT>
<FONT COLOR="#0000ff">2</FONT></TT>.

<P>
The <TT>
<FONT COLOR="#0000ff">continue</FONT></TT> statement is also useful for terminating
the current iteration of the loop. This means that if a <TT>
<FONT COLOR="#0000ff">continue</FONT></TT>
statement is encountered, execution will immediately continue from
the top of the loop, thus ignoring the remainder of the body of
the loop:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>10</code></font><code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>for&nbsp;i&nbsp;in&nbsp;0&nbsp;1&nbsp;2&nbsp;3&nbsp;4&nbsp;5&nbsp;6&nbsp;7&nbsp;8&nbsp;9&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;NEW_FILE=$1.BAK-$i</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if&nbsp;test&nbsp;-e&nbsp;$NEW_FILE&nbsp;&#059;&nbsp;then</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"backup-lots.sh:&nbsp;**warning**&nbsp;$NEW_FILE"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;already&nbsp;exists&nbsp;-&nbsp;skipping"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;continue</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;fi</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;cp&nbsp;$1&nbsp;$NEW_FILE</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>

<P>
Note that both <TT>
<FONT COLOR="#0000ff">break</FONT></TT> and <TT>
<FONT COLOR="#0000ff">continue</FONT></TT> work inside
<TT>
<FONT COLOR="#0000ff">for</FONT></TT>, <TT>
<FONT COLOR="#0000ff">while</FONT></TT>, and <TT>
<FONT COLOR="#0000ff">until</FONT></TT> loops.

<P>

<H1><A NAME="SECTION001050000000000000000">
7.5 Looping Over Glob Expressions</A>
</H1>

<P>
We know that the shell can expand file names when given <I>wildcards</I>. For
instance, we can type <TT>
<FONT COLOR="#0000ff">ls&nbsp;*.txt</FONT></TT> to list all files ending with <TT>
<FONT COLOR="#0000ff">.txt</FONT></TT>.
This applies equally well in any situation, for instance:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>for&nbsp;i&nbsp;in&nbsp;*.txt&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"found&nbsp;a&nbsp;file:"&nbsp;$i</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>

<P>
The <TT>
<FONT COLOR="#0000ff">*.txt</FONT></TT> is expanded to all matching files. <I>These files
are searched for in the current directory</I>. If you include an absolute path
then the shell will search in that directory:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>for&nbsp;i&nbsp;in&nbsp;/usr/doc/*/*.txt&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"found&nbsp;a&nbsp;file:"&nbsp;$i</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>

<P>
This example demonstrates the shell's ability to search for matching files and expand
an absolute path.

<P>

<H1><A NAME="SECTION001060000000000000000">
7.6 The <TT>
<FONT COLOR="#0000ff">case</FONT></TT> Statement</A>
</H1>

<P>
The <TT>
<FONT COLOR="#0000ff">case</FONT></TT> statement can make a potentially complicated program very
short. It is best explained with an example.

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>10</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>15</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>20</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>case&nbsp;$1&nbsp;in</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--test|-t)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"you&nbsp;used&nbsp;the&nbsp;--test&nbsp;option"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit&nbsp;0</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--help|-h)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"Usage:"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;myprog.sh&nbsp;[--test|--help|--version]"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit&nbsp;0</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--version|-v)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"myprog.sh&nbsp;version&nbsp;0.0.1"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit&nbsp;0</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-*)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"No&nbsp;such&nbsp;option&nbsp;$1"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"Usage:"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;myprog.sh&nbsp;[--test|--help|--version]"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit&nbsp;1</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>esac</code><br>
<code>&nbsp;</code><br>
<code>echo&nbsp;"You&nbsp;typed&nbsp;\"$1\"&nbsp;on&nbsp;the&nbsp;command-line"</code><br>
</FONT></TD></TR></TABLE><P>

<P>
Above you can see that we are trying to process the first argument to a program.
It can be one of several options, so using <TT>
<FONT COLOR="#0000ff">if</FONT></TT> statements will result
in a long program. The <TT>
<FONT COLOR="#0000ff">case</FONT></TT> statement allows us to specify several
possible statement blocks depending on the value of a variable. Note how each
statement block is separated by <TT>
<FONT COLOR="#0000ff">;;</FONT></TT>. The strings before the <TT>
<FONT COLOR="#0000ff">)</FONT></TT>
are glob expression matches. The first successful match causes that block to
be executed. The <TT>
<FONT COLOR="#0000ff">&#124;</FONT></TT> symbol enables us to enter several possible glob
expressions.

