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Debian

apt

Install & remove

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apt install <package>
apt install <package>=<version>    # pin to a specific version
apt install --no-install-recommends <package>

apt remove <package>               # remove but keep config files
apt purge <package>                # remove + config files
apt autoremove                     # remove unused dependencies

Update & upgrade

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apt update                         # refresh package index
apt upgrade                        # upgrade installed packages (no removals)
apt full-upgrade                   # upgrade + allow removals (dist-upgrade alias)

Search & inspect

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apt search <keyword>               # search package names/descriptions
apt show <package>                 # full package metadata
apt list --installed               # all installed packages
apt list --installed | grep <name> # filter installed
apt list --upgradable              # packages with available upgrades

Package cache

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apt-cache showpkg <package>        # show package versions and deps
apt-cache depends <package>        # direct dependencies
apt-cache rdepends <package>       # reverse dependencies (who depends on it)
apt-cache policy <package>         # installed vs candidate version, pinning

dpkg

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# Install a local .deb
dpkg -i package.deb

# List all installed packages
dpkg -l
dpkg -l | grep <name>

# List files installed by a package
dpkg -L <package>

# Which package owns a file
dpkg -S /usr/bin/cwebp
# webp: /usr/bin/cwebp

# Package status / info
dpkg -s <package>

# Reconfigure an installed package
dpkg-reconfigure <package>

Repositories

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# Sources list
/etc/apt/sources.list
/etc/apt/sources.list.d/*.list      # drop-in files (preferred)

# Format (old style)
deb http://deb.debian.org/debian bookworm main contrib non-free non-free-firmware
deb-src http://deb.debian.org/debian bookworm main

# Add a repo key (modern way)
curl -fsSL https://example.com/repo.gpg | gpg --dearmor \
    -o /usr/share/keyrings/example.gpg

# Reference the key in sources.list.d
echo "deb [signed-by=/usr/share/keyrings/example.gpg] \
    https://example.com/debian stable main" \
    > /etc/apt/sources.list.d/example.list

apt update

Package pinning

Prioritize or hold back packages via /etc/apt/preferences.d/.

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# /etc/apt/preferences.d/hold-nginx
Package: nginx
Pin: version 1.22.*
Pin-Priority: 1001

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# Hold a package at current version (simpler)
apt-mark hold <package>
apt-mark unhold <package>
apt-mark showhold               # list held packages

System info

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# Debian version
cat /etc/debian_version           # e.g. 12.5
lsb_release -a

# Which codename maps to what
# bookworm = Debian 12  |  bullseye = 11  |  buster = 10

# Kernel
uname -r

Useful packages

Package Provides
procps ps, top, free, vmstat
net-tools netstat, ifconfig
iproute2 ip, ss
dnsutils dig, nslookup
curl / wget HTTP clients
build-essential gcc, make, g++
lsof list open files/sockets
strace trace system calls
htop interactive process viewer
jq JSON processor