The apt-get command is one of the primary package management tools on Debian-based Linux distributions. Mastering apt-get update vs apt-get upgrade is critical for maintaining systems up-to-date and secure. This extensive 2600+ word guide will cover everything from the technical internals behind these operations to best practices for hardening your systems using apt.

Understanding APT – The Advanced Package Tool

Before diving into apt-get specifically, it helps to understand what Advanced Package Tool (APT) is and how Debian-based Linux systems manage software.

APT simplified:

  • High-level package management framework for Debian, Ubuntu and related distributions
  • Consists of multiple command line tools like apt-get and graphical interfaces
  • Automates tasks like installing, upgrading, configuring, and removing packages
  • Relies on dpkg low-level package manager for core functionality

Some key ways APT enhances the underlying dpkg tool:

  • Fetches packages automatically from configured repositories
  • Resolves package dependencies when installing
  • Provides signature validation for security
  • Offers command line and visual interfaces

Under the hood, APT handles a few key areas of package management:

Package Repositories

APT relies on designated internet servers to host downloadable .deb packages, typically organized by Debian/Ubuntu release. The main configuration file /etc/apt/sources.list specifies repository locations to sync package metadata and downloads from.

Local Package Cache

APT tools all utilize /var/cache/apt/archives/ as a local store of downloaded .deb package files to avoid re-downloading, with periodic cleaning. Package indexes and metadata get cached locally under /var/lib/apt/lists.

Metadata Storage

Information about packages like versions, dependencies, descriptions get stored in the APT database at /var/lib/apt. This metadata index gets updated by apt-get update and consulted when installing/upgrading packages to determine eligibility.

Now that we‘ve covered how APT manages Debian/Ubuntu packages overall, we can better understand the specific roles of apt-get update and apt-get upgrade.

Apt-Get Update In Depth

The apt-get update command serves the specific purpose of refreshing the local APT package metadata index with latest information from all configured repositories in /etc/apt/sources.list and /etc/apt/sources.list.d.

sudo apt-get update

Let‘s explore what exactly happens during an update:

  1. APT contacts all designated repositories checking for new signed Release files – these list available packages for the repo.

  2. New Packages and Sources index files containing package metadata get downloaded – these list specific versions, dependencies etc.

  3. Package hash summaries (Contents files) get updated locally.

  4. All index data gets cached locally under /var/lib/apt/lists with old index data cleared out.

  5. The consolidated package list is analyzed against currently installed packages to determine new updates available for upgrade.

Note that apt-get update only refreshes textual metadata indexes to reflect the latest upstream package state – no actual software packages get installed during this update process.

Visually, you can conceptualize the flow like this:

apt update flowchart

Apt update flowchart by author

The update output looks like this – fetching updated metadata from several repositories:

Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]         
Hit:2 http://archive.ubuntu.com/ubuntu focal InRelease
Get:3 http://archive.ubuntu.com/ubuntu focal-updates InRelease [114 kB]                 
Get:4 http://security.ubuntu.com/ubuntu focal-security/universe amd64 Packages [905 kB]
Get:5 http://archive.ubuntu.com/ubuntu focal-backports InRelease [108 kB] 
...                 
Fetched 2558 kB in 5s (633 kB/s)
Reading package lists... Done
Building dependency tree       
Reading state information... Done
42 packages can be updated. Run ‘apt list --upgradable‘ to see them.

As we can see, all repository metadata gets updated in the background while the system processes which currently installed packages have available upgrades – but still nothing installed yet!

When Should You apt-get Update?

Here are the most common use cases for running apt-get update:

  • Before upgrading packages: Ensures latest package versions visible
  • After editing /etc/apt/sources.list: Needed to refresh with new sources
  • When changing Debian/Ubuntu release channels (stable, testing etc) to pickup different package manifests
  • Setting up new Debian/Ubuntu server: Initial sync to prime for installs
  • Troubleshooting package errors: May fix inconsistencies like missing packages

As a general rule of thumb, its good practice to update the APT package indexes from repositories before any installs or upgrades via:

sudo apt-get update

This primes the system to avoid situations like installing outdated version of packages that newer versions exist upstream for already.

I recommend configuring a daily cron job to auto-run apt-get update in the background as well for low maintenance index refreshing:

/etc/cron.daily/apt-update

#!/bin/sh
/usr/bin/apt-get update

This ensures your system stays on top of new versions without frequent manual intervention. Don‘t worry, updates themselves are very low overhead by just updating metadata indices.

