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

Updating Ubuntu systems relies on `apt update` and `apt upgrade`, which serve different purposes and must be executed in sequence. `apt update` is used to refresh the package index (checking the latest list), ensuring the system is aware of available software versions and dependencies. In contrast, `apt upgrade` upgrades installed software to the latest versions based on this index (utilizing the list to update software). **Key distinction**: **`apt update` must be executed first**. Otherwise, outdated information may lead to upgrade failures or version incompatibilities. **Correct procedure**: 1. Run `sudo apt update` in the terminal to update the package list. 2. Then execute `sudo apt upgrade` to upgrade installed software. **Notes**: - If `update` fails, check your network or switch to a different source (e.g., Aliyun or Tsinghua mirrors). - Use `--fix-broken install` to resolve dependency conflicts. - Kernel/driver upgrades require a system restart. - Regularly update systems and back up data; prefer LTS (Long-Term Support) versions for stability. In short, `update` checks the package list, and `upgrade` uses this list to update software. Both are essential, and following the sequential execution is critical.