`read` with IFS empty was expected to set all parameters after the first
n filled variables to an empty string, but that was inconsistent with
the behavior of `read` everywhere else.
I'm not sure why fish differed from the spec with regards to the
behavior in the event of an empty IFS: we eschew IFS where possible, yet
here we adopt non-standard behavior splitting on every (unicode)
character instead of not splitting at all with IFS empty. We still do
that, but now the unset variables are treated as they normally would be,
i.e. cleared and not set to an empty string (which is what an empty
value between two IFS separators would contain).
This takes a string that is then split upon like `string split`.
Unlike $IFS, the string is used as one piece, not a set of characters.
There is still a fallback to IFS if no delimiter is given, that
behaves exactly as before.
Fixes#4156.
read_in_chunks does not clear the intermediate string 'str'
between iterations, so every chunk has every other chunk prepended
to it.
A secondary issue is that it calls str2wcstring() on an intermediate
chunk, which may split multi-byte sequences. This needs to be deferred
to the end.
Test added. Fixes#3756
The `--null` flag to `read` makes it split incoming lines on NUL instead
of newlines. This is intended for processing the output of a command
that uses NUL separators (such as `find -print0`).
Fixes#1694.
When $IFS is empty, command substitution no longer splits on newlines.
However we still want to trim off a single trailing newline, as most
commands will emit a trailing newline and it makes it harder to work
with their output.
Enhance the `read` builtin to support creating an array with the --array
flag. With --array, only a single variable name is allowed and the
entire input is tokenized and placed into that variable as an array.
Also add custom behavior if IFS is empty or unset. In that event, split
the input on every character, instead of the previous behavior of doing
no splitting at all.
