APT pinning: keeping one package on a chosen version or source with preferences files

What it is

APT assigns every candidate version of every package a numeric priority ("Pin-Priority"); when several sources offer a package, the highest-priority candidate wins. /etc/apt/preferences and fragment files under /etc/apt/preferences.d/ set explicit priorities per package, package glob or entire origin — this is what APT's own documentation calls "pinning". apt-mark hold/unhold is a separate, simpler mechanism that just excludes a package from any upgrade or dist-upgrade regardless of priority.

Why it matters

Without pinning, adding a second repository (a backports suite, a vendor PPA-equivalent, a testing pocket) can silently upgrade unrelated packages the next time apt upgrade runs, because APT prefers whichever source has the higher version by default. Pinning scopes that risk to exactly the packages an operator names.

How to apply

• Create one file per purpose under /etc/apt/preferences.d/, e.g. /etc/apt/preferences.d/pin-nginx:

  Package: nginx
  Pin: version 1.25.*
  Pin-Priority: 900
  

  A priority above 1000 would even force a downgrade; 100–500 competes with the normal candidate; below 0 prevents installation entirely.
• To prefer an entire suite (e.g. bookworm-backports) only for named packages, pin Pin: release a=bookworm-backports instead of a version.
• Inspect the effect before installing anything with apt-cache policy <package>, which the manual page says "is meant to help debug issues relating to the preferences file" and prints the priority of every available candidate.
• For a package that must never move, whatever its priority, use apt-mark hold <package> instead of a preferences file; apt-mark showhold lists everything currently held, and apt-mark unhold <package> reverses it. apt-mark itself is documented as a tool to "show, set and unset various settings for a package".
• Non-interactive use needs no special flag for either mechanism; both take effect immediately for the next apt run, no reboot or re-login required.

Pitfalls

• A preferences file with a typo'd Package: or Pin: line is silently ignored rather than rejected — always confirm with apt-cache policy after writing it.
• apt-mark hold blocks security updates for that package too; pair it with a written reminder to review the hold periodically.
• Priority pins and holds are independent: a hold survives even if a preferences file is later removed, and vice versa.