Managing Software Repositories

# View all configured apt sources
cat /etc/apt/sources.list
ls -la /etc/apt/sources.list.d/

# View the Ubuntu/Debian default sources
grep -v "^#" /etc/apt/sources.list | grep -v "^$"

# Add a third-party repository the safe modern way (Docker as example)
# Step 1: Create the keyrings directory
sudo mkdir -p /etc/apt/keyrings

# Step 2: Download and store the GPG key
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | \
  sudo gpg --dearmor -o /etc/apt/keyrings/docker.gpg
sudo chmod a+r /etc/apt/keyrings/docker.gpg

# Step 3: Add the repository source file
echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.gpg] \
  https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" | \
  sudo tee /etc/apt/sources.list.d/docker.list

# Step 4: Update and install
sudo apt update
sudo apt install docker-ce -y

# Remove a repository
sudo rm /etc/apt/sources.list.d/docker.list
sudo apt update

# /etc/yum.repos.d/myapp.repo — a typical .repo file structure

[myapp-stable]
name=MyApp Stable Repository
baseurl=https://repo.myapp.example.com/stable/el9/$basearch/
enabled=1
gpgcheck=1
gpgkey=https://repo.myapp.example.com/RPM-GPG-KEY-myapp
repo_gpgcheck=1
metadata_expire=86400

[myapp-testing]
name=MyApp Testing Repository
baseurl=https://repo.myapp.example.com/testing/el9/$basearch/
enabled=0
gpgcheck=1
gpgkey=https://repo.myapp.example.com/RPM-GPG-KEY-myapp

# List all configured repositories
dnf repolist all

# List only enabled repositories
dnf repolist

# Show detailed info about a specific repo
dnf repoinfo baseos

# Enable or disable a repository temporarily (for one command)
dnf install --enablerepo=epel-testing mypackage
dnf install --disablerepo=appstream mypackage

# Enable or disable a repository permanently
sudo dnf config-manager --enable epel
sudo dnf config-manager --disable myapp-testing

# Add a repository from a URL (downloads the .repo file automatically)
sudo dnf config-manager --add-repo https://repo.example.com/example.repo

# Add EPEL (Extra Packages for Enterprise Linux) — the most common third-party repo
sudo dnf install epel-release -y
sudo dnf repolist | grep epel

# View all keys stored in the apt keyring directory
ls -la /etc/apt/keyrings/

# Inspect a specific key file — shows fingerprint, UID, expiry
gpg --show-keys /etc/apt/keyrings/docker.gpg

# View keys in the legacy apt trusted keyring (pre-Ubuntu 22.04 style)
sudo apt-key list    # deprecated but still shows legacy keys

# Import a key for RHEL/Rocky (RPM format)
sudo rpm --import https://repo.example.com/RPM-GPG-KEY-example

# List all imported RPM keys
rpm -q gpg-pubkey --qf '%{NAME}-%{VERSION}-%{RELEASE}\t%{SUMMARY}\n'

# Remove an RPM key
rpm -q gpg-pubkey --qf '%{NAME}-%{VERSION}-%{RELEASE}\n' | grep old-vendor
sudo rpm -e gpg-pubkey-XXXXXXXX-XXXXXXXX

# ── Ubuntu PPAs ───────────────────────────────────────────────────

# Add a PPA (automatically handles key import and sources.list.d entry)
sudo add-apt-repository ppa:ondrej/nginx
sudo apt update

# List all configured PPAs
grep -r "ppa:" /etc/apt/sources.list.d/

# Remove a PPA and downgrade packages to the distro version
sudo add-apt-repository --remove ppa:ondrej/nginx
sudo apt install ppa-purge
sudo ppa-purge ppa:ondrej/nginx

# ── RHEL / Rocky EPEL ─────────────────────────────────────────────

# Install EPEL release package (adds repo config and GPG key in one step)
sudo dnf install epel-release -y

# For RHEL (not Rocky) — enable the CodeReady Linux Builder repo first
sudo subscription-manager repos --enable codeready-builder-for-rhel-9-x86_64-rpms

# Verify EPEL packages available
dnf repolist epel
dnf search htop

# Install a package from EPEL explicitly
sudo dnf install --enablerepo=epel htop -y

# Diagnose which repositories provide a specific package (apt)
apt-cache policy nginx

# Show all versions of a package available across all repos (apt)
apt-cache showpkg nginx

# Find which repo installed the currently-installed version (dnf)
dnf info nginx

# Check for duplicate packages across repos (dnf)
dnf --showduplicates list nginx

# Audit all repository source files for unexpected entries
grep -r "^deb" /etc/apt/sources.list /etc/apt/sources.list.d/ 2>/dev/null

# Full repo audit on RHEL — list all repos with their URLs
dnf repolist -v | grep -E "Repo-id|Repo-baseurl"

# Force refresh of all repository metadata caches
sudo apt update --allow-releaseinfo-change   # apt
sudo dnf makecache --refresh                  # dnf