Understanding Apt-Get Upgrade

While apt-get update fetches the metadata required for upgrades, the apt-get upgrade command actually installs updated package versions based on that refreshed metadata:

sudo apt-get upgrade

What apt-get upgrade does:

  • Consults the latest APT package indexes from apt-get update
  • Analyzes currently installed packages that have newer versions available
  • Calculates complete upgrade plan based on versioned dependencies between packages
  • Downloads required newer .deb archives from configured repositories
  • Calls dpkg to unpack and install each new .deb package
  • Repeats for all packages with updates available

So in summary, apt-get upgrade serves as the workhorse for actually downloading real software updates and installing them onto the operating system – powered by metadata from apt-get update.

A sample upgrade session looks like: 

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following NEW packages will be installed:
  linux-headers-5.4.0-121 linux-headers-5.4.0-121-generic
  linux-image-5.4.0-121-generic
The following packages will be upgraded:
  linux-headers-generic linux-image-generic
6 upgraded, 2 newly installed, 0 to remove and 36 not upgraded.
Need to get 48.6 MB of archives. 
After this operation, 348 MB of additional disk space will be used.
...

Here we see configuration details like packages upgraded, newly installed, additional space required all output before downloading and unpacking operations occur.

When Should You apt-get Upgrade?

It‘s important to remember upgrades should only be executed after first running apt-get update. This ensures the system knows about all eligible upgrades available.

Beyond that, it‘s good practice to periodically upgrade installed packages rather than just on an as-needed basis when problems arise. Some guidelines around scheduling upgrades:

  • Daily or weekly quick upgrades for things like security patches on servers
  • Before major apt operations like global dist-upgrade
  • On workstations, upgrade regularly based on user preference for latest software

Finding the right balance avoids both excessive upgrades when unneeded and risk of outdated packages.

Key Differences Between apt-get update vs upgrade

While it‘s easy to mix up apt-get update and apt-get upgrade based on their similar names, remembering their distinct purposes goes a long way:

Apt-Get Update Apt-Get Upgrade
Fetches package metadata/indices Installs actual package upgrades
Updates local package index Requires updated package lists from update
Non-intrusive metadata sync Can modify installed software behavior
Fast incremental updates May take time downloading/unpacking packages

The mnemonic I use is refresh knowledge, then apply knowledge. First update to refresh package indices, then upgrade to leverage that refreshed metadata to upgrade actual packages.

Always update first before upgrade!

Common Usage Combinations

Here are some common examples of using apt-get update and apt-get upgrade together for managing packages:

Update indexes then upgrade packages

sudo apt-get update
sudo apt-get upgrade

Update, upgrade, clean caches, then install new package 

sudo apt-get update
sudo apt-get upgrade
sudo apt-get clean
sudo apt-get install nginx

Update indexes and distribution upgrade together

sudo apt-get update && sudo apt-get dist-upgrade

The && operator chains the update and dist-upgrade together while ensuring update runs first.

Upgrade an individual package without full upgrade 

sudo apt-get update
sudo apt-get install python3

Targeted single package installs/upgrades can be done without upgrading everything.

Address failed package installs

sudo apt-get update
sudo apt --fix-broken install

Updates indexes then attempts to fix broken dependencies.

Best Practices and Analysis

Now that we‘ve covered the basics of using apt-get update and apt-get upgrade for package management, let‘s explore some best practices surrounding upgrades and updates from an expert perspective.

Frequency of Updates and Upgrades

Finding the right cadence between updating packaging indexes from repositories and actually upgrading installed packages boils down to balancing system stability with staying up-to-date.

For servers, I typically recommend:

  • Nightly indexing updates: apt-get update daily via cron
  • Weekly upgrades: Download patches, upgrades available each week

The nightly update allows gradual downloading of new metadata without risk of unexpected major changes. Then regular weekly upgrades apply those incremental changes.

For desktop systems, users tend to prefer latest software so can optionally upgrade more aggressively as new versions release.

Automating Updates and Upgrades

While ad-hoc manual commands work, creating crontab schedules for regular apt-get update and apt-get upgrade execution ensures consistency, reduces admin overhead, and minimizes security risk.

A sample auto-update crontab schedule:

/etc/cron.daily/apt

#!/bin/sh 

# Refresh repos nightly at 12AM 
0 0 * * * apt-get update  
30 0 * * 0 apt-get upgrade  # Weekly upgrades

This ensures nightly updates to prepare plus weekly batches of upgrade installs.

