How to find the last field using 'cut'

Without using sed or awk, only cut, how do I get the last field when the number of fields are unknown or change with every line?

You could try something like this:

echo 'maps.google.com' | rev | cut -d'.' -f 1 | rev

Explanation

• rev reverses "maps.google.com" to be moc.elgoog.spam
• cut uses dot (ie '.') as the delimiter, and chooses the first field, which is moc
• lastly, we reverse it again to get com

Use a parameter expansion. This is much more efficient than any kind of external command, cut (or grep) included.

data=foo,bar,baz,qux
last=${data##*,}

See BashFAQ #100 for an introduction to native string manipulation in bash.