Re: [BUG] Condition in [[ doesn't fire, with "if" it fires

[Peter Stephenson <p.stephenson@samsung.com>]

On Tue, 28 Feb 2017 03:27:36 -0800
Sebastian Gniazdowski <psprint3@fastmail.com> wrote:
> Checked that even:
> 
>     [[ "below" = "below" ]] && (( line ++ )) || (( line = stborder ))
> 
> doesn't fire. In the same way. It is the false option that is fired:

Ah, I see what you've done now.  I moved this to zsh-users because this
is an interesting point and the above should give folks enough context
to see what I'm talking about; look at zsh-workers for more background.

&& and || don't work the way they do in C, they are simply evaluated
left to right in a symmetric fashion.  (Yes, another shell oddity dating
from year zero for Ray to moan about.)  Have a look at the grammar tests
for some example.

In detail:

- Test succeeds.

- So (( line ++ )) run

- line was zero, and with a post increment the value is zero, so status
is 1.

- (This is the unexpected bit.)  On a non-zero status, the shell looks
forward for the next ||, ignoring any &&'s.  As I said, there is no
relative priority, just a strict left to right evaluation.

- So it then executes (( line = stborder ))

- I'm guessing stborder is 0, so that's what line is set to.

You *could* fix this up by switching to a pre-increment, but I'd
recommend just going to if/then/else --- it's much more obvious what the
logic actually means.

pws