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<TITLE>Linux Unleashed, Third Edition:Shell Programming</TITLE>
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<P><BR></P>
<H4 ALIGN="LEFT"><A NAME="Heading11"></A><FONT COLOR="#000077">The if Statement</FONT></H4>
<P>All three shells support nested <TT>if-then-else</TT> statements. These statements provide you with a way of performing complicated conditional tests in your shell programs. The syntax of the <TT>if</TT> statement is the same for <TT>bash</TT> and <TT>pdksh</TT> and is shown here:</P>
<!-- CODE //-->
<PRE>
if [ <I>expression</I> ]
then
<I>commands</I>
elif [ <I>expression2</I> ]
then
<I>commands</I>
else
<I>commands</I>
fi
</PRE>
<!-- END CODE //-->
<BLOCKQUOTE>
<P><FONT SIZE="-1"><HR><B>Note: </B><BR>The <TT>elif</TT> and <TT>else</TT> clauses are both optional parts of the <TT>if</TT> statement. Also note that <TT>bash</TT> and <TT>pdksh</TT> use the reverse of the statement name in most of their complex statements to signal the end of the statement. In this statement, the <TT>fi</TT> keyword is used to signal the end of the <TT>if</TT> statement.<HR></FONT>
</BLOCKQUOTE>
<P>The <TT>elif</TT> statement is an abbreviation of <TT>else if</TT>. This statement is executed only if none of the expressions associated with the <TT>if</TT> statement or any <TT>elif</TT> statements before it were true. The commands associated with the <TT>else</TT> statement are executed only if none of the expressions associated with the <TT>if</TT> statement or any of the <TT>elif</TT> statements were true.</P>
<P>In <TT>tcsh</TT>, the <TT>if</TT> statement has two different forms. The first form provides the same function as the <TT>bash</TT> and <TT>pdksh if</TT> statement. This form of <TT>if</TT> statement has the following syntax:</P>
<!-- CODE SNIP //-->
<PRE>
if (<I>expression1</I>) then
<I>commands</I>
else if (<I>expression2</I>) then
<I>commands</I>
else
<I>commands</I>
endif
</PRE>
<!-- END CODE SNIP //-->
<P>The second form of <TT>if</TT> statement provided by <TT>tcsh</TT> is a simple version of the first <TT>if</TT> statement. This form of <TT>if</TT> statement evaluates only a single expression. If the expression is true, it executes a single command; if the expression is false, nothing happens. The syntax for this form of <TT>if</TT> statement is the following:</P>
<!-- CODE SNIP //-->
<PRE>
if (<I>expression</I>) <I>command</I>
</PRE>
<!-- END CODE SNIP //-->
<P>This statement can be written using the first form of <TT>if</TT> statement by writing the <TT>if</TT> without any <TT>else</TT> or <TT>else if</TT> clauses. This form just saves a little typing.</P>
<P>The following is an example of a <TT>bash</TT> or <TT>pdksh if</TT> statement. This statement checks to see whether there is a <TT>.profile</TT> file in the current directory:</P>
<!-- CODE SNIP //-->
<PRE>
if [ -f .profile ]
then
echo “There is a .profile file in the current directory.”
else
echo “Could not find the .profile file.”
fi
</PRE>
<!-- END CODE SNIP //-->
<P>The same statement written using the <TT>tcsh</TT> syntax is shown here:</P>
<!-- CODE SNIP //-->
<PRE>
#
if ( { -f .profile } ) then
echo “There is a .profile file in the current directory.”
else
echo “Could not find the .profile file.”
