$ sudo lsof /var/lib/dpkg/lock-frontend
lsof: WARNING: can't stat() fuse.gvfsd-fuse file system /run/user/1000/gvfs
      Output information may be incomplete.
COMMAND  PID USER   FD   TYPE DEVICE SIZE/OFF     NODE NAME
apt     6675 root    4uW  REG  253,1        0 16789373 /var/lib/dpkg/lock-frontend

$ ps aux | grep -i apt
root      6673  0.0  0.0  92444  4888 ?        S    08:39   0:00 sudo apt install ubuntu-restricted-extras
root      6675  0.1  0.2 148688 89488 ?        S    08:39   0:01 apt install ubuntu-restricted-extras
root      6710  0.0  0.1  85872 55244 pts/1    Ss+  08:40   0:00 /usr/bin/dpkg --status-fd 71 --no-triggers --unpack --auto-deconfigure --recursive /tmp/apt-dpkg-install-GmUbnS
ms        9411  0.0  0.0  22664  1088 pts/2    S+   09:02   0:00 grep --color=auto -i apt