UNIX Bourne Shell Programming

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2. USER, SHELL, AND READ-ONLY SHELL VARIABLES
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2.3  Read-Only User Variables

The contents of the user variables and the shell variables can be
modified by the user.  It is possible to assign a new value to
them.  The new value can be assigned from the dollar ($) prompt or
from inside a BourneShell script.  Read-only variables are
different.  The value of read-only variables can not be changed.

The variable must be initialized to some value; and then, by
entering the following command, it can be made read only.


  Command format:    readonly variable_name                    
                                                               
  variable_name = name of the variable to be made read only    


Sample Session:


 $person=Kathleen                                              
 $readonly person                                              
 $echo $person                                                 
 Kathleen                                                      
 $person=Richard                                               
 person: is read only                                          
 $                                                             


The readonly command given without any arguments will display a
list of all the read-only variables.

Sample Session:


 $person=Kathleen                                              
 $readonly person                                              
 $example=Richard                                              
 $readonly example                                             
 $readonly                                                     
 readonly person                                               
 readonly example                                              
 $                                                             

2.4  Read-Only Shell Variables

The read-only shell variables are similar to the read-only user
variables; except the value of these variables is assigned by the
shell, and the user CANNOT modify them.


2.4.1  Name of the Calling Program

The shell will store the name of the command you used to call a
program in the variable named $0.

It has the number zero because it appears before the first argument
on the command line.

Sample Session:


 $cat name_ex                                                  
 echo 'The name of the command used'                           
 echo 'to execute this script was' $0                          
 $name_ex                                                      
 The name of the command used                                  
 to execute this script was name_ex                            
 $                                                             


2.4.2  Arguments

The BourneShell will store the first nine command line arguments
in the variables named $1, $2, ..., $9.  These variables appear in
this section because you cannot change them using the equal sign. 
It is possible to modify them using the set command.

Sample Session:


 $cat arg_ex                                                   
 echo 'The first five command line'                            
 echo 'arguments are' $1 $2 $3 $4 $5                           
 $arg_ex Richard Kathleen Douglas                              
 The first five command line                                   
 arguments are Richard Kathleen Douglas                        
 $                                                             


The script arg_ex will display the first five command-line
arguments.  The variables representing $4 and $5 have a null value.

The BourneShell variable $* represents all of the command-line
arguments as shown in the following example.

Sample Session:


 $cat display_all                                              
 echo $*                                                       
 $display_all Richard Kathleen Douglas                         
 Richard Kathleen Douglas                                      
 $                                                             


The BourneShell variable $# contains the number of arguments on
the command line.  This is a string variable that represents a
decimal number.  You can use the expr utility to perform
calculations with that number and test to perform logical tests on
it.

Sample Session:


 $cat num_args                                                 
 echo 'This script was called with'                            
 echo $# 'arguments'                                           
 $num_args Richard Kathleen Douglas                            
 This script was called with                                   
 3 arguments                                                   
 $                                                             

2.4.3  Shift

The shift command promotes each of the command-line arguments. 
The second argument, represented by $2, is now the first argument,
represented by $1.  The third becomes the second and so on until
the last argument becomes the next to last.  You can access only
the first nine command-line arguments (as $1 through $9).  The
shift command gives you access to the tenth, and the first becomes
unavailable.  There is no "unshift" command that will return the
arguments that are no longer available.

Sample Session: 


 $cat demo_shift                                               
 echo 'arg1='$1 '  arg2='$2 '   arg3='$3                       
 shift                                                         
 echo 'arg1='$1 '  arg2='$2 '   arg3='$3                       
 shift                                                         
 echo 'arg1='$1 '  arg2='$2 '   arg3='$3                       
 shift                                                         
 echo 'arg1='$1 '  arg2='$2 '   arg3='$3                       
 shift                                                         
 $demo_shift Richard Kathleen Douglas                          
 arg1=Richard  arg2=Kathleen  arg3=Douglas                     
 arg1=Kathleen arg2=Douglas   arg3=                            
 arg1=Douglas  arg2=          arg3=                            
 demo_shift: cannot shift                                     
 $                                                            


The BourneShell will display an error message when the script
executes a shift command after it has run out of variables.

2.4.4  Set


The Set command will display a list of all the variables that are
set when it has no arguments.

