Ordinarily, a successful lookup in a
database map causes the key to be replaced by its value. When the
intention is to merely verify that the key exists (not replace it),
the -m switch can be used to suppress replacement.
For example, the values that are returned from the
hosts.byname NIS database map are not generally
useful (they contain multiple hostnames). In looking up a key in this
database map (with $( and $),
Section 23.4), the -m switch
prevents those multiple names from wrongly replacing the single
hostname in the key. Note that the -a switch
(-a) can still be used to append a suffix
to a successful lookup. Also, the
$:default (Section 23.4.1) is still used if the lookup fails.