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<!-- TITLE=RED HAT LINUX 2ND EDITION //-->
<!-- AUTHOR=DAVID PITTS ET AL //-->
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<!-- PUBLICATION DATE=1998 //-->
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<A NAME="PAGENUM-411"><P>Page 411</P></A>
<H3><A NAME="ch21_ 1">
CHAPTER 21
</A></H3>
<H2>
Shell Programming
</H2>
<P>by Sanjiv Guha
</P>
<H3><A NAME="ch21_ 2">
IN THIS CHAPTER
</A></H3>
<UL>
<LI> Creating and Executing a Shell Program 412
<LI> Variables 413
<LI> Positional Parameters 415
<LI> Built-in Variables 416
<LI> Special Characters 416
<LI> Comparison of Expressions 419
<LI> Iteration Statements 426
<LI> Conditional Statements 430
<LI> Miscellaneous Statements 433
<LI> Functions 434
</UL>
<A NAME="PAGENUM-412"><P>Page 412</P></A>
<P>When you enter commands from the command line, you are entering commands one at a
time and getting the response from the system. From time to time you will need to execute
more than one command, one after the other, and get the final result. You can do so with a
shell program or shell script. A shell program is a series of Linux commands and utilities that
have been put into a file using a text editor. When you execute a shell program, each command
is interpreted and executed by Linux, one after the other.
</P>
<P>You can write shell programs and execute them like any other command under Linux. You
can also execute other shell programs from within a shell program if they are in the search path.
A shell program is like any other programming language and has its own syntax. You can
have variables defined and assign various values and so on. These are discussed in this
chapter.
</P>
<H3><A NAME="ch21_ 3">
Creating and Executing a Shell Program
</A></H3>
<P>Say you want to set up a number of aliases every time you log on. Instead of typing all
the aliases every time you log on, you can put them in a file using a text editor, such as
vi, and then execute the file.
</P>
<P>Here is a list of what is contained in a sample file created for this purpose,
myenv:
</P>
<!-- CODE SNIP //-->
<PRE>
alias ll `ls _l'
alias dir `ls'
alias copy `cp'
</PRE>
<!-- END CODE SNIP //-->
<P>myenv can be executed in a variety of ways under Linux.
</P>
<P>You can make myenv executable by using the
chmod command as follows, and then execute it as you would any other native Linux command:
</P>
<!-- CODE SNIP //-->
<PRE>
chmod +x myenv
</PRE>
<!-- END CODE SNIP //-->
<P>This turns on the executable permission of
myenv. There is one more thing you need to ensure before you can execute
myenv. The file myenv must be in the search path. You can get the
search path by executing
</P>
<!-- CODE SNIP //-->
<PRE>
echo $PATH
</PRE>
<!-- END CODE SNIP //-->
<P>If the directory where the file myenv is located is not in the current search path, then you
must add the directory name in the search path.
</P>
<P>Now you can execute the file myenv from the command line, as if it were a Linux command:
</P>
<!-- CODE SNIP //-->
<PRE>
myenv
</PRE>
<!-- END CODE SNIP //-->
<A NAME="PAGENUM-413"><P>Page 413</P></A>
<TABLE BGCOLOR=#FFFF99><TR><TD>NOTE</TD></TR><TR><TD><BLOCKQUOTE>
You must ensure that the first line in your shell program starts with a pound sign
(#). The pound sign tells the shell that the line is a comment. Following the pound sign, you
must have an exclamation point (!), which tells the shell to run the command following
the exclamation point and to use the rest of the file as input for that command. This is
common practice for all shell scripting.
</BLOCKQUOTE>
</TD></TR></TABLE>
<P>A second way to execute myenv under a particular shell, such as
pdksh, is as follows:
</P>
<!-- CODE SNIP //-->
<PRE>
pdksh myenv
</PRE>
<!-- END CODE SNIP //-->
<P>This invokes a new pdksh shell and passes the filename
myenv as a parameter to execute the file.
</P>
<P>You can also execute myenv from the command line as follows:
</P>
<TABLE WIDTH="360">
<TR><TD>
Command
</TD><TD>
Environment
</TD></TR>
</TABLE>
<HR>
<PRE>
. myenv pdksh and bash
source myenv tcsh
</PRE>
<P>The dot (.) is a way of telling the shell to execute the file
myenv. In this case, you do not have to ensure that execute permission of the file is set. Under
tcsh, you have to use the source command instead of the dot
(.) command.
</P>
<P>After executing the command myenv, you should be able to use
dir from the command line to get a list of files under the current directory, and
ll to get a list of files with various attributes displayed.
</P>
<H3><A NAME="ch21_ 4">
Variables
</A></H3>
<P>Linux shell programming is a full-fledged programming language and as such supports
various types of variables. There are three major types of variables: environment, built-in, and user.
</P>
<P>Environment variables are part of the system environment, and you do not have to define
them. You can use them in your shell program; some of them, like
PATH, can also be modified within a shell program.
</P>
<P>Built-in variables are provided by the system; unlike environment variables, you cannot
modify them.
</P>
<P>User variables are defined by you when you write a shell script. You can use and modify
them at will within the shell program.
</P>
<A NAME="PAGENUM-414"><P>Page 414</P></A>
<P>A major difference between shell programming and other programming languages is that
in shell programming, variables are not typecast; that is, you do not have to specify whether
a variable is a number or a string and so on.
</P>
<H4><A NAME="ch21_ 5">
Assigning a Value to a Variable
</A></H4>
<P>Say you want to use a variable called lcount to count the number of iterations in a loop
within a shell program. You can declare and initialize this variable as follows:
</P>
<TABLE WIDTH="360">
<TR><TD>
Command
</TD><TD>
Environment
</TD></TR>
</TABLE>
<HR>
<PRE>
lcount=0 pdksh and bash
set lcount = 0tcsh
</PRE>
<TABLE BGCOLOR=#FFFF99><TR><TD>NOTE</TD></TR><TR><TD>
<BLOCKQUOTE>
Under pdksh and bash, you must ensure that the equal sign
(=) does not have space before and after it.
</BLOCKQUOTE>
</TD></TR></TABLE>
<P>As shell programming languages do not use typed variables, it is possible that the same
variable can be used to store an integer value one time and a string another time. However, this is
not recommended. You should be careful not to do this.
</P>
<P>To store a string in a variable, you can use the following:
</P>
<TABLE WIDTH="360">
<TR><TD>
Command
</TD><TD>
Environment
</TD></TR>
</TABLE>
<HR>
<PRE>
myname=Sanjiv pdksh and bash
set myname = Sanjiv csh
</PRE>
<P>The preceding can be used if the string does not have embedded spaces. If a string has
embedded spaces, you can do the assignment as follows:
</P>
<TABLE WIDTH="360">
<TR><TD>
Command
</TD><TD>
Environment
</TD></TR>
</TABLE>
<HR>
<PRE>
myname='Sanjiv Guha' pdksh and bash
set myname = `Sanjiv Guha' tcsh
</PRE>
<H4><A NAME="ch21_ 6">
Accessing Variable Values
</A></H4>
<P>You can access the value of a variable by prefixing the variable name by a
$ (dollar sign). That is, if the variable name is
var, you can access the variable by using $var.
</P>
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