Bourne shell "if" syntax

[dteske at freebsd.org]

> -----Original Message-----
> From: owner-freebsd-questions at freebsd.org [mailto:owner-freebsd-
> questions at freebsd.org] On Behalf Of Tim Daneliuk
> Sent: Monday, June 10, 2013 12:17 PM
> To: dteske at freebsd.org
> Cc: freebsd-questions at freebsd.org
> Subject: Re: Bourne shell "if" syntax
> 
> On 06/10/2013 02:10 PM, dteske at freebsd.org wrote:
> >
> >
> >> -----Original Message-----
> >> From: owner-freebsd-questions at freebsd.org [mailto:owner-freebsd-
> >> questions at freebsd.org] On Behalf Of Tim Daneliuk
> >> Sent: Monday, June 10, 2013 12:06 PM
> >> To: freebsd-questions at freebsd.org
> >> Subject: Re: Bourne shell "if" syntax
> >>
> >> On 06/10/2013 01:59 PM, dteske at freebsd.org wrote:
> >>>
> >>>
> >>>> -----Original Message-----
> >>>> From: owner-freebsd-questions at freebsd.org [mailto:owner-freebsd-
> >>>> questions at freebsd.org] On Behalf Of lconrad at go2france.com
> >>>> Sent: Monday, June 10, 2013 11:53 AM
> >>>> To: freebsd-questions at 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
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