Bourne shell "if" syntax

[dteske at freebsd.org]

> -----Original Message-----
> From: Devin Teske [mailto:devin.teske at fisglobal.com] On Behalf Of
> dteske at freebsd.org
> Sent: Monday, June 10, 2013 12:19 PM
> To: tundra at tundraware.com; freebsd-questions at freebsd.org
> Cc: dteske at freebsd.org
> Subject: RE: Bourne shell "if" syntax
> 
> 
> 
> > -----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.
> 

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
> > _______________________________________________
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> 
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