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<P><BR></P>
<P><FONT SIZE="+1"><B>Using Iterative Structures</B></FONT></P>
<P>Iterative control structures allow you to write shell scripts that contain loops. The two basic types of loops are <TT>for</TT> and <TT>while</TT> loops.</P>
<P>With <TT>for</TT> loops, you specify a collection of files or values to use with some commands. To copy all the files whose names end with the characters .txt to the directory textdir, for example, use the following <TT>for</TT> loop:</P>
<!-- CODE SNIP //-->
<PRE>
for i in *.txt
do
cp $i textdir/$i
done
</PRE>
<!-- END CODE SNIP //-->
<P>The shell interprets the statement <TT>for i in *.txt</TT> and allows the variable <TT>i</TT> to take on the name of any file in the current directory whose name ends with .txt. You can then use the variable <TT>$i</TT> with any statements between the <TT>do</TT> and the <TT>done</TT> keywords.</P>
<P>The script in Listing 18.6 prints a collection of files, each with its own banner page. It also sends mail to the user concerning the status of the print requests. The characters <TT>$*</TT> represent all the parameters given to the shell command.</P>
<P><B>Listing 18.6</B> Processing Files with the for Command</P>
<!-- CODE //-->
<PRE>
# Name: Prntel
# Purpose: Print one or more files
# each with own title page
# Notify user which files were sent to the printer
# and which were not.
# Do this for all parameters to the command
for i in $*
do
if lp -t $i -dlasers $i > /dev/null
then
echo $i >> printed
else
echo $i >> notprinted
fi
done
# end of loop
if test -s printed
then
echo “These files were sent to the printer ” > mes
cat printed >> mes
mail $LOGNAME < mes
rm mes printed
f i
if test -s notprinted
then
echo “These files were not sent to the printer ” >mes
cat notprinted >> mes
mail $LOGNAME < mes
rm mes notprinted
fi
</PRE>
<!-- END CODE //-->
<P>A <TT>while</TT> loop looks at the exit status of a command in the same way the <TT>if</TT> statement looks at the status. The script in Listing 18.7 notifies users when they’ve received new mail. The script makes the assumption that if a mailbox changes, a user has new mail. The script uses the command <TT>diff</TT> to compare two files and then reports on the differences. If the files are the same, the exit status is zero (the command is successful).</P>
<P><B>Listing 18.7</B> Repeating Commands with while</P>
<!-- CODE //-->
<PRE>
# Name: checkmail
# Purpose: Notify user if their mail box has changed
# Suggestion: Run this in the background
# get a size of mail box for comparison
cp $MAIL omail # Get set for first time through
# MAIL is a “special” variable indicating the user’s mailbox
# while omail and $MAIL are the same, keep looping
while diff omail $MAIL > /dev/null
do
cp $MAIL omail
sleep 30 # sleep, pause for 30 seconds
done
# There must be a change in the files
echo “New mail!!” | write $LOGNAME
</PRE>
<!-- END CODE //-->
<P>You can see that some of the commands and concepts used with <TT>if…then…else</TT> statements can be transferred to <TT>while</TT> loops. The difference, of course, is that with <TT>while</TT> loops, you’re dealing with an iterative, repetitive process.</P>
<H3><A NAME="Heading30"></A><FONT COLOR="#000077">Customizing Linux Shells</FONT></H3>
<P>The shell starts when you log in. Tables 18.2 and 18.3 show you that special variables are given values by the shell to help define your shell environment. The shell sets some of these variables. You can change these settings and give other variables values by editing the file .profile if you’re using the Bourne or <TT>bash</TT> shell. If you’re using the C shell, you set the variables by editing the file .login. You can also use command aliasing to define aliases for commands.</P>
<P>Whenever you issue a command, a new shell starts; it inherits many of the characteristics—or much of the environment—of the existing shell. Note these two things about the new shell:</P>
<DL>
<DD><B>•</B> The new shell runs in your current directory. The <TT>pwd</TT> command returns the same value within a shell as it gives before the shell was started.
<DD><B>•</B> The new shell receives many of its variables from the existing shell. There are ways to make sure that variables set in the existing shell are exported to the new shell.
</DL>
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