Date: Sun, 10 Oct 2010 16:09:54 -0700 From: Devin Teske <dteske@vicor.com> To: Julian Elischer <julian@freebsd.org> Cc: freebsd-hackers@freebsd.org Subject: Re: sysrc -- a sysctl(8)-like utility for managing /etc/rc.conf et. al. Message-ID: <663E0B47-9318-4C75-BAEB-5C4F9EAFFDD1@vicor.com> In-Reply-To: <4CB14E2D.3070806@freebsd.org> References: <1286397912.27308.40.camel@localhost.localdomain> <AANLkTikoohMo5ng-RM3tctTH__P6cqhQpm=FPhSE9mMg@mail.gmail.com> <51B4504F-5AA4-47C5-BF23-FA51DE5BC8C8@vicor.com> <AANLkTim=BLkd229vdEst8U0ugpq3UsHPxjZZp2qaJxH-@mail.gmail.com> <238E0B24-AA12-4684-9651-84DA665BE893@vicor.com> <4CB14E2D.3070806@freebsd.org>
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On Oct 9, 2010, at 10:25 PM, Julian Elischer wrote:
> Ah grasshoppers...
>=20
> /me wonders if anyone will get the full significance of that..
>=20
>=20
> On 10/9/10 3:39 PM, Devin Teske wrote:
>> On Oct 9, 2010, at 1:25 PM, Garrett Cooper wrote:
>>=20
>>=20
>>> Why not just do...
>>>=20
>>> if [ "x$rc_conf_files" =3D x -o "x$varname" =3D x ]
>>> then
>>> return ${FAILURE-1}
>>> fi
>> I think you'll find (quite pleasantly) that if you intonate the =
lines...
>>=20
>> "rc_conf_files [is non-null] OR return failure"
>> "varname [is non-null] OR return failure"
>>=20
>> Sounds a lot better/cleaner than the intonation of the suggested =
replacement:
>>=20
>> "if x plus rc_conf_files expands to something that is not equal =
to x OR x plus the expansion of varname is not x then return failure"
>>=20
>=20
>=20
> For what it matters, I'v enever found the [ "x$foo" =3D "x" ] =
construct to be useful.
> the quoting seems to work for everything I've ever worked on.
I agree... I think that the "x" syntax came around for when people were =
using non-quoted syntax... for example...
[ x$foo =3D x ]
is still very useful in that it prevents the error when $foo expands to =
"-e".
However, enclosing the argument (as the 'x$foo' portion is really just =
the first argument to the '[' built-in) in quotes:
[ "$foo" =3D x ]
makes it so that the expansion is taken as:
[ "-n" =3D x ]
rather than:
[ -n =3D x ]
The former not causing an error, while the latter does.
Quite functionally, at a C-level, if you have your array, ...
argv[0] =3D "[\0";
argv[1] =3D "\"-n\"\0"; /* quoted syntax */
argv[2] =3D "=3D\0";
argv[3] =3D "x\0";
and
argv[0] =3D "[\0";
argv[1] =3D "-n\0"; /* non-quoted syntax */
argv[2] =3D "=3D\0";
argv[3] =3D "x\0";
The C-code that switch'es on the argv element can tell the difference =
between a leading quote and a leading dash and does indeed do the right =
thing for all expansions of the variable within a double-quote block =
("...").
Though, the non-quoted syntax should be avoided at all costs, imho =
unless you know for a fact that the expansion of the variable will never =
include a character from $IFS (defaults to space, tab, newline).
If it does, then it'll evaluate to a new positional argument for the =
C-code.
Take the following example:
$ foo=3D"abc =3D abc"
$ [ $foo ] && echo true
true
$ foo=3D"abc =3D 123"
$ [ $foo ] && echo true
# no output (not true)
Whereas the quoted syntax:
$ [ "$foo" ]
will always evaluate to true because there is an implied "-n" operator =
for the case where the first-and-last positional argument is a =
double-quoted string, and...
$ [ -n "$foo" ]
always evaluates to true in the above cases for foo ("abc =3D abc" then =
later "abc =3D 123").
> ...
>=20
> Now One thing that should be bourne in mind (heh) is that
> as there is a 'usual' form of format for perl there is one for sh as =
well so it's not "polite"
> to make one's sh code look like perl. :-)
Perusing sh(1) today... found examples of the lazy operators:
[ expr ] || expr
[ expr ] && expr
Search under "Short-Circuit List Operators"
I'd say that the description therein is a green-light to treat sh like =
perl ^_^
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> To unsubscribe, send any mail to =
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--
Cheers,
Devin Teske
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