Skip site navigation (1)Skip section navigation (2)
Date:      Mon, 10 Jun 2013 12:18:54 -0700
From:      <dteske@freebsd.org>
To:        <tundra@tundraware.com>, <freebsd-questions@freebsd.org>
Cc:        dteske@freebsd.org
Subject:   RE: Bourne shell "if" syntax
Message-ID:  <123201ce660f$59a7b9f0$0cf72dd0$@freebsd.org>
In-Reply-To: <51B62622.1070201@tundraware.com>
References:  <51b620a6.42f.2b6a6400.5605dcf0@go2france.com> <121701ce660c$9a9aa5b0$cfcff110$@freebsd.org> <51B62389.5000500@tundraware.com> <121b01ce660e$2abae5f0$8030b1d0$@freebsd.org> <51B62622.1070201@tundraware.com>

next in thread | previous in thread | raw e-mail | index | archive | help


> -----Original Message-----
> From: owner-freebsd-questions@freebsd.org [mailto:owner-freebsd-
> questions@freebsd.org] On Behalf Of Tim Daneliuk
> Sent: Monday, June 10, 2013 12:17 PM
> To: dteske@freebsd.org
> Cc: freebsd-questions@freebsd.org
> Subject: Re: Bourne shell "if" syntax
> 
> On 06/10/2013 02:10 PM, dteske@freebsd.org wrote:
> >
> >
> >> -----Original Message-----
> >> From: owner-freebsd-questions@freebsd.org [mailto:owner-freebsd-
> >> questions@freebsd.org] On Behalf Of Tim Daneliuk
> >> Sent: Monday, June 10, 2013 12:06 PM
> >> To: freebsd-questions@freebsd.org
> >> Subject: Re: Bourne shell "if" syntax
> >>
> >> On 06/10/2013 01:59 PM, dteske@freebsd.org wrote:
> >>>
> >>>
> >>>> -----Original Message-----
> >>>> From: owner-freebsd-questions@freebsd.org [mailto:owner-freebsd-
> >>>> questions@freebsd.org] On Behalf Of lconrad@go2france.com
> >>>> Sent: Monday, June 10, 2013 11:53 AM
> >>>> To: freebsd-questions@freebsd.org
> >>>> Subject: Bourne shell "if" syntax
> >>>>
> >>>>
> >>>>
> >>>> script fragment:
> >>>>
> >>>> PTR=`dig @some.dns +short +norec -x a.b.c.d`
> >>>>
> >>>> echo "$PTR"
> >>>>
> >>>> if  [  "$PTR"  ==  ""  ]  ;  then
> >>>>
> >>>
> >>> if [ "$PTR" = "" ]; then
> >>>
> >>> or
> >>>
> >>> if [ -z "$PTR" ]; then
> >>>
> >>> or
> >>>
> >>> if [ "$PTR" ]; then
> >>>
> >>> but _NOT_
> >>>
> >>> if [ "$PTR" == "" ]; then
> >>>
> >>
> >>
> >> I work across a bunch of different OSs and shells of many vintages.  As I
> > recall,
> >> the -z argument has problems of portability on older/broken shells and/or
> >> is not available in all environments (I cannot recall which at the moment).
> > So
> >> I achieve the same results by using a character sentinel that guarantees
that
> > the
> >> comparison always works:
> >>
> >>     f  [  _"$PTR"  ==  _  ]  ;  then
> >>
> >
> > Character sentinels are not required.
> >
> > FreeBSD's sh(1) knows (because "[" is a built-in) that when you quote a
> > parameter, that it is not (even if the value begins with "-") not an
operator.
> >
> 
> 
> That wasn't really my point.  I use sentinels because in the face of an
> empty string this:
> 
>     if [ $PTR = "" ]
> 
> Actually evaluates to:
> 
>     if [ = "" ]
> 

and hence why you shouldn't do that.

Instead do this:

if [ "$PTR" = "" ]

Which [potentially] evaluates to:

if [ "" = "" ]


> Which throws an error.  The character sentinel avoids this without having to
> use -z, which as I said, I've had problems with not being too portable across
> older machinery.
> 

Which again, is because you're not double-quoting your parameter.

The sentinel is not required if you double-quote your parameter (which you were
already doing in your example).

For example (with sentinel):

if [ _"$PTR" == (sic) _ ] ; then

Since you've already double-quoted the parameter, I'm letting you know that the
sentinel is unnecessary.
-- 
Devin


> 
> > All work as expected. It matters not the value of $foo. sh(1) in FreeBSD
knows
> > because of the double-quotes that it is not an operator.
> >
> > Furthermore...
> >
> > "==" is not the right operator. It's "=".
> >
> > Portability would surely be compromised if you were using "==" (which
doesn't
> > work on FreeBSD; or many other OSes I gather from experience).
> >
> 
> Ooops, I did catch that and you're quite right.
> --
> -----------------------------------------------------------------------
> Tim Daneliuk
> _______________________________________________
> freebsd-questions@freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-questions
> To unsubscribe, send any mail to "freebsd-questions-unsubscribe@freebsd.org"

_____________
The information contained in this message is proprietary and/or confidential. If you are not the intended recipient, please: (i) delete the message and all copies; (ii) do not disclose, distribute or use the message in any manner; and (iii) notify the sender immediately. In addition, please be aware that any message addressed to our domain is subject to archiving and review by persons other than the intended recipient. Thank you.



Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?123201ce660f$59a7b9f0$0cf72dd0$>