text = `ps -eo comm,rss`
#split the input to two colums
table=text.lines.collect {|l|
        [l.split[0..-2].join ,l.split[-1].to_i/1024]}

#summing rows with same progname
sum=Hash.new(0)
table.each {|i| sum[i[0]]+=i[1]}

# sort decrasing on second field (memory use)
table=sum.to_a.sort{|x,y| y[1]<=>x[1]}

#format and print output
puts table[0..4].inject("Memory usage (MB):\n"){|res,i|
         res+= i[0]+" ("+i[1].to_s+") "
}.chop