From owner-freebsd-questions@freebsd.org  Wed Aug  9 11:16:00 2017
Return-Path: <owner-freebsd-questions@freebsd.org>
Delivered-To: freebsd-questions@mailman.ysv.freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org
 [IPv6:2001:1900:2254:206a::19:1])
 by mailman.ysv.freebsd.org (Postfix) with ESMTP id 681A9DCA167
 for <freebsd-questions@mailman.ysv.freebsd.org>;
 Wed,  9 Aug 2017 11:16:00 +0000 (UTC)
 (envelope-from freebsd@qeng-ho.org)
Received: from bede.home.qeng-ho.org (bede.qeng-ho.org [217.155.128.241])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (Client CN "fileserver.home.qeng-ho.org",
 Issuer "fileserver.home.qeng-ho.org" (not verified))
 by mx1.freebsd.org (Postfix) with ESMTPS id 0B85F6D9C6
 for <freebsd-questions@freebsd.org>; Wed,  9 Aug 2017 11:15:59 +0000 (UTC)
 (envelope-from freebsd@qeng-ho.org)
Received: from arthur.home.qeng-ho.org (arthur.home.qeng-ho.org [172.23.1.2])
 by bede.home.qeng-ho.org (8.15.2/8.15.2) with ESMTP id
 v79AoO0q055765; Wed, 9 Aug 2017 11:50:24 +0100 (BST)
 (envelope-from freebsd@qeng-ho.org)
Subject: Re: Is multi-line assignment to eval possible ?
To: Manish Jain <bourne.identity@hotmail.com>,
 "freebsd-questions@freebsd.org" <freebsd-questions@freebsd.org>
References: <VI1PR02MB120024405E8AC219206B8F6CF68A0@VI1PR02MB1200.eurprd02.prod.outlook.com>
From: Arthur Chance <freebsd@qeng-ho.org>
Message-ID: <bb70ab65-3dcb-029d-7d74-2d824c2d04d8@qeng-ho.org>
Date: Wed, 9 Aug 2017 11:50:24 +0100
User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:52.0) Gecko/20100101
 Thunderbird/52.2.1
MIME-Version: 1.0
In-Reply-To: <VI1PR02MB120024405E8AC219206B8F6CF68A0@VI1PR02MB1200.eurprd02.prod.outlook.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-GB
Content-Transfer-Encoding: 8bit
X-BeenThere: freebsd-questions@freebsd.org
X-Mailman-Version: 2.1.23
Precedence: list
List-Id: User questions <freebsd-questions.freebsd.org>
List-Unsubscribe: <https://lists.freebsd.org/mailman/options/freebsd-questions>, 
 <mailto:freebsd-questions-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-questions/>
List-Post: <mailto:freebsd-questions@freebsd.org>
List-Help: <mailto:freebsd-questions-request@freebsd.org?subject=help>
List-Subscribe: <https://lists.freebsd.org/mailman/listinfo/freebsd-questions>, 
 <mailto:freebsd-questions-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 09 Aug 2017 11:16:00 -0000

On 08/08/2017 16:48, Manish Jain wrote:
> 
> Hi,
> 
> On my FreeBSD box, I was trying do something in a Bourne script which at 
> first seemed fairly trivial - but has now clouded my mind thoroughly.
> 
> a=A
> b=B
> 
> eval ${a}_${b}="something"

In this case you might want to check out the setvar builtin in /bin/sh:

setvar variable value
        Assigns the specified value to the specified variable.  The
        setvar command is intended to be used in functions that assign
        values to variables whose names are passed as parameters.  In
        general it is better to write “variable=value” rather than using
        setvar.

so that eval could be written as

setvar ${a}_${b} "something"

> No problems so far. I get a new variable A_B that stores "something"
> 
> But if I spread the assignment over 2 lines as under :
> 
> eval ${a}_${b}="some
> thing"
> 
> The shell barks at me that it could find no command called 'thing'. Is 
> there some hack that I could use to get eval to 'understand' that the 
> assignment has an embedded newline ?

Yes, but using setvar avoids the need:

setvar ${a}_${b} "some
thing"

works OK.

-- 
An amusing coincidence: log2(58) = 5.858 (to 0.0003% accuracy).