Remove old kernel versions from an Ubuntu Linux install.

removeoldkernel.py

#!/usr/bin/env python
import re
import subprocess

KERNEL_KEEP_MAX = 3
KERNEL_PACKAGE_REGEXP = re.compile(r'^[a-z]+ +(linux-image-[0-9][^ ]+) +([0-9][^ ]+[0-9])')


def get_installed_kernel_list():
	process = subprocess.Popen(['dpkg','--list'],stdout = subprocess.PIPE)

	kernel_list = []
	for line in process.stdout:
		match = KERNEL_PACKAGE_REGEXP.search(line)
		if (not match):
			continue

		kernel_list.append(match.group(1,2))

	return kernel_list

def get_sorted_kernel_list(kernel_list):
	# correctly sort a Linux kernel version string (e.g. [4.4.0-112.135])
	def sorter(a,b):
		def get_part_list(value):
			return re.split(r'[\.-]',value)

		b_list = get_part_list(b)
		for a_part in get_part_list(a):
			if (not b_list):
				# [a] greater than [b]
				return 1

			# compare parts - as integers if both are digits
			b_part = b_list.pop(0)
			result = (
				cmp(int(a_part),int(b_part))
				if (a_part.isdigit() and b_part.isdigit())
				else cmp(a_part,b_part)
			)

			if (result != 0):
				# we have a winner
				return result

		if (b_list):
			# [a] less than [b]
			return -1

		# same value
		return 0

	return sorted(
		kernel_list,
		sorter,
		key = lambda x: x[1],
		reverse = True
	)

def main():
	# fetch installed kernel list and sort
	kernel_list = get_sorted_kernel_list(
		get_installed_kernel_list()
	)

	counter = 0
	kernel_keep_list = []
	has_removed = False
	for kernel_package,kernel_version in kernel_list:
		counter += 1

		if (counter > KERNEL_KEEP_MAX):
			has_removed = True
			print('Removing kernel: {0}'.format(kernel_package))

			# remove kernel and the 'extra' package
			subprocess.call([
				'sudo','apt','purge',
				kernel_package,
				re.sub(r'^(linux-image-)',r'\1extra-',kernel_package)
			])

		else:
			kernel_keep_list.append(kernel_package)

	# display remaining kernel versions
	if (has_removed):
		print('')

	print('Remaining kernel versions:\n{0}'.format('\n'.join(kernel_keep_list)))


if (__name__ == '__main__'):
	main()