ch13.htm

linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

fi

</FONT></PRE>

<P>The same statement written using the <TT>tcsh</TT> syntax is shown here:<FONT

COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">#

if ( { -f .profile } ) then

      echo &quot;There is a .profile file in the current directory.&quot;

else

      echo &quot;Could not find the .profile file.&quot;

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">

endif

</FONT></PRE>



<DL>

	<DT><FONT COLOR="#0066FF"><B></B></FONT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading21<FONT COLOR="#000077"><B>NOTE:</B> </FONT>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>



</DL>



<CENTER>

<H4><A NAME="Heading22<FONT COLOR="#000077">The case Statement</FONT></H4>

</CENTER>

<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 compare only 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>

<PRE><FONT COLOR="#0066FF">

case string1 in

      str1)

            commands;;

      str2)

            commands;;

      *)

            commands;;

</FONT></PRE>

<PRE><FONT COLOR="#0066FF">

esac

</FONT></PRE>

<P><TT>string1</TT> is compared to <TT>str1</TT> and <TT>str2</TT>. If one of these

strings matches <TT>string1</TT>, the commands up until 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</TT> <TT>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:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">switch (string1)

      case  str1:

            statements

      breaksw

      case  str2:

            statements

      breaksw

      default:

            statements

      breaksw



endsw

</FONT></PRE>

<P>This behaves in the same manner as the <TT>bash</TT> and <TT>pdksh</TT> <TT>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 following it up until

the <TT>breaksw</TT> keyword is executed. If none of the strings matches, the code

following the default keyword up until the <TT>breaksw</TT> keyword is executed.</P>

<P>The following code is an example of a <TT>bash</TT> or <TT>pdksh</TT> <TT>case</TT>

statement. This code checks to see if the first command-line option was <TT>-i</TT>

or <TT>-e</TT>. If it was <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

i. If the first option was <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 was not <TT>-i</TT> or <TT>-e</TT>, the program

prints a brief error message to the screen.<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">case $1 in

   -i)

      count='grep ^i $2 | wc -l'

      echo &quot;The number of lines in $2 that start with an i is $count&quot;

      ;;

   -e)

      count='grep ^e $2 | wc -l'

      echo &quot;The number of lines in $2 that start with an e is $count&quot;

      ;;

   * )

      echo &quot;That option is not recognized&quot;

      ;;

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">esac

</FONT></PRE>

<P>The same example written in <TT>tcsh</TT> syntax is shown here:</P>

<PRE><FONT COLOR="#0066FF">

# 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 &quot;The number of lines in $2 that begin with i is $count&quot;

   breaksw

   case -e | e:

      set count = 'grep ^e $2 | wc -l'

      echo &quot;The number of lines in $2 that begin with e is $count&quot;

   breaksw

   default:

      echo &quot;That option is not recognized&quot;

   breaksw

</FONT></PRE>

<PRE><FONT COLOR="#0066FF">

endsw

</FONT></PRE>

<CENTER>

<H3><A NAME="Heading23<FONT COLOR="#000077">Iteration Statements</FONT></H3>

</CENTER>

<P>The shell languages also provide several iteration or looping statements. The

most commonly used of these is the <TT>for</TT> statement.

<CENTER>

<H4><A NAME="Heading24<FONT COLOR="#000077">The for Statement</FONT></H4>

</CENTER>

<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.



<DL>

	<DT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading25<FONT COLOR="#000077"><B>NOTE: </B></FONT>The <TT>for</TT>

	statement syntax is the same in both <TT>bash</TT> and <TT>pdksh</TT>.

<HR>



</DL>



<P>The first form of <TT>for</TT> statement that <TT>bash</TT> and <TT>pdksh</TT>

support has the following syntax:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">for var1 in list

do

      commands

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">done

</FONT></PRE>

<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 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:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">for var1

do

      statements

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">done

</FONT></PRE>

<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 in to 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:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">for var1 in &quot;$@&quot;

do

      statements

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">done

</FONT></PRE>

<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</TT>

<TT>for</TT> statement. The syntax of the <TT>foreach</TT> statement is the following:<FONT

COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">foreach name (list)

      commands



end

</FONT></PRE>

<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.<FONT

COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">for file

do

tr a-z A-Z &lt; $file &gt;$file.caps

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">done

</FONT></PRE>

<P>The same example written in <TT>tcsh</TT> shell language is shown next:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">#

foreach file ($*)

   tr a-z A-Z &lt; $file &gt;$file.caps

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">end

</FONT></PRE>

<CENTER>

<H4><A NAME="Heading26<FONT COLOR="#000077">The while Statement</FONT></H4>

</CENTER>

<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:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">while expression

do

      statements

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">done

</FONT></PRE>

<P>The syntax for the <TT>while</TT> statement in <TT>tcsh</TT> is the following:<FONT

COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">while (expression)

      statements

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">end

</FONT></PRE>

<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.<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">count=1

while [ -n &quot;$*&quot; ]

do

      echo &quot;This is parameter number $count $1&quot;

      shift

      count='expr $count + 1'



done

</FONT></PRE>

<P>As you will see in the section titled &quot;The <TT>shift</TT> Command,&quot;

the <TT>shift</TT> command moves the command-line parameters over one to the left.</P>

<P>The same program written in the <TT>tcsh</TT> language is shown next:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">#

set count = 1

while ( &quot;$*&quot; != &quot;&quot; )

      echo &quot;This is parameter number $count $1&quot;

      shift

      set count = 'expr $count + 1'

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">end

</FONT></PRE>

<CENTER>

<H4><A NAME="Heading27<FONT COLOR="#000077">The until Statement</FONT></H4>

</CENTER>

<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<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">until expression

do

      commands

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">done

</FONT></PRE>

<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:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">count=1

until [ -z &quot;$*&quot; ]

do

      echo &quot;This is parameter number $count $1&quot;

      shift

      count='expr $count + 1'

</FONT></PRE>



<PRE><FONT COLOR="#0066FF">done

</FONT></PRE>

<P>The only difference between this example and the <TT>while</TT> statement example