Linux Survival Basics

Check the Installed and Candidate Package Version

A package has an update available, and you need to see the installed version, candidate version, and repository source.

Command

apt policy nginx

What changed

Nothing changes. apt prints installed and candidate versions plus repository priority lines.

Danger

safe

When to use it

Use when a package version looks wrong, a security pocket matters, or a host is pinned to an older package.

When not to use it

Do not use it as proof the service is running; package state and service state are separate.

Undo or recovery

No undo needed because the command is read-only.

Expected output

Installed and candidate package versions with version table entries.

demo script

Disposable terminal steps

apt policy nginx
apt policy openssl

Terminal transcript Demo JSON

simulated output

What it looks like

disposable vessel

::fixture-ready::
$ apt policy nginx
nginx:
  Installed: 1.24.0-2ubuntu7.3
  Candidate: 1.24.0-2ubuntu7.4
  Version table:
     1.24.0-2ubuntu7.4 500
        500 http://archive.ubuntu.com/ubuntu noble-updates/main amd64 Packages
 *** 1.24.0-2ubuntu7.3 100
        100 /var/lib/dpkg/status
::exit-code::0
$ apt policy openssl
openssl:
  Installed: 3.0.13-0ubuntu3.5
  Candidate: 3.0.13-0ubuntu3.6
  Version table:
     3.0.13-0ubuntu3.6 500
        500 http://security.ubuntu.com/ubuntu noble-security/main amd64 Packages
 *** 3.0.13-0ubuntu3.5 100
        100 /var/lib/dpkg/status
::exit-code::0

YouTube Short

Find apt's candidate.

When a package update looks suspicious, apt policy shows installed version, candidate version, and the repository apt would choose.

experiments

A/B tests to run

Metric: average_view_duration

A: Where would apt get nginx?

B: Installed vs candidate version.