moar_parens.rb

#!/usr/bin/env ruby

count = ARGV[0] ? ARGV[0].to_i : 1

opens = "(" * count
txt = STDIN.read.chomp
closes = ")" * count

puts(opens + txt + closes)

# $ echo "this needs more parens" | moar_parens
# (this needs more parens)
# $ echo "this needs more parens" | moar_parens 3
# (((this needs more parens)))

This is O(n^2):

Computing opens and closes is O(n), and the concatenation is O(n^2). Fortunately, there's an easy fix, just do

print(opens) 
print(txt)
puts(closes)