ch13.htm

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

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.



<DL>

	<DT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading28<FONT COLOR="#000077"><B>NOTE:</B> </FONT><TT>tcsh</TT> does

	not have an equivalent of the <TT>until</TT> statement other than rewriting it as

	a <TT>while</TT> loop.

<HR>



</DL>



<CENTER>

<H4><A NAME="Heading29<FONT COLOR="#000077">The shift Command</FONT></H4>

</CENTER>

<P><TT>bash</TT>, <TT>pdksh</TT>, and <TT>tcsh</TT> all support a command called

<TT>shift</TT>. The <TT>shift</TT> command moves the current values stored in the

positional parameters to the left one position. For example, if the values of the

current positional parameters are<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">$1 = -r  $2 = file1  $3 = file2

</FONT></PRE>

<P>and you executed the <TT>shift</TT> command<FONT COLOR="#0066FF"></FONT>

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

</FONT></PRE>

<P>the resulting positional parameters would be as shown here:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">$1 = file1  $2 = file2

</FONT></PRE>

<P>You can also move the positional parameters over more than one place by specifying

a number with the <TT>shift</TT> command. The following command would shift the positional

parameters two places:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">shift 2

</FONT></PRE>

<P>This is a very useful command when you have a shell program that needs to parse

command-line options. This is true because options are typically preceded by a hyphen

and a letter that indicates what the option is to be used for. Because options are

usually processed in a loop of some kind, you often want to skip to the next positional

parameter once you have identified which option should be coming next. For example,

the following shell program expects two command-line options--one that specifies

an input file and one that specifies an output file. The program reads the input

file, translates all the characters in the input file into uppercase, then stores

the results in the specified output file.



<DL>

	<DT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading30<FONT COLOR="#000077"><B>NOTE: </B></FONT>The following example

	was written using <TT>bash</TT>, <TT>pdksh</TT> syntax.

</DL>





<DL>

	<DD><FONT COLOR="#0066FF">while [ &quot;$1&quot; ]<BR>

	do<BR>

	      if [ &quot;$1&quot; = &quot;-i&quot; ] then<BR>

	            infile=&quot;$2&quot;<BR>

	            shift 2<BR>

	      elif [ &quot;$1&quot; = &quot;-o&quot; ]<BR>

	      then<BR>

	            outfile=&quot;$2&quot;<BR>

	            shift 2<BR>

	      else<BR>

	            echo &quot;Program $0 does not recognize option $1&quot;<BR>

	      fi<BR>

	done<BR>

	<BR>

	tr a-z A-Z &lt;$infile &gt;$outfile</FONT>

<HR>

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

</DL>



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

<CENTER>

<H4><A NAME="Heading31<FONT COLOR="#000077">The select Statement</FONT></H4>

</CENTER>

<P><TT>pdksh</TT> offers one iteration statement that neither <TT>bash</TT> nor <TT>tcsh</TT>

provides. This is the <TT>select</TT> statement. This is actually a very useful statement.

It is quite a bit different from the other iteration statements because it actually

does not execute a block of shell code repeatedly while a condition is true or false.

What the <TT>select</TT> statement does is enable you to automatically generate simple

text menus. The syntax for the <TT>select</TT> statement is<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">select menuitem [in list_of_items]

do

      commands



done

</FONT></PRE>

<P>where square brackets are used to enclose the optional part of the statement.</P>

<P>When a <TT>select</TT> statement is executed, <TT>pdksh</TT> creates a numbered

menu item for each element in the list_of_items. This list_of_items can be a variable

that contains more than one item, such as <TT>choice1 choice2</TT>, or it can be

a list of choices typed in the command. For example:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">select menuitem in choice1 choice2 choice3

</FONT></PRE>

<P>If the <TT>list_of_items</TT> is not provided, the <TT>select</TT> statement uses

the positional parameters just as with the <TT>for</TT> statement.</P>

<P>Once the user of the program containing a <TT>select</TT> statement picks one

of the menu items by typing the number associated with it, the <TT>select</TT> statement

stores the value of the selected item in the <TT>menuitem</TT> variable. The statements

contained in the <TT>do</TT> block can then perform actions on this menu item.</P>

<P>The following example illustrates a potential use for the <TT>select</TT> statement.

This example displays three menu items, and when the user chooses one of them it

asks whether that was the intended selection. If the user enters anything other than

<TT>y</TT> or <TT>Y</TT>, the menu is redisplayed.<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">select menuitem in pick1 pick2 pick3

do

      echo &quot;Are you sure you want to pick $menuitem&quot;

      read res

      if [ $res = &quot;y&quot; -o $res = &quot;Y&quot; ]

      then

            break

      fi

</FONT></PRE>



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

</FONT></PRE>

<P>A few new commands are introduced in this example. The <TT>read</TT> command is

used to get input from the user. It stores anything that the user types into the

specified variable. The <TT>break</TT> command is used to exit a <TT>while</TT>,

<TT>until</TT>, <TT>repeat</TT>, <TT>select</TT>, or <TT>for</TT> statement.

