UNIX Bourne Shell Programming

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4.8  for

The format for this construct is:

  Command Format:  for loop-index in argument-list            
                        do                                     
                        commands                               
                        done                                   


This structure will assign the value of the first item in the
argument list to the loop index and executes the commands between
the do and done statements.  The do and done statements indicate
the beginning and end of the for loop.

After the structure passes control to the done statement, it
assigns the value of the second item in the argument list to the
loop index and repeats the commands.  The structure will repeat
the commands between the do and done statements once for each
argument in the argument list.  When the argument list has been
exhausted, control passes to the statement following the done.
Sample Session:


 $cat find_henry1                                              
 for x in project1 project2 project3                           
 do                                                            
 grep henry $x                                                 
 done                                                          


Sample Session:
 

 $head project?                                                
 ==> project1 <==                                              
 henry                                                         
 joe                                                           
 mike                                                          
 sue                                                           
                                                               
 ==> project2 <==                                              
 joe                                                           
 mike                                                         
 sue                                                           
                                                               
 ==> project3 <==                                             
 joe                                                           
 mike                                                          
 sue                                                           
 henry                                                         
                                                               
 ==> project4 <==                                              
 joe                                                           
 mike                                                          
                                                               
 $find_henry                                                   
 henry                                                         
 henry                                                         
 $                                                             



Each file in the argument list was searched for the string, henry.
When a match was found, the string was printed.

4.9  while

The format for this construct is:

  Command Format:  while expression                            
                        do                                     
                        commands                               
                        done                                   


As long as the expression returns a true exit status, the structure
continues to execute the commands between the do and the done
statement.  Before each loop through the commands, the structure
executes the expression.  When the exit status of the expression
is false (non-zero), control is passed to the statement following
the done statement.

The commands to be executed must change the expression test or an
infinite loop can result.


4.10  until

The format for this construct is:

  Command Format:  until expression                            
                       do                                     
                        commands                               
                       done                                   


The until and while structures are very similar.  The only
difference is that the test is at the top of the loop.  The until
structure will continue to loop until the expression returns true
or a nonerror condition.  The while loop will continue as long as
a true or nonerror condition is returned.

The commands to be executed must change the expression test or an
infinite loop can result.
Sample Session:


 $cat until_ex                                                 
 secretname='jenny'                                            
 name='noname'                                                 
 echo 'Try to guess the secret name!'                          
 echo                                                          
 until (test "$name" = "$secretname")                          
     do                                                        
     echo 'Your guess:  \c'                                    
     read name                                                 
     done                                                      
 echo 'You did it!'                                            
 $                                                             


The until loop will continue until name is equal to the secret
name.


Sample Session:


 $chmod a+x until_ex                                           
 $until_ex                                                     
 Try to guess the secret name!                                 
                                                              
 Your guess: gaylan                                            
 Your guess: art                                               
 Your guess: richard                                           
 Your guess: jenny                                             
 You did it!                                                   
 $                                                             

4.11  case

The format for this construct is:

  Command Format:  case test-string in                        
                        pattern-1 ) commands-1 ;;              
                        pattern-2 ) commands-2 ;;              
                        pattern-3 ) commands-3 ;;              
                        .                                      
                        .                                      
                        .                                      
                        *)          commands   ;;              
                   esac                                        


The case structure allows a multiple-branch decision mechanism.
The path that is taken depends on a match between the test-string
and one of the patterns.
Sample Session:
                                                  

 $cat case_ex                                                  
 echo 'Enter A, B, or C: \c'                                   
 read letter                                                   
 case $letter in                                               
    A) echo 'You entered A' ;;                                 
    B) echo 'You entered B' ;;                                 
    C) echo 'You entered C' ;;                                 
    *) echo 'You did not enter A, B, or C' ;;                  
 esac                                                          
 $chmod a+x case_ex                                            
 $case_ex                                                      
 Enter A, B, or C: B                                           
 You entered B                                                 
 $case_ex                                                      
 Enter A, B, or C: b                                           
 You did not enter A, B, or C                                  
 $                                                             


This example uses the value of a character that the user entered
as the test string.  The value is represented by the variable
letter.  If letter has the value of A, the structure will execute
the command following A.  If letter has a value of B or C, then
the appropriate commands will be executed.  The asterisk indicates
any string of characters; and it, therefore, functions as a
catchall for a no-match condition.  The lowercase b in the second
sample session is an example of a no match condition.


Workshop 4

This workshop will reinforce your understanding of the ideas
presented in Chapter 4. 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.   Which utility will evaluate an expression and then return
          a condition indicating whether or not the expression is
          true (equal to zero) or false (not equal to zero)?


     2.   What are the operators for character string comparisons?

COMPUTER EXERCISES 


     3.   Use the "if then" construct to write a BourneShell script
          that will check for at least two parameters being present
          on the command line. Output an appropriate error message.


     4.   Write a BourneShell script using the "if then else"
          construct that will check for equality of two strings
          that are supplied as parameters to the script. Output a
          message stating if the strings are equal or not equal.


     5.   Write a BourneShell script using the "if then elif"
          construct that will check two numbers, input as
          parameters, and tell if the first parameter is greater
          than, equal to, or less than the second number. Output
          appropriate error messages.


     6.   Write a BourneShell script using the "for" construct that
          has a loop index called "fruit" and an argument list as
          follows: apples oranges bananas pears. Echo the name of
          each argument to the monitor screen and when the last
          argument is listed output an appropriate message.


     7.   Write a BourneShell script using the "while" construct
          that will add all the numbers between 0 and 9 and display
          the result. The sum of the digits 0 through 9 is 45.


      8.  Write a BourneShell script using the "until" construct
          similar to the example in the manual except compare
          numbers instead of strings.


      9.  Write a BourneShell script using the "case" statement
          that will ask you to enter the day of the week and then
          echo that day to the monitor screen. Be sure to include
          an appropriate message if you enter in a string other
          than a valid day of the week.


EXTRA MILE


     10.  Write a BourneShell script called "dir_num" that will
          test all of the files in the current directory and print
          all the files that prove to be a directory.
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