inspect a docker image's commands without having its dockerfile

ceto.rb

#!/usr/bin/env ruby

# ceto
# from cetology: the scientific study of whales
# inspect a docker image's commands without having its dockerfile

# bash scripting stuff that probably needs to be broken up for visibility
SPECIALS = %w(
  $\(
  \)
  |
  if
  then
  else
  fi
)

def cleanup(c)
  c.gsub!(/^\/bin\/sh -c #\(nop\)\s+/, '')
  c.gsub!(/^(\|\d+ DOCKER_GROUP=\d+ )?\/bin\/sh -c/, 'RUN')
  c.gsub!('&&', "&& \\\n")
  c.gsub!(';', "; \\\n")
  c.gsub!(/!\n\s\s+/, " \\\n")
  SPECIALS.each { |s| c.gsub! s, "#{s} \\\n" }
  c
end

id = ARGV.first
if !id || (%w(-h --help).include? id)
  print <<~MSG
    Usage: ceto <docker_image_id>
  MSG
  exit 1
end

raw = `docker history --format '{{.CreatedBy}}' --no-trunc #{id}`
raw_cmds = raw.split("\n").reverse
clean_cmds = raw_cmds.map { |c| cleanup c }
puts clean_cmds