endif
</PRE>
<!-- END CODE SNIP //-->
<BLOCKQUOTE>
<P><FONT SIZE="-1"><HR><B>Note: </B><BR>Notice that in the <TT>tcsh</TT> example the first line starts with a <TT>#</TT>. This is required for <TT>tcsh</TT> to recognize the file containing the commands as a <TT>tcsh</TT> script file.<HR></FONT>
</BLOCKQUOTE>
<H4 ALIGN="LEFT"><A NAME="Heading12"></A><FONT COLOR="#000077">The case Statement</FONT></H4>
<P>The <TT>case</TT> statement enables you to compare a pattern with several other patterns and execute a block of code if a match is found. The shell <TT>case</TT> statement is quite a bit more powerful than the <TT>case</TT> statement in Pascal or the <TT>switch</TT> statement in C. This is because in the shell <TT>case</TT> statement you can compare strings with wildcard characters in them, whereas with the Pascal and C equivalents, you can only compare enumerated types or integer values.</P>
<P>Once again, the syntax for the <TT>case</TT> statement is identical for <TT>bash</TT> and <TT>pdksh</TT> and different for <TT>tcsh</TT>. The syntax for <TT>bash</TT> and <TT>pdksh</TT> is the following:</P>
<!-- CODE SNIP //-->
<PRE>
case string1 in
str1)
<I>commands</I>;;
str2)
<I>commands</I>;;
*)
<I>commands</I>;;
esac
</PRE>
<!-- END CODE SNIP //-->
<P><TT>string1</TT> is compared to <TT>str1</TT> and <TT>str2</TT>. If one of these strings matches <TT>string1</TT>, all commands up to the double semicolon (<TT>;;</TT>) are executed. If neither <TT>str1</TT> nor <TT>str2</TT> matches <TT>string1</TT>, the commands associated with the asterisk are executed. This is the default case condition because the asterisk matches all strings.</P>
<P>The <TT>tcsh</TT> equivalent of the <TT>bash</TT> and <TT>pdksh case</TT> statement is called the <TT>switch</TT> statement. This statement’s syntax closely follows the C <TT>switch</TT> statement syntax. Here it is:</P>
<!-- CODE //-->
<PRE>
switch (string1)
case str1:
<I>statements</I>
breaksw
case str2:
<I>statements</I>
breaksw
default:
<I>statements</I>
breaksw
endsw
</PRE>
<!-- END CODE //-->
<P>This behaves in the same manner as the <TT>bash</TT> and <TT>pdksh case</TT> statement. Each string following the keyword <TT>case</TT> is compared with <TT>string1</TT>. If any of these strings matches <TT>string1</TT>, the code follows it until the <TT>breaksw</TT> keyword is executed. If none of the strings match, the code follows the default keyword until the <TT>breaksw</TT> keyword is executed.</P>
<P>The following code is an example of a <TT>bash</TT> or <TT>pdksh case</TT> statement. This code checks to see if the first command-line option is <TT>-i</TT> or <TT>-e</TT>. If it is <TT>-i</TT>, the program counts the number of lines in the file specified by the second command-line option that begins with the letter <TT>i</TT>. If the first option is <TT>-e</TT>, the program counts the number of lines in the file specified by the second command-line option that begins with the letter e. If the first command-line option is not <TT>-i</TT> or <TT>-e</TT>, the program prints a brief error message to the screen.</P>
<!-- CODE //-->
<PRE>
case $1 in
-i)
count=‘grep ^i $2 | wc -l‘
echo “The number of lines in $2 that start with an i is $count”
;;
-e)
count=‘grep ^e $2 | wc -l‘
echo “The number of lines in $2 that start with an e is $count”
;;
* )
echo “That option is not recognized”
;;
esac
</PRE>
<!-- END CODE //-->
<P>The same example written in <TT>tcsh</TT> syntax is shown here:</P>
<!-- CODE //-->
<PRE>
# remember that the first line must start with a # when using tcsh
switch ( $1 )
case -i | i:
set count = ‘grep ^i $2 | wc -l‘
echo “The number of lines in $2 that begin with i is $count”
breaksw
case -e | e:
set count = ‘grep ^e $2 | wc -l‘
echo “The number of lines in $2 that begin with e is $count”
breaksw
default:
echo “That option is not recognized”
breaksw
endsw
</PRE>
<!-- END CODE //-->
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