Command substitution is indicated by a command enclosed in ``'. The output
from such a command is broken into separate words at blanks, tabs and newlines,
and null words are discarded. The output is variable and command substituted
and put in place of the original string.
Command substitutions inside double
quotes (`"') retain blanks and tabs; only newlines force new words. The single
final newline does not force a new word in any case. It is thus possible for a
command substitution to yield only part of a word, even if the command outputs
a complete line.