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From: Eli Schwartz <eschwartz@archlinux.org>
Newsgroups: gnu.bash.bug
Subject: Re: Not missing, but very hard to see (was Re: Backslash missing in brace expansion)
Date: Thu, 12 Dec 2019 22:03:33 -0500
Lines: 124
Approved: bug-bash@gnu.org
Message-ID: <mailman.767.1576206236.1979.bug-bash@gnu.org>
References: <20191205201157.cd481936f76d95bbdfabc73c@schrader-schulte.de> <662e2328-f331-c554-afcf-fd3819f6beab@case.edu> <20191206055304.076d6115afa3a4f2a6a21c34@schrader-schulte.de> <5b5064a8-7175-42e7-1eb5-6374dee6c11e@redhat.com> <21761e28-c496-ff67-d7b7-628c9325085f@iki.fi> <9dd3a388-39b1-c059-de99-813f1e411764@case.edu> <5DF2987E.5000309@tlinx.org> <568aeaaa-22b3-c7b9-0e18-a92bef6d2ffb@iki.fi> <5DF2FE31.9070406@tlinx.org> <0ff3a920-94c2-b0c9-5631-0964955657aa@archlinux.org>
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On 12/12/19 9:57 PM, L A Walsh wrote:
>=20
>=20
> On 2019/12/12 13:01, Ilkka Virta wrote:
>> On 12.12. 21:43, L A Walsh wrote:
>> =C2=A0
>>> On 2019/12/06 14:14, Chet Ramey wrote:
>>>
>>> Seems very hard to print out that backquote though.=C2=A0 Closest I g=
ot
>>> was bash converting it to "''":
>>> =C2=A0=C2=A0=C2=A0=20
>>
>> The backquote is in [6], and the backslash disappears, you just get
>> the pair of quotes in [2] because that's how printf %q outputs an
>> empty string.
>> =C2=A0=20
> -----
>=20
> =C2=A0=C2=A0 I'm sorry, but you are mistaken.

How so?

> =C2=A0=C2=A0 The characters from 'Z' (0x5A) through 'z' (0x61) are:
>=20
> 0x5A 0x5B 0x5C 0x5D 0x5E 0x5F 0x60 0x61
> Z=C2=A0=C2=A0=C2=A0 [=C2=A0=C2=A0=C2=A0 \=C2=A0=C2=A0=C2=A0 ]=C2=A0=C2=A0=
=C2=A0=C2=A0 ^=C2=A0=C2=A0 _=C2=A0=C2=A0=C2=A0=C2=A0 `=C2=A0=C2=A0=C2=A0 =
a
>=20
> the backslash comes between the two square brackets.
>=20
> Position [6] is the "Grave Accent" (or backquote).
>=20
> It is quoted properly.

But... that's exactly what was said.

> As for %q printing an empty string for 0x5C
>=20
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "%q" causes=C2=A0 printf to =
output the corresponding argument in a
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 format that can be reused as=
 shell input.
>=20
> =C2=A0=C2=A0 For that string to be empty would mean there is no charact=
er at hex
> value 0x5C (unicode U+005C), which isn't so.

But there isn't. An empty string was passed as an argument to printf,
because the backslash was *converted* via escaping, into an empty
string, *before* it was passed on the command line as an argv element to
the printf builtin.

Do you think that because printf is a builtin, and you didn't use
/bin/printf, that somehow means it is exempt from the usual rule of how
shells work, and gets to see its own argv before the parser reinterprets =
it?

>> =C2=A0
>>>> =C2=A0read -r -a a< <(printf "%q " {Z..a})
>>>> =C2=A0my -p a
>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=20
>>> declare -a a=3D([0]=3D"Z" [1]=3D"\\[" [2]=3D"''" [3]=3D"\\]" [4]=3D"\=
\^" [5]=3D"_"
>>> [6]=3D"\\\`" [7]=3D"a")
>>> =C2=A0=C2=A0=C2=A0=20
>>
>>
>> =C2=A0=20


--=20
Eli Schwartz
Bug Wrangler and Trusted User


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