Relative times within log files

relative_time_logs

#!/usr/bin/env ruby

# Takes a log where lines have [HH:MM:SS] in front of the lines, and turns them
# into +HH:MM:SS since start of the script (ie. relative times).
#
# Any line that has no time at the start will be printed as is.
#
# Usage: relative_time_logs < some.log

time_regex = /^\[(?<hours>\d\d):(?<minutes>\d\d):(?<seconds>\d\d)\]/

STDIN.each_line.reduce(nil) do |start_time, line|
  match = line.match(time_regex)

  # If the line doesn't start with a time, just leave it alone.
  unless match
    puts line
    next start_time
  end

  hours = match[:hours].to_i
  minutes = match[:minutes].to_i
  seconds = match[:seconds].to_i

  line_time = (hours * 60 * 60) + (minutes * 60) + seconds
  start_time = line_time if start_time.nil?

  seconds_diff = line_time - start_time

  puts line.sub(time_regex, "[+#{Time.at(seconds_diff).utc.strftime('%H:%M:%S')}]")
  start_time
end