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From: krs@uts.amdahl.com (Kris Stephens [Hail Eris!])
Newsgroups: comp.unix.shell
Subject: Re: sh loop variable and "double indirection"
Message-ID: <a21801wv11X.00@amdahl.uts.amdahl.com>
Date: 28 Jan 91 04:09:16 GMT
References: <1991Jan27.044258.18779@shibaya.lonestar.org>
Reply-To: krs@amdahl.uts.amdahl.com (Kris Stephens [Hail Eris!])
Organization: Amdahl Corporation, Sunnyvale CA
Lines: 62

In article <1991Jan27.044258.18779@shibaya.lonestar.org> afc@shibaya.lonestar.org (Augustine Cano) writes:
>I am trying to specify (at run time) an upper limit for a loop in a shell
>script.  In pseudo-code the ideal would be something like this:
>
>read i
>for 0 to i
>do
>...
>done

Let me give you the ksh version first, then the (slower) sh mods -- it's
slower because each iteration requires a fork/exec not required in ksh.

:
# ksh version
# (note -- in commentary, Augustine requests that we stop at i-1)
typeset -i i j	# make things run faster by naming these as ints
j=0		# could be done on the typeset, but needed for the sh version

echo "enter limit: \c"
read i dummy	# in case we get two words; also, you should test for a bad $i
while [ $j -lt $i ]
do
	eval 'var'$j'="$REAL_VALUE'$j'"'
	eval 'echo "var$j is $var'$j'"'
	((j = $j + 1))
done
exit

The sh version is the same except for two parts:  delete the typeset,
because it's not in sh, and replace   ((j = $j + 1))  with a call
j=`expr $j + 1`  (which is the extra fork/exec).

>Not very elegant since a limit of 10 iterations is hard-wired.  Can anybody
>think of a more concise way to do this?  Using PERL is not an option, this
>must be portable sh code.
>
>The next problem is the thorny one.  Some shell variables having been
>previously set up, say:
>
>var0=REAL_VALUE0
>var1=REAL_VALUE1
>var2=REAL_VALUE2
>var3=REAL_VALUE3
>var4=REAL_VALUE4
>
>I want to manipulate variable names inside the above loop such that
>I could display the "REAL VALUEx" based on the current value of $i.

Assuming that all possible REAL_VALUE* variables were defined somehow,
the two eval instructions inthe sample loop will store their contents
in the variables  var0 through var${i-1} and print them out.  If you're
a Rexx programmer, eval in the shell is like Rexx's interpret.  The args
to eval are evaluated once (losing one level of quotation), the 'eval '
is stripped, and the resulting string is evaluated and executed.

...Kris
-- 
Kristopher Stephens, | (408-746-6047) | krs@uts.amdahl.com | KC6DFS
Amdahl Corporation   |                |                    |
     [The opinions expressed above are mine, solely, and do not    ]
     [necessarily reflect the opinions or policies of Amdahl Corp. ]