Re: document that read built-in can't return zero-length string in the middle of input

[alex xmb sw ratchev]

On Mon, Jan 15, 2024, 19:03 Chet Ramey <chet.ramey@case.edu> wrote:

> On 1/11/24 7:29 PM, Ángel wrote:
> > On 2024-01-11 at 09:29 -0500, Chet Ramey wrote:
> >> On 1/11/24 2:37 AM, ilya Basin wrote:
> >>> Dear.
> >>> I needed to read 16 bytes from a binary file and tried to replace a
> >> hexdump call with read built-in. I expected that with "-N1" if a NUL
> >> character is encountered bash would assign an empty string, however
> >> there's no indication that a NUL character was there and it simply
> >> assigns the next non-NUL character to the variable.
> >>
> >> The read builtin skips over NUL characters because you can't store them
> >> as part of the value of a shell variable. That seems obvious.
> >
> > While doing  read -N 16 from a file bigger than 16 bytes and ending up
> > with less than that (e.g. only 10 bytes because the other 6 turned out
> > to be NULs) would be surprising, the assumption of the OP that when
> > using -N1 a NUL byte would become an empty string doesn't seem far-
> > fetched.
>
> The -N option doesn't make `read' immune from the other rules it follows.
> If you run
>
> printf '\\sa\n' | { read -N 1 var ; echo "$var"; }
> or
> printf '\\\n\sa\n' | { read -N 1 var ; echo "$var"; }
>
> you'll get `s'.
>
> >> What would you like to see documented? That NUL characters don't
> >> count towards the number of characters read to satisfy -N? Doesn't
> >> that follow from the above?
> >
> > Not eccessarily. The phrase "reading exactly nchars characters" depends
> > on (a) what you consider to be reading, as you are read(2)ing more
> > bytes than stated with -N; and (b) what you consider to be a character.
>
> `read' reads multibyte characters; should -N 1 read the first byte or
> the entire character? Anyway, the description of `read -N' doesn't say
> `bytes', it says `characters'. Maybe there should be something in there
> that says -N n stops after consuming enough data to store n characters
> in the named variable.
>

maybe locale based ..

> I would tend to add a line below saying e.g. "NUL characters
> > cannot be stored in bash variables and are always ignored by read".
>
> That's reasonable, except you have to mention the -d '' case. It might
> also be worthwhile to add something to the description of shell variables
> that says they can't store NUL bytes.
>
> Chet
> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>                  ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/
>
>