<P>

<H1><A NAME="SECTION001070000000000000000">
7.7 Using Functions: the <TT>
<FONT COLOR="#0000ff">function</FONT></TT> Keyword</A>
</H1>

<P>
<A NAME="sec:functions"></A>
<P>
So far, our programs execute mostly from top to bottom. Often, code needs to
be repeated, but it is considered bad programming practice to repeat groups
of statements that have the same functionality. Function definitions provide
a way to group statement blocks into one. A function
groups a list of commands and assigns it a name. For example:

<P>

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>10</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>15</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>20</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>25</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>&nbsp;</code><br>
<code>function&nbsp;usage&nbsp;()</code><br>
<code>{</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"Usage:"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;myprog.sh&nbsp;[--test|--help|--version]"</code><br>
<code>}</code><br>
<code>&nbsp;</code><br>
<code>case&nbsp;$1&nbsp;in</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--test|-t)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"you&nbsp;used&nbsp;the&nbsp;--test&nbsp;option"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit&nbsp;0</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--help|-h)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;usage</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--version|-v)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"myprog.sh&nbsp;version&nbsp;0.0.2"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit&nbsp;0</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-*)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"Error:&nbsp;no&nbsp;such&nbsp;option&nbsp;$1"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;usage</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit&nbsp;1</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>esac</code><br>
<code>&nbsp;</code><br>
<code>echo&nbsp;"You&nbsp;typed&nbsp;\"$1\"&nbsp;on&nbsp;the&nbsp;command-line"</code><br>
</FONT></TD></TR></TABLE><P>

<P>
Wherever the <TT>
<FONT COLOR="#0000ff">usage</FONT></TT> keyword appears, it is effectively substituted
for the two lines inside the <TT>
<FONT COLOR="#0000ff">&#123;</FONT></TT> and <TT>
<FONT COLOR="#0000ff">&#125;</FONT></TT>. There are obvious
advantages to this approach: if you would like to change the program <I>usage</I>
description, you only need to change it in one place in the code. Good programs
use functions so liberally that they never have more than 50 lines of program
code in a row.

<P>

<H1><A NAME="SECTION001080000000000000000">
7.8 Properly Processing Command-Line Arguments: the <TT>
<FONT COLOR="#0000ff">shift</FONT></TT> Keyword</A>
</H1>

<P>
Most programs we have seen can take many command-line arguments, sometimes in any order.
Here is how we can make our own shell scripts with this functionality. The command-line
arguments can be reached with <TT>
<FONT COLOR="#0000ff">$1</FONT></TT>, <TT>
<FONT COLOR="#0000ff">$2</FONT></TT>, etc. The script,

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>&nbsp;</code><br>
<code>echo&nbsp;"The&nbsp;first&nbsp;argument&nbsp;is:&nbsp;$1,&nbsp;second&nbsp;argument&nbsp;is:&nbsp;$2,&nbsp;third&nbsp;argument&nbsp;is:&nbsp;$3"</code><br>
</FONT></TD></TR></TABLE><P>
can be run with

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>myfile.sh&nbsp;dogs&nbsp;cats&nbsp;birds</code><br>
</FONT></TD></TR></TABLE><P>
and prints

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>The&nbsp;first&nbsp;argument&nbsp;is:&nbsp;dogs,&nbsp;second&nbsp;argument&nbsp;is:&nbsp;cats,&nbsp;third&nbsp;argument&nbsp;is:&nbsp;birds</code><br>
</FONT></TD></TR></TABLE><P>

<P>
Now we need to loop through each argument and decide what to do with it. A script
like

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>for&nbsp;i&nbsp;in&nbsp;$1&nbsp;$2&nbsp;$3&nbsp;$4&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#060;statments&#062;</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>
doesn't give us much flexibilty. The <TT>
<FONT COLOR="#0000ff">shift</FONT></TT> keyword is meant to make
things easier. It shifts up all the arguments by one place so that <TT>
<FONT COLOR="#0000ff">$1</FONT></TT>
gets the value of&nbsp;<TT>
<FONT COLOR="#0000ff">$2</FONT></TT>, <TT>
<FONT COLOR="#0000ff">$2</FONT></TT> gets the value of <TT>
<FONT COLOR="#0000ff">$3</FONT></TT>,
and so on. (<TT>
<FONT COLOR="#0000ff">!=</FONT></TT> tests that the <TT>
<FONT COLOR="#0000ff">&#34;$1&#34;</FONT></TT> is not
equal to <TT>
<FONT COLOR="#0000ff">&#34;&#34;</FONT></TT>, that is, whether it is empty and is hence past
the last argument.) Try

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>while&nbsp;test&nbsp;"$1"&nbsp;!=&nbsp;""&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;$1</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;shift</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>
and run the program with lots of arguments.

