The Linux Documentation Project & Linux man pages online.
Shell Check - Find bugs in your shell scripts.
commandlinefu - command-line gems.
Google Shell Style guide - Short, sensible advice on coding style.
The Linux Cookbook - Tips and Techniques.
Linux Shell Scripting Tutorial - LSST.
Administration shell scripts - Dawid Michalczyk.
grymoire.com - home for UNIX wizards.
Introduction to text manipulation on UNIX-based systems.
LWN - Linux Weekly News.
TCP and UDP port numbers - Wikipedia.
The Tragedy of systemd - Benno Rice at linux.conf.au
Explain Shell - Enter an Ubuntu (bash) command-line to see the help text that matches each argument.
Unix/Linux/BSD Toolbox - Concise summary of commands.
FreeBSD Man pages
Greg Wooledge Bash Guide and FAQ
Bash Guide for beginners - Machtelt Garrels.
GNU Bash Manual - gzip archives.
Documentation and Examples - bash cook book.
bash tips - Configure the bash terminal.
Bash pitfalls - GreyCat's wiki.
Hyperpolyglot - Comparison of bash vs cmd vs PowerShell.
Introduction to the Ubuntu terminal
Shell Check - Static analysis tool - alerts for many common beginner's syntax errors.
Dell - Linux workstations and Laptops.
System76 - Ubuntu Linux workstations, servers and Laptops.
UbuntuForums - HowTo guides and forum.
comp.unix.shell - Shell scripting.
comp.os.linux - General linux.
SS64 Discussion Forum
Bash downloads - Latest versions and patches.
Distrowatch - Compare major distributions.
Elementary.io - macOS clone.
Linux.Dell.com (Red Hat)
Linux Mint - Multimedia out of the box - based on Debian/Ubuntu - xfce is a lightweight desktop, good for older hardware.
SUSE Linux Enterprise Server
Open Suse Tumbleweed (rolling distribution) or Leap (stable release model).
Live CDs & Wikipedia Live CDs
Ubuntu & Gnome
st - simple terminal
A comparison of Terminal emulators - lwn.net
Most (if not all) of these applications should be available to install via your package manager e.g. Apt-Get or Aptitude.
Beyond grep - A grep-like tool, aimed at programmers.
Asunder - Audio CD ripper and encoder for Linux.
Audacity / Tenacity - CD Ripper, Sound editor.
AutoFsck - Configure boot disc checks (Ubuntu).
Beyond Compare - Directory and file comparison utility (cross platform).
cabextract - Free Software for extracting Microsoft cabinet files.
CalcPad - Calculator for Linux (clone of Numi MacOS) .
Clonezilla - Partition and disk imaging/cloning program.
CSV kit - Utilities for working with CSV files.
Dar - Disk Archive/backup.
Duf - Disk Usage/Free Utility.
Easy Tag - Tag editor for MP3, Ogg Vorbis files.
FreeCAD - Sketch 2D shapes and create high quality drawings / 3D models.
GoAccess - Web Log file analyzer.
GNU Parted | Partimage - Partition managers.
Gftp multithreaded ftp
HTML Tidy - Check and fix HTML and XML files.
htop - Interactive process viewer.
i3 window manager
Inkscape - Vector Graphics (Illustrator).
Insync - Google Drive integration.
ImageMagick - Edit/Convert bitmap images(PNG, JPEG, GIF) on the command line (scripts).
k3b.org - The CD/DVD Kreator.
KDE - Plasma desktop, Krita artwork, Kontact email/calendar.
KNode newsreader - for K desktop.
KVM - Virtualization software (Kernel-based Virtual Machine.)
K3b - CD/DVD Kreator for Linux - optimized for KDE.
Micro - Terminal-based text editor that aims to be easy to use and intuitive.
Midnight Commander - File Manager.
MPV - Media player (fork of mplayer).
Nautilus/Files - Simple file manager for GNOME
Network Monitoring Tools - SLAC.
OBS Studio - Video recording and live streaming (Free, open source).
Partition Image - re-image OS.
PhotoRec - Data recovery software - hard disk, CD-ROM & Memory Cards.
PDFtk - Command-line tool for processing PDFs.
PostgreSQL - Database.
Performance monitoring tools: htop, iftop
rDesktop - RDP client for Windows (Terminal Services).
Sectools.org - Top 100 Network Security Tools.
ShellCheck - Shell script static analysis tool.
Shotcutapp - Video editor, free, open source.
TeamViewer - Screen sharing/Remote control (Mac/PC/Linux).
Terminator - Gnome Terminal management.
Textadept - Text editor.
thttpd - HTTP Web Server.
Unite - GNOME Shell extension to provide a Ubuntu Unity Shell look/feel.
VLC Media Player
WebP utilities - Encode/decode WebP animated images, Google.
Wink - Capture screenshots/video, add help text.
Xfce - Lightweight window manager.
GNU FTP Sites - GNU programs and Manuals.
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UNIX and Linux System Administration Handbook 5th Edition, Aug 2017 by Evi Nemeth, Garth Snyder, Trent R. Hein, Ben Whaley
The latest edition of the world’s best-selling UNIX system administration book has been made even better by adding coverage of the leading Linux distributions: Ubuntu, openSUSE, and RHEL.
Linux in a Nutshell 6th Edition, Sept 2009
A command reference for Linux, find the command syntax you need. Also UNIX in a Nutshell, Fourth edition (2008) by Arnold Robbins
the standard desktop reference. (it even inspired a spoof cover)
bash Cookbook: Solutions and Examples for bash Users O'Reilly Nov 2017 by Carl Albing, JP Vossen
A substantial trove of tips and useful scripts, covers bash 3.2
Essential System Administration, Third Edition Aug 2002 by Æleen Frisch
Describes Unix tools in the context of a system administrator's job. More than just covering syntax, tools and concepts, this book shows how to use administrative tools intelligently and efficiently.
UNIX Power Tools by Jerry D Peak, Tim O'Reilly et al
Useful tricks and techniques, each chapter describes the different ways of performing a task. e.g. the page on 'Wildcards' has 10 ways to select a group of files - only two of which involve the grep command. [Archive of shell scripts]
Shell Programming in Unix, Linux and OS X, Fourth Edition (Paperback)
by Stephen Kochan, Patrick Wood
Detailed coverage of shell programming: regular expressions, the kernel and utilities.
“Talk is cheap. Show me the code” - Linus Torvalds