For even more control, look at dedicated APT tools like Unattended Upgrades.

Potential Issues When Neglecting Updates

Failing to regularly apt-get update package indexes and apt-get upgrade software leaves systems exposed on multiple fronts:

Security vulnerabilities: Unpatched CVEs get exploited
Buggy software: Stability/reliability regressions missed
Missing features: Can‘t leverage new versions without upgrading
Errors when installing packages: Indexes outdated

I‘ve seen neglected Debian servers with four year uptime and extremely outdated packages that experience all the above issues. Don‘t let that happen!

While upgrading does introduce some change management overhead, it pays off tremendously in security and stability.

Diagnosing and Fixing Common Apt Problems

While APT handles dependencies automatically, it‘s not entirely foolproof – packages can still get broken especially when mixing repositories like third party PPAs. Some common failure scenarios:

Could not resolve dependencies

Unable to correct problems, you have held broken packages.

Fix by updating indices then using apt --fix-broken to rebuild dependency tree.

GPG key errors

NO_PUBKEY A14703C3F756E6E7

Indicates missing repository signing key. Fetch key from keyserver and re-update.

404 Not Found errors

Repositories have moved or been retired – adjust sources list.

Outdated index issues

Failed to fetch http://old-ubuntu.com/ubuntu/dists/focal/InRelease

Repository no longer active – replace sources. Retry installs after updating indices.

Carefully reading error output provides clues on how best to resolve. Don‘t hesitate to retry installs after updating indexes with apt-get update as a first troubleshooting step.

Apt vs Aptitude vs Apt-Get

While apt-get remains the most popular tool for package management from the command line in 2023, Debian and Ubuntu systems also come with two other primary interfaces – apt and aptitude.

Here is a comparison between the major APT tools:

Tool Can Update Indices Can Upgrade Packages Additional Capabilities
apt-get Yes Yes Mature, well-tested CLI
apt Yes Yes Enhanced CLI, progress bars
aptitude Yes Yes Ncurses GUI, visualization

The apt-get command provides the underlying core functionality and stability. apt builds on it with a more modern CLI experience like multi-arch support, colorized output, and progress reporting.

And aptitude layers an ncurses graphical package manager on top of CLI apt with visual interfaces for update management and task automation.

So while all three leverage the same APT libraries, they each provide slightly different interfaces or enhancements around the underlying functionality.

For servers, apt-get tends to still be the most universal and compatible interface. But apt brings better user experience for human administrators. Either is a safe option.

Security Hardening for APT Repositories

Since APT pulls package updates directly from designated repositories, some good security practices around properly curating, maintaining, and validating those sources are:

  • Comment out deb-src entries if unused to reduce attach surface
  • Validate GPG release signing keys with packet maintainers
  • Monitor for emergency repo redirects indicating compromises
  • Limit less audited third party PPAs when possible
  • Periodically review sources list for stale/unneeded repos
  • Ensure local APT caches like /var/cache/apt have restricted permissions to protect against tampering

Proactively managing repository configurations defends against supply chain attacks through compromised packages or indices.

Conclusions and Summary

We‘ve covered a tremendous amount of ground around effectively utilizing APT apt-get update and apt-get upgrade for managing packages – from key technical details to security best practices.

To recap the key points:

apt-get update

  • Refreshes package metadata indices from repositories
  • Important before upgrading packages or installing new ones
  • Lightweight, downloads only index data for checking
  • Run at least daily either manually or via cron

apt-get upgrade

  • Actually upgrades installed packages based on refreshed metadata
  • Does not install completely new packages
  • Downloads and installs newer .deb archives
  • Ideally run at least weekly after updates

Core Differences

Apt-Get Update Apt-Get Upgrade
Updates package liste Performs actual upgrades
Prepares system for upgrades Relies on updated indices
Fast metadata syncs Intensive disk and cpu

Common Combinations

  • sudo apt update && sudo apt upgrade (update then upgrade)
  • sudo apt update && sudo apt install nginx (update before installing new package)

Best Practices

  • Always update before upgrading
  • Automate regular updates/upgrades via crontab
  • Upgrade packages before major apt operations
  • Address errors promptly and retry installs

Productive package management boils down to regularly refreshing metadata with apt-get update and selectively applying upgrades using apt-get upgrade to balance stability and currency. This guide should give you deep knowledge on using these pivotal apt-get tools effectively.

Let me know if you have any other best practices around apt-get!

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