<CENTER>

<H4><A NAME="Heading32<FONT COLOR="#000077">The repeat Statement</FONT></H4>

</CENTER>

<P><TT>tcsh</TT> has an iteration statement that has no equivalent in <TT>pdksh</TT>

or <TT>bash</TT>. This is the <TT>repeat</TT> statement. The <TT>repeat</TT> statement

executes a single command a specified number of times. The syntax for the <TT>repeat</TT>

statement is the following:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">repeat count command

</FONT></PRE>

<P>The following is an example of the <TT>repeat</TT> statement. It takes a set of

numbers as command-line options and prints that number of periods to the screen.

This program acts as a very primitive graphing program.<FONT COLOR="#0066FF"></FONT>

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

foreach num ($*)

      repeat $num echo -n &quot;.&quot;

      echo &quot;&quot;



end

</FONT></PRE>



<DL>

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

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading33<FONT COLOR="#000077"><B>NOTE: </B></FONT>Any <TT>repeat</TT>

	statement can be rewritten as a <TT>while</TT> or <TT>for</TT> statement. The <TT>repeat</TT>

	syntax is simply more convenient.

<HR>



</DL>



<CENTER>

<H3><A NAME="Heading34<FONT COLOR="#000077">Functions</FONT></H3>

</CENTER>

<P>The shell languages enable you to define your own functions. These functions behave

in much the same way as functions you define in C or other programming languages.

The main advantage of using functions as opposed to writing all of your shell code

in line is for organizational purposes. Code written using functions tends to be

much easier to read and maintain and also tends to be smaller, because you can group

common code into functions instead of putting it everywhere it is needed.</P>

<P>The syntax for creating a function in <TT>bash</TT> and <TT>pdksh</TT> is the

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

<PRE><FONT COLOR="#0066FF">fname () {

      shell commands



}

</FONT></PRE>

<P><TT>pdksh</TT> also allows the following syntax:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">function fname {

      shell commands

</FONT></PRE>



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

<P>Both of these forms behave in the same way.</P>

<P>Once you have defined your function using one of these forms, you can invoke it

by entering the following command:</P>

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

fname [parm1 parm2 parm3 ...]

</FONT></PRE>



<DL>

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

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading35<FONT COLOR="#000077"><B>NOTE: </B></FONT>The <TT>tcsh</TT>

	shell does not support functions.

<HR>



</DL>



<P>Notice that you can pass any number of parameters to your function. When you do

pass parameters to a function, it sees those parameters as positional parameters,

just as a shell program does when you pass it parameters on the command line. For

example, the following shell program contains several functions, each of which is

performing a task associated with one of the command-line options. This example illustrates

many of the topics covered in this chapter. It reads all the files that are passed

on the command line and--depending on the option that was used--writes the files

out in all uppercase letters, writes the files out in all lowercase letters, or prints

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

<PRE><FONT COLOR="#0066FF">upper () {

      shift

      for i

      do

          tr a-z A-Z &lt;$1 &gt;$1.out

          rm  $1

          mv $1.out $1

          shift

      done; }



lower () {

      shift

      for i

      do

          tr A-Z a-z &lt;$1 &gt;$1.out

          rm $1

          mv $1.out $1

          shift

      done; }



print () {

      shift

      for i

      do

            lpr $1

            shift

      done; }



usage_error () {

      echo &quot;$1 syntax is $1 &lt;option&gt; &lt;input files&gt;&quot;

      echo &quot;&quot;

      echo &quot;where option is one of the following&quot;

      echo &quot;p  -- to print frame files&quot;

      echo &quot;u  -- to save as uppercase&quot;

      echo &quot;l  -- to save as lowercase&quot;; }



case $1

in

      p | -p)     print $@;;

      u | -u)           upper $@;;

      l | -l)     lower $@;;

      *)          usage_error $0;;

</FONT></PRE>



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

esac

</FONT></PRE>

<CENTER>

<H3><A NAME="Heading36<FONT COLOR="#000077">Summary</FONT></H3>

</CENTER>

<P>This chapter introduced you to many of the features of the <TT>bash</TT>, <TT>pdksh</TT>,

and <TT>tcsh</TT> programming languages. As you become familiar with using Linux,

you will find that you use shell programming languages more and more often.</P>



<P>Even though the shell languages are very powerful and also quite easy to learn,

you might run into some situations where shell programs are not suited to the problem

you are solving. In these cases you may want to investigate the possibility of using

one of the other languages available under Linux. Some of your options are C and

C++, which are described in Chapters 27, &quot;Programming in C,&quot; and 28, &quot;Programming

in C++&quot;; <TT>gawk</TT>, which is described in Chapter 26, &quot;<TT>gawk</TT>&quot;;

and Perl, which is described in Chapter 29, &quot;Perl.&quot;









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