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 12:06 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.

So doing things like:

foo=-gt
if [ "$foo" = "" ]; then

or

foo=-gt
if [ -z "$foo" ]; then

or

if [ ! "$foo" ]; then

or even the following (flipping the conditional):

if [ "$foo" != "" ]; then
if [ -n "$foo" ]; then
if [ "$foo" ]; then

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).
-- 
Devin


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