A command enclosed in parentheses preceded by a dollar sign, like
‘$(...)’, or quoted with grave
accents, like ‘`...`’, is replaced with its standard output, with
any trailing newlines deleted.
If the substitution is not enclosed in double quotes, the
output is broken into words using the IFS parameter.
The substitution ‘$(cat foo)’ may be replaced
by the faster ‘$(<foo)’. In this case foo
undergoes single word shell expansions (parameter expansion,
command substitution and arithmetic expansion), but not
filename generation.
If the option GLOB_SUBST is set, the result of any unquoted command
substitution, including the special form just mentioned, is eligible for
filename generation.