<P>
Now we can put any sort of condition
statements within the loop to process the arguments in turn:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>10</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>15</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>20</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>25</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>30</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>#!/bin/sh</code><br>
<code>&nbsp;</code><br>
<code>function&nbsp;usage&nbsp;()</code><br>
<code>{</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"Usage:"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;myprog.sh&nbsp;[--test|--help|--version]&nbsp;[--echo&nbsp;&#060;text&#062;]"</code><br>
<code>}</code><br>
<code>&nbsp;</code><br>
<code>while&nbsp;test&nbsp;"$1"&nbsp;!=&nbsp;""&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;case&nbsp;$1&nbsp;in</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--echo|-e)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"$2"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;shift</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--test|-t)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"you&nbsp;used&nbsp;the&nbsp;--test&nbsp;option"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--help|-h)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;usage</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit&nbsp;0</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--version|-v)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"myprog.sh&nbsp;version&nbsp;0.0.3"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit&nbsp;0</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-*)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"Error:&nbsp;no&nbsp;such&nbsp;option&nbsp;$1"</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;usage</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit&nbsp;1</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#059;&#059;</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;esac</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;shift</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>

<P>
<TT>
<FONT COLOR="#0000ff">myprog.sh</FONT></TT> can now run with multiple arguments on the command-line.

<P>

<H1><A NAME="SECTION001090000000000000000">
7.9 More on Command-Line Arguments: <TT>
<FONT COLOR="#0000ff">$@</FONT></TT> and <TT>
<FONT COLOR="#0000ff">$0</FONT></TT></A>
</H1>

<P>
Whereas <TT>
<FONT COLOR="#0000ff">$1</FONT></TT>, <TT>
<FONT COLOR="#0000ff">$2</FONT></TT>, <TT>
<FONT COLOR="#0000ff">$3</FONT></TT>, etc. expand to the individual
arguments passed to the program, <TT>
<FONT COLOR="#0000ff">$@</FONT></TT> expands to <I>all</I> arguments.
This behavior is useful for passing all remaining arguments onto a second command. For
instance,

<P>

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>if&nbsp;test&nbsp;"$1"&nbsp;=&nbsp;"--special"&nbsp;&#059;&nbsp;then</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;shift</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;myprog2.sh&nbsp;"$@"</code><br>
<code>fi</code><br>
</FONT></TD></TR></TABLE><P>
<TT>
<FONT COLOR="#0000ff">$0</FONT></TT> means the name of the program itself and not any command-line argument.
It is the command used to invoke the current program. In the above cases, it
is <TT>
<FONT COLOR="#0000ff">./myprog.sh</FONT></TT>. Note that <TT>
<FONT COLOR="#0000ff">$0</FONT></TT> is immune to <TT>
<FONT COLOR="#0000ff">shift</FONT></TT> operations.

<P>

<H1><A NAME="SECTION0010100000000000000000">
7.10 Single Forward Quote Notation</A>
</H1>

<P>
Single forward quotes <TT>
<FONT COLOR="#0000ff">&nbsp;'&nbsp;</FONT></TT> <I>protect</I> the enclosed text from
the shell. In other words, you can place any odd characters inside forward quotes,
and the shell will treat them literally and reproduce your text exactly. For
instance, you may want to <TT>
<FONT COLOR="#0000ff">echo</FONT></TT> an actual <TT>
<FONT COLOR="#0000ff">$</FONT></TT> to the screen to
produce an output like&nbsp;<TT>
<FONT COLOR="#0000ff">costs&nbsp;$1000</FONT></TT>. You can use <TT>
<FONT COLOR="#0000ff">echo&nbsp;'costs
$1000'</FONT></TT> instead of <TT>
<FONT COLOR="#0000ff">echo&nbsp;&#34;costs&nbsp;$1000&#34;</FONT></TT>.

