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

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Date:   Wed, 5 Jun 2019 01:55:54 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     David Laight <David.Laight@...lab.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

On Tue, Jun 4, 2019 at 6:01 PM David Laight <David.Laight@...lab.com> wrote:
>
> 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.
>

I will use this:

$(shell command -v $(c)gcc 2>/dev/null)

Make does not handle redirection by itself.

'2>/dev/null' is easy to understand,
and might be useful as extra safety.




-- 
Best Regards
Masahiro Yamada