linux-kernel - RE: [PATCH] kbuild: use more portable 'command -v' for cc-cross-prefix

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Date:   Tue, 4 Jun 2019 09:01:31 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Masahiro Yamada' <yamada.masahiro@...ionext.com>
CC:     "linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>,
        "Vineet Gupta" <vgupta@...opsys.com>,
        Alexey Brodkin <abrodkin@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        linux-stable <stable@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] kbuild: use more portable 'command -v' for
 cc-cross-prefix

From: Masahiro Yamada
> Sent: 04 June 2019 04:31
...
> > > > You could use:
> > > >         $(shell sh -c "command -v $(c)gcc")
> > > > or maybe:
> > > >         $(shell command$${x:+} -v $(c)gcc)
> > >
> > >
> > > How about this?
> > >
> > >           $(shell : ~; command -v $(c)gcc)
> >
> > Overcomplicated ....
> >
> > I've not looked at the list of 'special characters' in make,
> > but I suspect any variable expansion is enough.
> > Since ${x:+} always expands to the empty string (whether or
> > not 'x' is defined) it can't have any unfortunate side effects.
> 
> 
> Probably, my eyes are used to Makefile.
> ":" is a no-op command, and it is used everywhere in kernel Makefiles
> in the form of "@:'
> 
> It depends on people which solution seems simpler.
> So, this argument tends to end up with bikesheding.

I am fully aware of ':', it is a shell builtin that always return success.
Usually used when you want the side-effects of substitutions without
executing anything (eg : ${foo:=bar} ), to change the result of a
sequence of shell commands or as a dummy (eg while :; do :; done; )
Very annoyingly bash parses !: as something other than 'not true'.

$(shell command$${x:+} -v $(c)gcc) will be marginally faster
because it is less parsing.

	David
 

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