systemd-analyze dot (only the top 30 units with the most appearances)

systemd-analyze-dot-only-n.rb

require "set"

unless ARGV.length == 1
  warn("usage: systemd-analyze dot | systemd-analyze-dot-only-n N")
  exit(1)
end
n = ARGV.shift.to_i(10)

txt = ARGF.read
pairs_unit_count = Hash.new(0)

txt.scan(/^\t"(.+?)"->"(.+?)" \[color="\w+"\];$/) do |left, right|
  pairs_unit_count[left] += 1
  pairs_unit_count[right] += 1
end

n_units = pairs_unit_count.sort_by { |_unit, count| -count }.first(n).map { |unit, _count| unit }.to_set

# n_units.add("postfix.service")
# n_units.delete("shutdown.target")

txt.split(/\r?\n/) do |line|
  unless line.start_with?("\t")
    puts(line)
    next
  end
  match_data = /^\t"(.+?)"->"(.+?)" \[color="\w+"\];$/.match(line)
  if match_data.nil?
    warn("unexpected line: #{line}")
    next
  end
  next unless n_units.include?(match_data[1]) && n_units.include?(match_data[2])
  puts(line)
end

systemd-analyze dot | ruby systemd-analyze-dot-only-n.rb 30 | dot -Tsvg > /tmp/systemd.svg
# with:
# n_units.add("postfix.service")
# n_units.delete("shutdown.target")