Essential for System Updates: The Difference Between `apt update` and `upgrade` in Ubuntu

Updating software on Ubuntu is a crucial step to keep your system secure and performant. While apt update and apt upgrade are the most commonly used commands, many beginners confuse their roles. Today, we’ll break down their differences and correct usage in the simplest way possible.

Why Update the System?¶

Think of your phone apps: if they’re outdated for too long, they might have vulnerabilities or missing features. The same applies to your computer: updates fix security flaws (e.g., patching bugs hackers could exploit), add new functionality (e.g., software shortcuts), and improve system stability.

`apt update`: Update the Software Package “Latest List”¶

The core purpose of apt update is to refresh the package index. The “index” is like a system-wide “software manual” that lists each package’s name, version, size, and dependencies.

For example, Ubuntu’s official repositories maintain this “manual.” When you run apt update, your system downloads the latest version of these indexes from the source servers, ensuring you know:
- Which software has new versions available?
- What are the version numbers and update details of these new versions?
- What dependencies are required to install the new versions?

Example: If you want to install a new app but can’t find the latest version, apt update is like checking the app store’s “new arrivals list” first—so you know which apps to upgrade later.

`apt upgrade`: Actually Upgrade Installed Software¶

The core purpose of apt upgrade is to upgrade all installed packages to their latest versions. It uses the “latest list” from apt update to check your system’s current software and upgrade them to the most stable versions available.

Example: If your system has an old text editor (version 1.0), and the latest index shows version 2.0 is available, apt upgrade will replace the old 1.0 with the new 2.0.

Key Difference: “Check the List” First, Then “Upgrade Software”¶

Command	Function	Analogy	Execution Dependency
`apt update`	Update the “latest package list”	Checking the latest tracking number on a logistics website	Must run first
`apt upgrade`	Upgrade installed software to the latest version	Upgrading an old package to a new one	Depends on `apt update` results

Simple Summary: apt update is like “checking the latest news,” while apt upgrade is “using that news to update software.” Skipping apt update and running apt upgrade directly may cause failures or upgrade to incompatible versions due to outdated information.

Correct Usage Steps: Update First, Then Upgrade¶

Update the package list:
Open the terminal and run:

   sudo apt update

(sudo requires admin privileges; you’ll be prompted for your password.)
After execution, the system will download the latest package indexes, and you’ll see messages like “XX sources updated.”

Upgrade installed software:
Once the package list is updated, run:

   sudo apt upgrade

The system will list all packages ready for upgrade. Type y to confirm, then wait for the upgrade to complete.

Common Issues & Notes¶

What if apt update fails?
- Check your internet connection (e.g., Wi-Fi is disconnected).
- If sources are temporarily unavailable, switch to a domestic mirror (e.g., Alibaba Cloud, Tsinghua Source) by editing the /etc/apt/sources.list file.
What if apt upgrade freezes or errors?
- This may be due to dependency conflicts. Run:

     sudo apt --fix-broken install

 This command automatically resolves dependency issues.

Do I need to restart after upgrading?
- Usually not necessary, but if the kernel (system core) or graphics drivers are updated, the system will prompt for a restart—follow the on-screen instructions.
Can I skip update and upgrade directly?
- Not recommended! This may cause version conflicts or software failures.

Final Reminders¶

System updates are beneficial, but:
- Non-LTS versions (e.g., Ubuntu 22.10) may have compatibility issues. Prioritize LTS versions (e.g., 22.04 LTS).
- Back up important data (e.g., documents, photos) before upgrading to avoid accidents.

By understanding the difference between apt update and apt upgrade, you’ll never feel confused about system updates again! For stability, make these commands part of your regular maintenance routine to keep Ubuntu running “up-to-date” at all times.