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<TITLE>Linux Unleashed, Third Edition:Shell Programming</TITLE>
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<P><BR></P>
<H3><A NAME="Heading13"></A><FONT COLOR="#000077">Iteration Statements</FONT></H3>
<P>The shell languages also provide several iteration or looping statements. The most commonly used of these is the <TT>for</TT> statement. In addition to the <TT>for</TT> loop, there are several others (such as <TT>while</TT> and <TT>until</TT>) but they are all variations of the same approach. The <TT>for</TT>loop is by far the most commonly used in shell programs.</P>
<H4 ALIGN="LEFT"><A NAME="Heading14"></A><FONT COLOR="#000077">The for Statement</FONT></H4>
<P>The <TT>for</TT> statement executes the commands that are contained within it a specified number of times. <TT>bash</TT> and <TT>pdksh</TT> have two variations of the <TT>for</TT> statement. The <TT>for</TT> statement syntax is the same in both <TT>bash</TT> and <TT>pdksh</TT>.</P>
<P>The first form of <TT>for</TT> statement that <TT>bash</TT> and <TT>pdksh</TT> support has the following syntax:</P>
<!-- CODE SNIP //-->
<PRE>
for var1 in list
do
<I>commands</I>
done
</PRE>
<!-- END CODE SNIP //-->
<P>In this form, the <TT>for</TT> statement executes once for each item in the list. This list can be a variable that contains several words separated by spaces or it can be a list of values that is typed directly into the statement. Each time through the loop, the variable <TT>var1</TT> is assigned to the current item in the list until the last one is reached.</P>
<P>The second form of <TT>for</TT> statement has the following syntax:</P>
<!-- CODE SNIP //-->
<PRE>
for var1
do
<I>statements</I>
done
</PRE>
<!-- END CODE SNIP //-->
<P>In this form, the <TT>for</TT> statement executes once for each item in the variable <TT>var1</TT>. When this syntax of the <TT>for</TT> statement is used, the shell program assumes that the <TT>var1</TT> variable contains all the positional parameters that were passed into the shell program on the command line.</P>
<P>Typically, this form of <TT>for</TT> statement is the equivalent of writing the following <TT>for</TT> statement:</P>
<!-- CODE SNIP //-->
<PRE>
for var1 in “$@”
do
<I>statements</I>
done
</PRE>
<!-- END CODE SNIP //-->
<P>The equivalent of the <TT>for</TT> statement in <TT>tcsh</TT> is called the <TT>foreach</TT> statement. It behaves in the same manner as the <TT>bash</TT> and <TT>pdksh for</TT> statement. The syntax of the <TT>foreach</TT> statement is the following:</P>
<!-- CODE SNIP //-->
<PRE>
foreach name (<I>list</I>)
<I>commands</I>
end
</PRE>
<!-- END CODE SNIP //-->
<P>The following is an example of the <TT>bash</TT> or <TT>pdksh</TT> style of <TT>for</TT> statement. This example takes as command-line options any number of text files. The program reads in each of these files, converts all the letters to uppercase, and then stores the results in a file of the same name but with a <TT>.caps</TT> extension.</P>
<!-- CODE SNIP //-->
<PRE>
for file
do
tr a-z A-Z < $file >$file.caps
done
</PRE>
<!-- END CODE SNIP //-->
<P>The same example written in <TT>tcsh</TT> shell language is shown next:</P>
<!-- CODE SNIP //-->
<PRE>
#
foreach file ($*)
tr a-z A-Z < $file >$file.caps
end
</PRE>
<!-- END CODE SNIP //-->
<H4 ALIGN="LEFT"><A NAME="Heading15"></A><FONT COLOR="#000077">The while Statement</FONT></H4>
<P>Another iteration statement offered by the shell programming language is the <TT>while</TT> statement. This statement causes a block of code to be executed while a provided conditional expression is true. The syntax for the <TT>while</TT> statement in <TT>bash</TT> and <TT>pdksh</TT> is the following:</P>
<!-- CODE SNIP //-->
<PRE>
while <I>expression</I>
do
<I>statements</I>
done
</PRE>
<!-- END CODE SNIP //-->
<P>The syntax for the <TT>while</TT> statement in <TT>tcsh</TT> is the following:</P>
<!-- CODE SNIP //-->
<PRE>
while (<I>expression</I>)
<I>statements</I>
end
</PRE>
<!-- END CODE SNIP //-->
<P>The following is an example of the <TT>bash</TT> and <TT>pdksh</TT> style of <TT>while</TT> statement. This program lists the parameters that were passed to the program, along with the parameter number.</P>
<!-- CODE SNIP //-->
<PRE>
count=1
while [ -n “$*” ]
do
echo “This is parameter number $count $1”
shift
count=‘expr $count + 1‘
done
</PRE>
<!-- END CODE SNIP //-->
<P>As you will see in the section titled “The <TT>shift</TT> Command,” the <TT>shift</TT> command moves the command-line parameters over one space to the left.</P>
<P>The same program written in the <TT>tcsh</TT> language is shown next:</P>
<!-- CODE SNIP //-->
<PRE>
#
set count = 1
while ( “$*” != “” )
echo “This is parameter number $count $1”
shift
set count = ‘expr $count + 1‘
end
</PRE>
<!-- END CODE SNIP //-->
<H4 ALIGN="LEFT"><A NAME="Heading16"></A><FONT COLOR="#000077">The until Statement</FONT></H4>
<P>The <TT>until</TT> statement is very similar in syntax and function to the <TT>while</TT> statement. The only real difference between the two is that the <TT>until</TT> statement executes its code block while its conditional expression is false, and the <TT>while</TT> statement executes its code block while its conditional expression is true. The syntax for the <TT>until</TT> statement in <TT>bash</TT> and <TT>pdksh</TT> is</P>
<!-- CODE SNIP //-->
<PRE>
until <I>expression</I>
do
<I>commands</I>
done
</PRE>
<!-- END CODE SNIP //-->
<P>The same example that was used for the <TT>while</TT> statement can be used for the <TT>until</TT> statement. All you have to do to make it work is negate the condition. This is shown in the following code:</P>
<!-- CODE SNIP //-->
<PRE>
count=1
until [ -z “$*” ]
do
echo “This is parameter number $count $1”
shift
count=‘expr $count + 1‘
done
</PRE>
<!-- END CODE SNIP //-->
<P>The only difference between this example and the <TT>while</TT> statement example is that the <TT>-n</TT> test command option (which means that the string has nonzero length) was removed, and the <TT>-z</TT> test option (which means that the string has zero length) was put in its place.</P>
<P>In practice, the <TT>until</TT> statement is not very useful because any <TT>until</TT> statement you write can also be written as a <TT>while</TT> statement. <TT>tcsh</TT> does not have an equivalent of the <TT>until</TT> statement other than rewriting it as a <TT>while</TT> loop.</P><P><BR></P>
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