Sample Session:


 $set                                                          
 HOME=/user0/teacher                                          
 IFS=                                                         
                                                               
 LOGNAME=richard                                               
 MAIL=/usr/mail/richard                                        
 MAILCHECK=600                                                 
 PATH=:/bin:/usr/bin                                           
 PS1=$                                                         
 PS2=>                                                         
 SHELL=/bin/sh                                                 
 TERM=vt100                                                    
 TZ=MST7MDT                                                    
 $                                                             


When set is called with arguments, it sets the value of the
command-line arguments ($1-$n) to the arguments.  The example sets
the first three arguments.

Sample Session:


 $cat set_ex                                                   
 set who really cares                                          
 echo $#: $*                                                   
 $set_ex                                                       
 3: who really cares                                           
 $                                                             


2.4.5  expr

The expr command will perform arithmetic in the BourneShell.
   Command format:  expr expression                            
                                                               
   See Appendix C for a complete list of expressions           


The arguments are taken as an expression. After the evaluation has
taken place, the result is written to standard output.  The terms
of the expression must be separated by blanks.  Special characters
to the shell must be escaped.  Strings containing blanks or other
special characters must be quoted.

Sample Session:


 $expr 7 + 8 + 10                                              
 25                                                            
 $expr 10 - 8                                                  
 2                                                             
 $expr 10 '*' 4                                                
 40                                                            
 $expr 135 / 5                                                 
 27                                                            
 $                                                             


expr will also work with user defined variables as in the following
example:

Sample Session:


 $cat data                                                     
 8                                                             
 15                                                            
 25                                                            
 $cat express                                                  
 count=0                                                       
 tot=0                                                         
 for a in `cat data`                                           
 do                                                            
 tot=`expr $tot + $a`                                          
 count=`expr $count + 1`                                       
 done                                                          
 avg=`expr $tot / $count`                                      
 echo "The average is $avg"                                    
 $                                                             

Let's execute the script "express" with tracing on so we can follow
the execution.

Sample Session:


 $sh -x express                                                
 count=0                                                       
 tot=0                                                         
 + cat data                                                    
 + expr 0 + 8                                                  
 tot=8                                                         
 + expr 0 + 1                                                 
 count=1                                                       
 + expr 8 + 15                                                 
 tot=23                                                        
 + expr 1 + 1                                                  
 count=2                                                      
 + expr 23 + 25                                                
 tot=48                                                        
 + expr 2 + 1                                                  
 count=3                                                       
 + expr 48 / 3                                                 
 avg=16                                                        
 + echo The average is 16                                      
 The average is 16                                             
 $                                                             


Workshop 2

This workshop will reinforce your understanding of the ideas
presented in Chapter 2. Login to the Multimax using the username
and password given to you by the instructor.  Each student is to
complete the entire workshop.

DESK EXERCISES 

     1.   Any series of non-blank characters can be assigned to a
          user variable.

                            True/False


     2.   How can you insert a space into a user variable?



     3.   What utility can be used to display the contents of a
          user variable to standard output?



     4.   The backslash (\) character is used to remove the special
          meaning of some characters.

                            True/False


     5.   What other character can be used to prevent the shell
          from doing the substitution?


     6.   Double quote marks will prevent the shell from making
          the substitution.

                            True/False

                     Continue on the next page
     7.   What do the following shell variables do?


          HOME

          IFS

          MAIL

          MAILPATH

          MAILCHECK

          PATH

          PS1

          PS2


     8.   What is the command to create a read-only user variable?


     9.   What is the read-only shell variable that represents the
          calling program?


     10.  What do $1,$2,...,$9 represent?


     11.  What BourneShell variable represents all of the command
          line arguments?


     12.  What does the shift command do?


     13.  What is displayed when you enter set with no arguments?


COMPUTER EXERCISES 


     14.  Login to the Multimax (domax1) using the username and
          password given to you by the instructor.


     15.  Create a subdirectory called sub_dir.

     16.  Modify your .profile to include the following:


          a) Change the home directory to sub_dir

          b) Set the internal-field separator to a comma

          c) Have mail messages saved into mail1.

          d) Set the PATH to look for programs in the following  
             directories:

               $HOME/bin
               /bin
               /usr/bin

          e) Change the prompt to reflect the name of the system

          f) Change the secondary prompt to 'More?'


     17.  Execute the .profile
          Enter $. .profile


     18.  Verify that the changes are correct. If you have extra
           time go to the next page.

Extra Mile


Change the .profile file so the date/time and a list of all
users that are currently logged in will be displayed on your
monitor screen automatically when you login.
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