Isso parece um truque de mingw:
$ test=$(ls | wc -l); echo test="$test"
test=45
$ bash --version
GNU bash, version 4.3.43(1)-release (x86_64-redhat-linux-gnu)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.