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Full Discussion: Dynamic variables and eval
Top Forums Shell Programming and Scripting Dynamic variables and eval Post 302413970 by yonderboy on Sunday 18th of April 2010 09:46:53 PM
Old 04-18-2010
Dynamic variables and eval
Hi,

I need some direction with the following. The below code is semi-psuedo code which will hopefully make it easier to understand what I am trying to achieve:

Code:
 
for i in `echo ${testarray[*]}`
        do
                let c=c+1
                eval "first$c=$i"
                while :
                do
                        let n=n+1
                        echo "Please enter a value for second var as first has already been obtained from array:"                     
                        read second
                        echo "Please enter a value for third var"
                        read third
                        echo "Please enter a value for fourth var"
                        read fourth
                        eval "second$n=${second}
                        echo "Would you like to run the while loop again?"
                        while read input
                        do
                                if [[ $input == "y" || $input == "Y" ]];then
                                        eval "first$c\second$n=${second}${n}"
                                        eval "first$c\second$n\third=${third}"
                                        eval "first$c\second$n\fourth=${fourth}"
                                        break
                                elif [[ $input == "n" || $input == "N" ]];then
                                        break 2
                                else
                                        echo "Invalid response!"
                                fi
                        done
                done
        done


First of all I am looping through an array ( let's call it ${testarray}) with a "for" loop and assigning the value of that element of the array for that iteration to the ubiquitous ${i}. I am also setting up a loopcount variable called ${c} which keeps track of the number of times the "for" loop has been called.

The first stage of the for loop simply assigns the value of ${i} to a more meaningful name (${first}${c}).

I then start a nested while loop with yet another counter called ${n}which, (while true) reads in another variable called "${second}", another called "${third}" and yet another called ${fourth}.

I then use eval to give ${second} a unique name (depending on the iteration. (e.g. for the first iteration, ${second} would be renamed ${second1}, for the second iteration, ${second2}, then ${second3} etc..). ${third} and ${fourth} do not need to be unique for the iteration.

Then, depending on user input I want to EITHER start again at the top of the while loop (thereby keeping the iterative value for ${first}${c}) , but increment the count value of ${second};
OR start the "for" loop again and increment the value of ${first}${c}, followed once again by the other values.

What I hope to end up with is a bunch of dynamic variables of the form (as an example) ${first1second1third} and ${first1second1fourth) or ${first1second2third} etc....

What I am having trouble with is getting the dynamic variables to be named correctly and evaluated by the shell correctly.

Hope this makes sense.....

Thanks in advance
Last edited by yonderboy; 04-19-2010 at 12:48 AM.. Reason: clarity
 

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LEARN ABOUT MINIX

break

break(1)                                                           User Commands                                                          break(1)

NAME

       break, continue - shell built-in functions to escape from or advance within a controlling while, for, foreach, or until  loop

SYNOPSIS

   sh
       break [n]

       continue [n]

   csh
       break

       continue

   ksh
       *break [n]

       *continue [n]

DESCRIPTION

   sh
       The break utility exits from the enclosing for or while loop, if any. If n is specified, break n levels.

       The continue utility resumes the next iteration of the enclosing for or while loop. If n is specified, resume at the n-th enclosing loop.

   csh
       The  break  utility  resumes  execution after the end of the nearest enclosing foreach or while loop. The remaining commands on the current
       line are executed. This allows multilevel breaks to be written as a list of break commands, all on one line.

       The continue utility continues execution of the next iteration of the nearest enclosing while or foreach loop.

   ksh
       The break utility exits from the enclosed for, while, until, or select loop, if any. If n is specified,  then  break  n  levels.  If  n  is
       greater than the number of enclosing loops, the outermost enclosing loop shall be exited.

       The  continue utility resumes the next iteration of the enclosed for, while, until, or select loop. If n is specified then resume at the n-
       th enclosed loop. If n is greater than the number of enclosing loops, the outermost enclosing loop shall be used.

       On this man page, ksh(1) commands that are preceded by one or two * (asterisks) are treated specially in the following ways:

       1.  Variable assignment lists preceding the command remain in effect when the command completes.

       2.  I/O redirections are processed after variable assignments.

       3.  Errors cause a script that contains them to abort.

       4.  Words that follow a command preceded by ** that are in the format of a variable assignment are expanded with the same rules as a  vari-
           able  assignment.  This means that tilde substitution is performed after the = sign, and also that word splitting and file name genera-
           tion are not performed.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE         |      ATTRIBUTE VALUE        |
       +-----------------------------+-----------------------------+
       |Availability                 |SUNWcsu                      |
       +-----------------------------+-----------------------------+

SEE ALSO

       csh(1), exit(1), ksh(1), sh( 1), attributes(5)

SunOS 5.10                                                          17 Jul 2002                                                           break(1)
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