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IFS= read -rd '' output < <(cmd)

LC_ALL=C IFS= read -rd '' output < <(cmd)

(LC_ALL=C needed to work around some bugs in some versions of bash).

To get the exit status of cmd, you can do wait "$!"; ret=$? in bash but not in zsh.

To get the exit status of cmd, you can do wait "$!"; ret=$? in some versions of bash but not in zsh though in zsh, you can write it cmd | IFS= read -rd '' output and get the exit status in $pipestatus[1].

An alternative is to do:

Since version 3.4.0 (released in March 2022), fish also supports $(...) which behaves like (...) except that it can also be used inside double quotes in which case it behaves like in the POSIX shell: the output is not split on lines but all trailing newline characters are removed.

Since version 3.4.0 (released in March 2022), you can do instead:

set output (cmd | string collect --allow-empty --no-trim-newlines)

With older versions, you could do:

With the caveat that $output is an empty list instead of a list with one empty element if there's no output.

Version 3.4.0 also added support for $(...) which behaves like (...) except that it can also be used inside double quotes in which case it behaves like in the POSIX shell: the output is not split on lines but all trailing newline characters are removed.