How to fix E: Unable to write to /var/cache/apt/ in APT

The most common cause is running apt without administrative privileges. Always prefix apt commands with sudo:

sudo apt update
sudo apt install <package-name>

Apt requires root permissions to write to system directories like /var/cache/apt/.

If using sudo does not resolve the issue, check if another package manager instance is currently running:

ps aux | grep -i apt

If you see an apt or apt-get process, wait for it to complete. You can also check if the Software Update manager is open and close it before trying again.

If no apt process is running but lock files remain, remove them:

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

Then reconfigure the package database:

sudo dpkg --configure -a

If the error message includes "Read-only file system", run a filesystem check:

sudo touch /var/cache/apt/test-file

If this fails with read-only error, your filesystem needs repair. Reboot into recovery mode or use a live USB, then run:

sudo fsck -y /dev/sdXn

Replace /dev/sdXn with your actual partition. After repair, reboot and verify apt works.

If your cache is corrupted from an interrupted operation:

sudo rm -rf /var/lib/apt/lists/*
sudo mkdir -p /var/lib/apt/lists/partial
sudo apt update

This removes all cached package lists and rebuilds them from repositories.

If issues persist, verify the cache directory ownership:

ls -la /var/cache/apt/

The directory should be owned by root. If not, fix permissions:

sudo chown -R root:root /var/cache/apt/
sudo chmod -R 755 /var/cache/apt/

Then retry your apt command.