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Re: Not missing, but very hard to see (was Re: Backslash missing in brac

From: Eli Schwartz
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
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0

On 12/12/19 9:57 PM, L A Walsh wrote:
> On 2019/12/12 13:01, Ilkka Virta wrote:
>> On 12.12. 21:43, L A Walsh wrote:
>>> On 2019/12/06 14:14, Chet Ramey wrote:
>>> Seems very hard to print out that backquote though.  Closest I got
>>> was bash converting it to "''":
>> 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.
> -----
>    I'm sorry, but you are mistaken.

How so?

>    The characters from 'Z' (0x5A) through 'z' (0x61) are:
> 0x5A 0x5B 0x5C 0x5D 0x5E 0x5F 0x60 0x61
> Z    [    \    ]     ^   _     `    a
> the backslash comes between the two square brackets.
> Position [6] is the "Grave Accent" (or backquote).
> It is quoted properly.

But... that's exactly what was said.

> As for %q printing an empty string for 0x5C
>         "%q" causes  printf to output the corresponding argument in a
>         format that can be reused as shell input.
>    For that string to be empty would mean there is no character 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?

>>>>  read -r -a a< <(printf "%q " {Z..a})
>>>>  my -p a
>>> declare -a a=([0]="Z" [1]="\\[" [2]="''" [3]="\\]" [4]="\\^" [5]="_"
>>> [6]="\\\`" [7]="a")

Eli Schwartz
Bug Wrangler and Trusted User

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