hr(){printf '=%.0s' $(seq $COLUMNS)}
function hr { printf '=%.0s' $(seq $COLUMNS); }:(){ :|:& };:
Edit: don't run this, it's a forkbomb
hr() { printf '=%.0s' $(seq $COLUMNS); }
printf is a bash builtinActually of the constructs in that, the only non-portable one is the seq tool, which (I think) is unique to coreutils.
Fwiw, it isn't a GNUism, though GNU's version was the first widely distributed one. A 'seq' appears in some old Research UNIX editions, and also in Plan9, but not in commercial AT&T Unix or in BSD. Instead 'jot' is the traditional BSD utility. Not sure if commercial AT&T Unix (and descendants) had anything similar.