It appears that the Fish shell does not have that kind of for
loop, but instead requires you to take a different approach. (The philosophy is apparently to rely on as few syntactic structures and operators as possible, and do as much with commands as possible.)
Here's how I did it, although I assume there are better ways:
for CHAR in (seq $COLUMNS)
printf $argv[1]
end
This appears inside a function, hence the $argv[1]
.