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World L2: Keys & Staff

Packages with apt

You will learn: to find, install and remove programs safely from Debian's own store.

Checked with Linux (Debian 13) on 1 Nov 2026

The problem

You want a nicer way to see folders: a program called tree. You type tree. The server says -bash: tree: command not found. What now?

Watch

You are hereyou@kedai:~$

Folder ~

  • kedai/drwxrwxr-xkitchen menu

+ 3 hidden (ls -a)

Services

  • cronrunningstarts at boot
  • sshrunningstarts at boot
  • nginxrunningstarts at boot

A package: A ready-to-install program in one box, from the Debian store. apt fetches and installs it. is a program in a box from Debian’s store. apt show tree reads the label: version, size, what it does. apt is the store clerk.

Predict

You type apt install tree without sudo. It ends with are you root?. What does that mean?

Check

Put the install routine in order.

First step at the top. Put these in order. Use the arrows, or drag.

  • apt show tree: read the label
  • sudo apt update: fetch the latest shelf list
  • tree: use it
  • sudo apt install tree: bring it in

Try it

Refresh the list, install tree, then run it: sudo apt update, then sudo apt install tree, then tree.

You are hereyou@kedai:~$

Folder ~

  • kedai/drwxrwxr-xkitchen menu

+ 3 hidden (ls -a)

Services

  • cronrunningstarts at boot
  • sshrunningstarts at boot
  • nginxrunningstarts at boot

Check

update, upgrade or remove? Pick the right one.

Drag a card into a box. Or tap the card, then tap the box.

        Check

        Safe on a server, or risky?

        Swipe right for good, left for needs work. Or use the buttons.

        1 / 4sudo apt install htop

        You did it!

        You can now find, install and remove programs safely with apt.

        Go deeper (optional)

        Real rep (optional): on a Debian or Ubuntu VM, try apt search monitor. Install security updates often: sudo apt update && sudo apt upgrade.

        Where the story breaks: real staff can say no. A process does exactly what its program says, and root can do anything. That is why keys matter so much.