> -----Original Message----- > From: Devin Teske [mailto:[email protected]] On Behalf Of > [email protected] > Sent: Monday, June 10, 2013 12:19 PM > To: [email protected]; [email protected] > Cc: [email protected] > Subject: RE: Bourne shell "if" syntax > > > > > -----Original Message----- > > From: [email protected] [mailto:owner-freebsd- > > [email protected]] On Behalf Of Tim Daneliuk > > Sent: Monday, June 10, 2013 12:17 PM > > To: [email protected] > > Cc: [email protected] > > Subject: Re: Bourne shell "if" syntax > > > > On 06/10/2013 02:10 PM, [email protected] wrote: > > > > > > > > >> -----Original Message----- > > >> From: [email protected] [mailto:owner-freebsd- > > >> [email protected]] On Behalf Of Tim Daneliuk > > >> Sent: Monday, June 10, 2013 12:06 PM > > >> To: [email protected] > > >> Subject: Re: Bourne shell "if" syntax > > >> > > >> On 06/10/2013 01:59 PM, [email protected] wrote: > > >>> > > >>> > > >>>> -----Original Message----- > > >>>> From: [email protected] [mailto:owner-freebsd- > > >>>> [email protected]] On Behalf Of [email protected] > > >>>> Sent: Monday, June 10, 2013 11:53 AM > > >>>> To: [email protected] > > >>>> 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. >
Actually, there's another reason you should also avoid the above (unquoted parameter), and that's in the case of a multi-word value. For example: foo="abc 123" if [ $foo = "" ]; then Produces: sh: line 0: [: too many arguments -- Devin > Instead do this: > > if [ "$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 > > _______________________________________________ > > [email protected] mailing list > > http://lists.freebsd.org/mailman/listinfo/freebsd-questions > > To unsubscribe, send any mail to "freebsd-questions- > [email protected]" > > _____________ > 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. _____________ 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. _______________________________________________ [email protected] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[email protected]"
