apt-cache: Usage, Options & Common CI Errors

apt-cache inspects the package index - search, show details, and check available versions.

apt-cache is the read-only counterpart to apt-get: it answers "does this package exist, what version, and from which repo?" In CI it is invaluable for debugging why an install picks the wrong version.

What it does

apt-cache queries the local APT index (populated by apt-get update). It does not touch the network or install anything. apt-cache policy shows which version would be installed and the priority of each source; apt-cache madison lists all available versions of a package.

Common usage

apt-cache search nginx
apt-cache show nginx
apt-cache policy nginx              # candidate version + sources
apt-cache madison docker-ce        # all versions, for pinning
apt-cache depends curl

Common errors in CI

Empty output from apt-cache search or "Unable to locate package" downstream usually means apt-get update has not run, so the index is empty. apt-cache policy showing "Candidate: (none)" means no source provides an installable version for your release - you likely need to add a repo or PPA. When an install grabs an unexpected version, apt-cache policy reveals the pin priorities causing it. Note apt (not apt-get) prints a "this CLI is unstable for scripts" warning; use apt-cache/apt-get in scripts.

Options

Related guides