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From: brad@SSD.CSD.HARRIS.COM (Brad Appleton)
Newsgroups: comp.unix.questions
Subject: Re: unix shell programming question
Summary: use "eval"
Message-ID: <3106@hcx1.SSD.CSD.HARRIS.COM>
Date: 27 Feb 90 15:29:59 GMT
References: <504@lkbpyr.UUCP>
Sender: news@hcx1.SSD.CSD.HARRIS.COM
Organization: Harris Computer Systems, Fort Lauderdale, FL
Lines: 87

In article <504@lkbpyr.UUCP> jonas@lkbpyr.UUCP (Jonas Heyman) writes:
>Hello,
>
>Could anyone give me some help:
>
>On the lines with "echo" I want to have the same output.
>Each time the "for loop" goes trough I want to list $str1 and $str2.
>How can I do that.
>
>Sincerely Jonas.
>
>----------------------------------------
>str1="string 1"
>str2="string 2"
>
>...
>...  
>...
>
>echo $str1        #This is OK   "string 1"
>echo $str2 	  #This is OK   "string 2"
>
>for loop in 1 2
>do
>	aa=$str$loop     #Here I want "string 1" and "string 2"
>	echo $aa
>
>	echo $str$loop   #Here I want "string 1" and "string 2"
>done
>-----------------------------------------
>
You need to use "eval" to accomplish this. Eval is a shell builtin which
may be used to:

* Process the result of an expansion or substitution by a step that
  proceeds it during command processing

* Find the value of a parameter/variable whose value is the name of 
  another parameter/variable

* Execute a line that was read from standard input


Using "eval" in your example would result in the following
(NOTE:  I removed the leading '$' from '$str" since "str" is 
not the name of a shell variable):

--------------------------------------------------------------------
     str1="string 1"
     str2="string 2"
	.
	.
	.
     for loop in 1 2
     do
     	aa=str$loop
     	echo `eval echo '$'$aa`   ## prints "string 1" when loop=1
                                  ## and prints "string 2" when loop=2
     
     	echo `eval echo '$'str$loop`   ## same effect as above
     done
--------------------------------------------------------------------

I'm not sure of what it is you want to "echo" however. The output from 
the script segment above would be:

     string 1
     string 1
     string 2
     string 2

not:

     string 1
     string 2
     string 1
     string 2

Hope this helps!


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