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Step 9 — Package Management (Ubuntu 24)

Type along exactly as shown. Focuses on APT (Debian/Ubuntu). Optional bits cover snap and flatpak.
Estimated time: ~25–30 minutes

What you’ll learn

Discover, install, remove, and upgrade software with APT
Inspect package metadata, versions, deps, and files
Roll back to a specific version, hold packages, and clean cache
Diagnose broken packages and locks; read APT/DPKG logs
(Optional) Add third‑party repositories safely with modern keyrings
(Optional) Use snap and flatpak where appropriate

Setup: Use a lab VM or a machine where you can install harmless utilities like cowsay, sl, tree. Production hosts should follow change control.

0) Refresh indexes & basic system hygiene

sudo apt update                    # refresh package lists
apt list --upgradable              # see pending upgrades (no sudo needed)
sudo apt upgrade -y                # safe upgrade of installed packages
sudo apt autoremove -y             # remove unused deps
sudo apt clean                     # drop cached .deb files

Tip: Use sudo apt full-upgrade (aka dist-upgrade) to allow kernel or dependency changes that add/remove packages.

1) Find packages

Search by name/description:

apt search htop | sed -n '1,20p'         # quick text search

Show detailed metadata:

apt show htop | sed -n '1,40p'

See available versions & repository origin:

apt policy htop

List reverse deps (who depends on whom):

apt-cache rdepends --installed bash | sed -n '1,20p'

2) Install & remove packages

Install with confirmation and see what else will be pulled in:

sudo apt install -y htop tree

Remove software but keep config files:

sudo apt remove -y tree

Remove and purge config files:

sudo apt purge -y tree

Verify installation contents

Which files did a package install?

dpkg -L htop | sed -n '1,40p'    # list files owned by installed package

Which package owns a file path?

dpkg -S /usr/bin/htop

Find package for an uninstalled file path (requires apt-file):

sudo apt install -y apt-file
sudo apt-file update
apt-file search bin/ncdu | head -10

3) Versions: pin, hold, and roll back

See available versions across repos:

apt policy openssh-server

Install a specific version:

sudo apt install openssh-server=1:9.6p1-3ubuntu13    # example; use a version shown by `apt policy`

Temporarily hold at current version (don’t upgrade):

sudo apt-mark hold openssh-server
apt-mark showhold
# later, unhold
sudo apt-mark unhold openssh-server

Advanced: pin priority (optional)

Create a pin to prefer Ubuntu main over a PPA for a package (edit safely):

sudo mkdir -p /etc/apt/preferences.d
sudo tee /etc/apt/preferences.d/openssh.pref >/dev/null <<'EOF'
Package: openssh-server
Pin: release a=noble
Pin-Priority: 600
EOF

Rules of thumb: Priority >1000 forces downgrade if needed; 500 is default; 600 prefers that source.

4) Inspect dependencies & health

Show dependencies:

apt-cache depends htop | sed -n '1,40p'

Simulate an install (no changes):

sudo apt -s install ncdu

Check package status (installed?, config state):

dpkg -s htop | sed -n '1,40p'

5) Fix common issues

Broken dependencies / interrupted dpkg

sudo dpkg --configure -a    # finish interrupted installs
sudo apt -f install         # attempt to fix broken deps

Lock is held

If another process is using APT/DPKG (GUI updater, unattended‑upgrades):

ps aux | egrep 'apt|dpkg|unattended' | egrep -v 'egrep'
# Wait for it to finish or stop the offending process if safe.

Stuck half‑installed package

sudo apt purge -y <packagename>
sudo apt install -y <packagename>

Where to read logs

sudo tail -n 100 /var/log/apt/history.log
sudo tail -n 100 /var/log/dpkg.log

6) Repositories the right way (modern keyrings) — optional

Avoid legacy apt-key. Use per‑repo keyrings and signed-by=.

Example: add a vendor repo safely (replace placeholders with a real vendor’s URLs):

sudo mkdir -p /etc/apt/keyrings
curl -fsSL https://vendor.example.com/keys/repo.gpg | \
  sudo gpg --dearmor -o /etc/apt/keyrings/vendor.gpg

# Add source list (adjust codename e.g., noble, jammy)
. /etc/os-release
echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/vendor.gpg] \
https://vendor.example.com/apt $VERSION_CODENAME main" | \
  sudo tee /etc/apt/sources.list.d/vendor.list >/dev/null

sudo apt update
apt policy vendor-package-name

Remove the repo cleanly later:

sudo rm -f /etc/apt/sources.list.d/vendor.list /etc/apt/keyrings/vendor.gpg
sudo apt update

PPAs (Launchpad) — optional

sudo apt install -y software-properties-common
sudo add-apt-repository -y ppa:graphics-drivers/ppa
sudo apt update

Remove:

sudo add-apt-repository -r ppa:graphics-drivers/ppa
sudo apt update

7) Kernel & reboot awareness

List kernels and check if a reboot is pending:

dpkg -l 'linux-image*' | sed -n '1,20p'
[ -f /var/run/reboot-required ] && echo 'Reboot required' || echo 'No reboot pending'

8) Snap & Flatpak (optional)

Snap

snap list
snap find jq | head -10
sudo snap install jq
sudo snap remove jq

Flatpak

sudo apt install -y flatpak
sudo flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak search org.gnome.Calculator | head -5
sudo flatpak install -y flathub org.gnome.Calculator
flatpak list
sudo flatpak uninstall -y org.gnome.Calculator

When to use: Prefer APT for system packages; snap/flatpak for sandboxed desktop apps or when newer versions are only available there.

9) Cleanup & space management

sudo apt autoremove --purge -y
sudo apt clean
sudo du -h /var/cache/apt/archives | tail -1

10) Practice tasks (do these now)

Search for ncdu, inspect details (apt show), and install it.
Use dpkg -L ncdu to list files; find which package owns /usr/bin/ncdu (dpkg -S).
Install jq at a specific version if multiple are available (apt policy, then apt install jq=…).
Hold jq, show holds, then unhold it.
Install apt-file, search which package provides bin/htop.
(Optional) Add a PPA, install a package from it, then remove the PPA and revert using pins or by installing the Ubuntu version.
Read the last 50 lines of both APT and DPKG logs.

11) Troubleshooting quick guide

Broken deps → sudo dpkg --configure -a, sudo apt -f install, then inspect /var/log/apt/history.log.
Held or pinned versions blocking upgrades → check apt-mark showhold, and files under /etc/apt/preferences.d.
Repo GPG errors → ensure you used keyrings + signed-by=…; check permissions on /etc/apt/keyrings/*.gpg (0644).
“Package not found” → verify apt update, correct codename in source (noble, jammy), and architecture matches.
Lock files busy → another process is running (unattended-upgrades, GUI), or left stale lock after crash. Confirm with ps and wait/kill cautiously.

12) Quick quiz (1 minute)

Which command shows all available versions for a package and where they come from?
What’s the difference between remove and purge?
How do you prevent a package from upgrading?
Which logs help you reconstruct what APT did recently?
Why is apt-key deprecated and what should you use instead?

Answers: apt policy; remove keeps config files, purge deletes them; apt-mark hold (and pins for repo preference); /var/log/apt/history.log and /var/log/dpkg.log; use per‑repo keyrings with signed-by=.

Next Step

Proceed to Step 10 — Users & Authentication (local users, groups, passwords, SSH basics). If your curriculum orders differ, update the previous step’s “Next Step” pointers accordingly.

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