Date: Mon, 10 Jun 2013 14:16:50 -0500 From: Tim Daneliuk <tundra@tundraware.com> To: dteske@freebsd.org Cc: freebsd-questions@freebsd.org Subject: Re: Bourne shell "if" syntax Message-ID: <51B62622.1070201@tundraware.com> In-Reply-To: <121b01ce660e$2abae5f0$8030b1d0$@freebsd.org> References: <51b620a6.42f.2b6a6400.5605dcf0@go2france.com> <121701ce660c$9a9aa5b0$cfcff110$@freebsd.org> <51B62389.5000500@tundraware.com> <121b01ce660e$2abae5f0$8030b1d0$@freebsd.org>
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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 [ = "" ]
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.
> 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
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