Bourne shell "if" syntax

[Teske, Devin]

On Jun 11, 2013, at 2:56 AM, Jan Henrik Sylvester wrote:

> On 06/10/2013 21:10, dteske at freebsd.org wrote:
>> 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.
> 

Appears I was wrong (and I can admit that).

Further testing/investigation shows that it's not based on double-quoting (although this DOES play a role -- but only in the fact that you're ensuring that the program -- [ -- only gets one argument -- sans trailing square-bracket).

It appears that the divide is the number of arguments.

For example, compare:

if [ "-gt 1" ]; then # arg1 treated as a string; returns success

to

if [ "-gt" "1" ]; then # arg1 treated as an operator; syntax error

to

if [ "-gt" ]; then # arg1 treated as a string; returns success


Or compare the equally divided:

A=-gt

if [ "$A 1" ]; then # arg1 treated as a string; returns success

to

if [ "$A" "1" ]; then # arg1 treated as an operator; syntax error

to

if [ "$A" ]; then # arg1 treated as a string; returns success

When the program -- [ -- (regardless of built-in or external) sees only one argument in it's ARGV array, it will *not* treat the argument as a flag but instead return true if it's non-NULL or return false if NULL.





> What you are saying here is at least misleading. I just started up sh on
> 9.1 RELEASE and tried:
> 
> $ A=-z
> $ if [ "$A" "" ] ; then echo z ; fi
> z
> $ if [ "$A" "1" ] ; then echo z ; fi
> $ if /bin/[ "$A" "" ] ; then echo z ; fi
> z
> $ if /bin/[ "$A" "1" ] ; then echo z ; fi
> $
> 

Interesting that I do not get the same results on 9.0-R for the external usage.

devin at mick.vicor.com ~ $ if /bin/[ "$A" "" ] ; then echo z ; fi
[: ]: unknown operand
devin at mick.vicor.com ~ $ if /bin/[ "$A" "1" ] ; then echo z ; fi
[: ]: unknown operand

Must be a bug in 9.0-R with the ending square-bracket (`]'). If I remove the ending square-bracket it runs…

devin at mick.vicor.com ~ $ if /bin/[ "$A" "" ; then echo z ; fi
z
devin at mick.vicor.com ~ $ if /bin/[ "$A" 1 ; then echo z ; fi





> Although "-z" is quoted, it is seen as an operator. It does not seem to
> have anything to do with whether the build-in or external "[" is used.
> 

You're at least partially right… number of arguments makes a difference.

A=-gt
if [ "$A" "" ]; then echo z; fi

sh: line 0: [: -gt: unary operator expected

===

Meanwhile, it knows to treat it as a string when it's the only argument…

A=-gt
if [ "$A" ]; then echo z; fi

z

-- 
Devin

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