$(cmd) is unquoted, that's a split+glob operator. The shell retrieves the output of
cmd, removes all the trailing newline characters, then splits that based on the value of the
$IFS special parameter, and then performs filename generation (for instance turns
*.txt into the list of non-hidden txt files in the current directory) on the resulting words (that latter part not with
zsh) and in the case of
ksh also performs brace expansion (turns
ac for instance).
The default value of
$IFS contains the SPC, TAB and NL characters (also NUL in
zsh, other shells either remove the NULs or choke on them). Those (not NUL) also happen to be IFS-whitespace characters¹, which are treated specially when it comes to IFS-splitting.
If the output of
" a b\nc \n", that split+glob operator will generate a
"c" arguments to
./input. With IFS-white-space characters, it's impossible for
split+glob to generate an empty argument because sequences of one or more IFS-whitespace characters are treated as one delimiter. To generate an empty argument, you'd need to choose a separator that is not an IFS-whitespace character. Actually, any non-whitespace character will do (best to also avoid multi-byte characters which are not supported by all shells here).
So for instance if you do:
IFS=: # split on ":" which is not an IFS-whitespace character
set -o noglob # disable globbing (also brace expansion in ksh)
a::b\n, then that split+glob operator will result in
"b" arguments (note that the
"s are not part of the value, I'm just using them here to help show the values).
a:b:\n, depending on the shell, that will result in
"". You can make it consistent across all shells with
(which also means that for an empty output of
cmd (or an output consisting only of newline characters),
./input will receive one empty argument as opposed to no argument at all).
printf 'a b:: c\n'
printf 'I got %d arguments:\n' "$#"
[ "$#" -eq 0 ] || printf ' - <%s>\n' "$@"
set -o noglob
I got 3 arguments:
- <a b>
- < c>
Also note that when you do:
" are part of the shell syntax, they are shell quoting operators. Those
" characters are not passed to
¹ IFS whitespace characters, per POSIX being the characters classified as
[:space:] in the locale and that happen to be in
$IFS though in ksh88 (on which the POSIX specification is based) and in most shells, that's still limited to SPC, TAB and NL. The only POSIX compliant shell in that regard I found was
bash (since 5.0) also include other whitespace (such as CR, FF, VT...), but limited to the single-byte ones (beware on some systems like Solaris, that includes the non-breaking-space which is single byte in some locales)