How to fix E: Unable to lock directory /var/lib/apt/lists/ in APT

First, check if the system's automatic update daemon is running. It runs periodically and will release the lock when finished.

ps aux | grep apt

If you see apt.systemd.daily in the output, the automatic update is running. Simply wait a few minutes for it to complete, then try your command again.

If another apt process is stuck or not responding, use fuser to safely terminate it. This is the recommended approach as it's more controlled than killing processes directly.

sudo fuser -vik -TERM /var/lib/dpkg/lock /var/lib/dpkg/lock-frontend /var/lib/apt/lists/lock

This command will:
- Identify which processes are holding these lock files
- Prompt you to confirm termination
- Send a TERM signal (graceful shutdown) to those processes

After this completes, try your apt command again.

If the graceful termination doesn't work, use the -KILL signal for a forceful shutdown:

sudo fuser -vik -KILL /var/lib/dpkg/lock /var/lib/dpkg/lock-frontend /var/lib/apt/lists/lock

After terminating the processes, reconfigure the package system:

sudo dpkg --configure -a
sudo apt update

The first command reconfigures any interrupted package operations.

If the above approaches don't work and you're confident no apt process is running, you can manually remove lock files:

sudo rm /var/lib/apt/lists/lock
sudo rm /var/lib/dpkg/lock
sudo rm /var/lib/dpkg/lock-frontend
sudo rm /var/cache/apt/archives/lock

After removing the locks, reconfigure the package system:

sudo dpkg --configure -a
sudo apt update

Note: Only do this as a last resort when you're certain no package manager process is running, as manually deleting lock files can lead to package database corruption if processes are still active.

If lock files persist even after checking for running processes, a system restart will clear them:

sudo reboot

After reboot, the lock files will be cleaned up and you can run apt commands normally. This is a safe recovery option when lock files are truly stale.