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

[Chet Ramey]

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.

> 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/

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