text = `ps -eo comm,rss`

#split the input into two colums: [progname,memusage]
table=text.lines.collect {|l|  [l.split[0..-2].join ,l.split[-1].to_i/1024]}

# sort decrasing on the second field (memory usage)
table.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+")"+" "
}