<P>

<H1><A NAME="SECTION0010110000000000000000">
7.11 Double-Quote Notation</A>
</H1>

<P>
Double quotes <TT>
<FONT COLOR="#0000ff">&nbsp;&#34;&nbsp;</FONT></TT> have the opposite sense of single quotes.
They allow <I>all</I> shell interpretations to take place inside them. The
reason they are used at all is only to group text containing whitespace into
a single word, because the shell will usually break up text along whitespace
boundaries. Try,

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>for&nbsp;i&nbsp;in&nbsp;"henry&nbsp;john&nbsp;mary&nbsp;sue"&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;"$i&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;is&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;a&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;person"</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>
compared to

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>for&nbsp;i&nbsp;in&nbsp;henry&nbsp;john&nbsp;mary&nbsp;sue&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;$i&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;is&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;a&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;person</code><br>
<code>done</code><br>
</FONT></TD></TR></TABLE><P>

<P>

<H1><A NAME="SECTION0010120000000000000000">
7.12 Backward-Quote Substitution</A>
</H1>

<P>
<A NAME="sec:backquote"></A>
<P>
Backward quotes <TT>
<FONT COLOR="#0000ff">&nbsp;`&nbsp;</FONT></TT> have a special meaning to the shell. When a command
is inside backward quotes it means that the command should be run and its <I>output</I>
substituted in place of the backquotes. Take, for example, the <TT>
<FONT COLOR="#0000ff">cat</FONT></TT> command.
Create a small file, <TT>
<FONT COLOR="#0000ff">to_be_catted</FONT></TT>, with only the text <TT>
<FONT COLOR="#0000ff">daisy</FONT></TT>
inside it. Create a shell script

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>X=`cat&nbsp;to_be_catted`</code><br>
<code>echo&nbsp;$X</code><br>
</FONT></TD></TR></TABLE><P>

<P>
The value of <TT>
<FONT COLOR="#0000ff">X</FONT></TT> is set to the output of the <TT>
<FONT COLOR="#0000ff">cat</FONT></TT> command, which in this
case is the word <TT>
<FONT COLOR="#0000ff">daisy</FONT></TT>. This is a powerful tool. Consider the <TT>
<FONT COLOR="#0000ff">expr</FONT></TT>
command:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>X=`expr&nbsp;100&nbsp;+&nbsp;50&nbsp;'*'&nbsp;3`</code><br>
<code>echo&nbsp;$X</code><br>
</FONT></TD></TR></TABLE><P>

<P>
Hence we can use <TT>
<FONT COLOR="#0000ff">expr</FONT></TT> and backquotes to do mathematics inside our shell
script. Here is a function to calculate factorials. Note how we enclose the <TT>
<FONT COLOR="#0000ff">*</FONT></TT>
in forward quotes. They prevent the shell from expanding the <TT>
<FONT COLOR="#0000ff">*</FONT></TT> into matching file names:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>10</code></font><code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>function&nbsp;factorial&nbsp;()</code><br>
<code>{</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;N=$1</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;A=1</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;while&nbsp;test&nbsp;$N&nbsp;-gt&nbsp;0&nbsp;&#059;&nbsp;do</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A=`expr&nbsp;$A&nbsp;'*'&nbsp;$N`</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;N=`expr&nbsp;$N&nbsp;-&nbsp;1`</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;done</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;echo&nbsp;$A</code><br>
<code>}</code><br>
</FONT></TD></TR></TABLE><P>

<P>
We can see that the square braces used further above can actually suffice for
most of the times where we would like to use <TT>
<FONT COLOR="#0000ff">expr</FONT></TT>. (However, <TT>
<FONT COLOR="#0000ff">$[]</FONT></TT>
notation is an extension of the GNU shells and is not
a standard feature on all varients of U<SMALL>NIX</SMALL>.) We can now run <TT>
<FONT COLOR="#0000ff">factorial
20</FONT></TT> and see the output. If we want to assign the output to a variable,
we can do this with <TT>
<FONT COLOR="#0000ff">X=`factorial&nbsp;20`</FONT></TT>.

<P>
Note that another notation which gives the effect of a backward quote is

<TT>
<FONT COLOR="#0000ff">$(</FONT></TT><I>command</I><TT>
<FONT COLOR="#0000ff">)</FONT></TT>, which is identical to <TT>
<FONT COLOR="#0000ff">`</FONT></TT><I>command</I><TT>
<FONT COLOR="#0000ff">`</FONT></TT>.
Here, I will always use the older backward quote style.

<P>

<P>
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