How to fix apt-get: relocation error: /usr/lib/x86_64-linux-gnu/libapt-pkg.so.6.0: symbol _ZN3apt3deb5HeaderC1EPKc version APTPKG_6.0 not defined in file libapt-pkg.so.6.0 with link time reference in APT

First, check which repositories are enabled and look for anything from non-standard sources:

cat /etc/apt/sources.list
ls -la /etc/apt/sources.list.d/

Look for any PPAs or third-party repositories that might provide their own apt packages. If you find suspicious entries, disable them:

# Comment out problematic lines in /etc/apt/sources.list
sudo nano /etc/apt/sources.list

# Or remove third-party repo files
sudo rm /etc/apt/sources.list.d/problematic-repo.list

Only keep the official Ubuntu or Debian repositories for your release.

Attempt to repair broken packages using dpkg. This can work even when apt is broken:

sudo dpkg --configure -a

This tells dpkg to complete any partial installations. If this succeeds, try:

sudo apt --fix-broken install

If apt still doesn't work, proceed to the next step.

If dpkg alone doesn't fix it, manually reinstall the apt packages. First, try to download the package files from the Ubuntu/Debian mirror:

For Ubuntu/Debian online systems:

# Try to download and install directly
sudo apt-get install --reinstall apt libapt-pkg6.0

If that fails (because apt doesn't work), use dpkg directly:

# Check your system's release
lsb_release -cs

Then download the matching packages from https://packages.ubuntu.com or https://packages.debian.org and install with dpkg:

# Example for Ubuntu 22.04 (jammy)
cd /tmp
wget http://security.ubuntu.com/ubuntu/pool/main/a/apt/apt_2.4.14_amd64.deb
wget http://security.ubuntu.com/ubuntu/pool/main/a/apt/libapt-pkg6.0_2.4.14_amd64.deb

# Install using dpkg
sudo dpkg -i ./apt_2.4.14_amd64.deb ./libapt-pkg6.0_2.4.14_amd64.deb

Replace the version numbers and URLs with those appropriate for your distribution release.

If dpkg reports conflicts during installation, use the auto-deconfigure flag:

sudo dpkg -i --auto-deconfigure apt_*.deb libapt-pkg6.0_*.deb

This tells dpkg to automatically deconfigure conflicting packages first, then install the new versions. This can help resolve dependency entanglement.

If your system was corrupted by a problematic PPA, prevent it from causing issues again:

# If the PPA package is still installed
sudo apt-get remove ppa-purge
sudo ppa-purge ppa:problematic/repo

# Or manually remove the repo
sudo rm /etc/apt/sources.list.d/problematic-repo.list

# Clean APT cache
sudo apt-get clean
sudo apt-get autoclean

After cleaning, verify your sources list contains only official repositories:

cat /etc/apt/sources.list
ls /etc/apt/sources.list.d/

Then try apt again:

sudo apt-get update

Once apt is working again, bring your system fully up to date:

sudo apt-get update
sudo apt-get upgrade

If you want a more aggressive upgrade (useful after fixing from a corrupted state):

sudo apt-get dist-upgrade

This ensures all packages are at compatible versions. After the upgrade, verify everything works:

apt --version
apt-cache search test
sudo apt-get install --dry-run vim  # Test without installing

If none of the above works and your system is unbootable, use a live USB:

1. Download Ubuntu or Debian live ISO matching your original release
2. Boot from USB: insert USB and restart, then select boot from USB
3. Once in the live environment, open a terminal

4. Mount your root filesystem:

sudo mount /dev/sdXX /mnt  # Replace sdXX with your root partition
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys
sudo mount --bind /dev /mnt/dev

5. Chroot into your system:

sudo chroot /mnt /bin/bash

6. Once inside, repeat steps 1-5 from this guide:

dpkg --configure -a
apt --fix-broken install
apt-get install --reinstall apt libapt-pkg6.0

7. Exit chroot and reboot:

exit
sudo reboot

This approach works when your main system cannot boot due to apt corruption.