FYI,

> bash -x linux-purge --auto-only --keep=2 --yes
+ set -u -e
+ export LC_ALL=C
+ LC_ALL=C
+ export TMPDIR=/dev/shm
+ TMPDIR=/dev/shm
+ readonly program_name=linux-purge 'program_full_name=Linux Purge for Ubuntu' program_version=1.0.110-d5962da copyright_years=2017-2020 MB=1000000 width=72
+ program_name=linux-purge
+ program_full_name='Linux Purge for Ubuntu'
+ program_version=1.0.110-d5962da
+ copyright_years=2017-2020
+ MB=1000000
+ width=72
++ uname -r
++ stat --format=%m /boot
+ readonly 'w_error=Terminal screen not wide enough. Minimum is 72 columns.' 'h_error=Not enough lines in terminal screen.' lockfile=/var/lib/dpkg/lock current=4.15.0-115-generic boot=/boot
+ w_error='Terminal screen not wide enough. Minimum is 72 columns.'
+ h_error='Not enough lines in terminal screen.'
+ lockfile=/var/lib/dpkg/lock
+ current=4.15.0-115-generic
+ boot=/boot
+ readonly 'cs=;'
+ cs=';'
++ mktemp /dev/shm/linux-purge.error_XXX
+ readonly error_file=/dev/shm/linux-purge.error_ZxP
+ error_file=/dev/shm/linux-purge.error_ZxP
+ keep=
+ auto_only=
+ clear_boot=
+ choose=
+ debug=
+ debug_file=
+ show_debug_filename=
+ fix=
+ info=
+ simulate=
+ yes=
+ manual=
+ legend=1
+ list=
+ optimize=
+ interface=d
+ operation=purge
+ update_grub_disabled=
+ update_grub=
+ unset -v LatestReleases ManualReleases HoldReleases
+ trap finish EXIT
+ trap 'finish INT; trap - INT; kill -s INT -- $$' INT
+ env -u GETOPT_COMPATIBLE getopt --test
+ [[ 4 -eq 4 ]]
++ env -u GETOPT_COMPATIBLE getopt -o abcd::fhi:k:lmnosrvy -l auto-only,clear-boot,choose,debug::,fix,help,info,interface:,keep:,list,manual,no-legend,optimize,remove,simulate,version,yes --name '[linux-purge] ERROR' -- --auto-only --keep=2 --yes
+ params=' --auto-only --keep '\''2'\'' --yes --'
+ eval set -- ' --auto-only --keep '\''2'\'' --yes --'
++ set -- --auto-only --keep 2 --yes --
+ :
+ case ${1-} in
+ auto_only=1
+ shift
+ :
+ case ${1-} in
+ keep=1
+ n=2
+ shift
+ shift
+ :
+ case ${1-} in
+ yes=-y
+ shift
+ :
+ case ${1-} in
+ shift
+ break
+ [[ 0 -eq 0 ]]
+ [[ -n '' ]]
+ [[ -n '' ]]
+ [[ -n 1 ]]
+ [[ 2 =~ ^[0-9]+$ ]]
+ [[ -n 1 ]]
+ [[ ! -n 1 ]]
+ [[ -n 1 ]]
+ [[ -n '' ]]
+ [[ -n '' ]]
+ [[ -n '' ]]
+ [[ -z '' ]]
+ [[ 0 -ne 0 ]]
+ [[ -n '' ]]
++ command -v update-grub
+ update_grub=/usr/sbin/update-grub
+ [[ -n /usr/sbin/update-grub ]]
+ chmod +x /usr/sbin/update-grub
+ [[ d = d ]]
+ hash dialog
+ [[ -n '' ]]
+ operate
+ [[ -n '' ]]
+ purge_normally
++ dpkg-query -W '-f=${Package} ${db:Status-Abbrev}\n'
+++ literalize 4.15.0-115-generic
+++ printf %s 4.15.0-115-generic
+++ sed -e 's/[^^]/[&]/g' -e 's/\^/\\^/g'
++ grep -qE '^linux-image-(unsigned-)?[4][.][1][5][.][0][-][1][1][5][-][g][e][n][e][r][i][c] [ih]i $'
dpkg-query: error: error writing to '<standard output>': Broken pipe
++ dpkg-query -W '-f=${Package} ${Status}\n' 'linux-*'
++ awk '$3!="ok" || ($4!="installed" && $4!="not-installed" && $4!="config-files")
	 '
+ Pkgs=
+ [[ -n '' ]]
++ dpkg-query -W '-f=${db:Status-Abbrev} ${Package}\n'
++ sed -nr 's/^[ih]i  linux-image-(unsigned-)?([0-9]+\..+)/\2/p'
++ linux-version sort
+ Releases='4.15.0-55-generic
4.15.0-106-generic
4.15.0-107-generic
4.15.0-112-generic
4.15.0-115-generic'
+ [[ -n '' ]]
+ [[ -z 1 ]]
+ ReleasesToKeep=
++ get_latest_releases
+++ get_meta_kernels
+++ dpkg-query -W -f '${db:Status-Abbrev} ${Package}\n'
+++ awk '/^[ih].. linux-(.+-)?image-/{print $2}'
+++ grep -Ev '[[:digit:]]+\.[[:digit:]]+\.[[:digit:]]+'
++ local meta_kernels=
++ [[ -z '' ]]
+ LatestReleases=
+ [[ -n '' ]]
+ error 'There are no meta-kernels so can not decide which kernels to keep.'
+ printf '[%s] ERROR: %s\n' linux-purge 'There are no meta-kernels so can not decide which kernels to keep.'
+ fold -s
[linux-purge] ERROR: There are no meta-kernels so can not decide which kernels 
to keep.
+ [[ -n '' ]]
+ echo 1
+ exit 1
+ finish
+ err=1
+ trap - EXIT
+ [[ -n 1 ]]
+ err=0
+ rm -- /dev/shm/linux-purge.error_ZxP
+ restore_update_grub
+ [[ -n '' ]]
+ [[ -n '' ]]
+ [[ null == INT ]]
+ ((  err != 0 && err != 141  ))
+ return 0