Linux Commands Necessary for Rasberry Pi?

You have a lot of questions, which are clearly too much to be done much justice in a forum, and this doesn't help you. As a suggestion, you may need to read some books to get up-to-speed in any useful/significant detail, and then ask more narrower, specific questions.

But, to get you started:

RTOS: If you've been working for 7 years with microcontrollers then you should already know most of this: Basically, when you're designing certain things (airplane, washing machine) the system may have elements that are time-critical (i.e. as opposed to a key-press on a keyboard, which you won't mind to be sluggish occasionally). They are known as real-time systems, and traditionally they are coded as a main program loop (e.g. while(forever)) which cycles round, executing tasks in a fixed sequence, one after the other. This works to a degree, but isn't really maintainable if you want to add features, or if a task takes some uncertain time (e.g. how long to wait for a motor to spin up? What if the motor is replaced?). When to quit waiting and continue with the remainder tasks? It ends up messing with the timings of the entire system!.

A solution is to execute each task concurrently, i.e. multi-task. One approach is to use interrupts, but there are a number of reasons why this is not always recommended (although for small programs it is often ideal). Another approach is to use an RTOS, which basically lets you write several small programs that can all execute in place of the single large program. The RTOS provides functions to allow each individual program to communicate with each other. In other words, the RTOS replaces the big control loop and jumps that traditionally controlled the overall program execution.

Beyond that, there is a lot of theory that you'd need to read to understand such OS's. There are good books on the subject (I've not found good web resources on the subject). One simple book to read is by Jean J. Labrosse (it is unfortunately only geared to his own particular OS) but at least it gives you some background. There are more advanced books.

Linux can be (and is) used to implement real-time systems but isn't traditionally known as a real-time OS. It does offer ways to create threads (of program flow) and communicate between them. This is a fairly advanced topic for a beginner. I'm guessing your C knowledge may already be fine? GCC is fine if you wish to use C. You can use the fairly regular tutorial on C/C++ in the blogs section, to brush up. It's as good as a textbook so far.

Doing things like image processing requires you to either become very competant at programming and at maths, or requires you to be able to take some pre-existing library of code and use it; the latter is preferred usually. It still requires you to know a programming language first. Sometimes people go for more simpler methods like line-tracking using a reflective sensor rather than a VGA camera.

Learning Linux commands is fairly essential for any programmer, since a large quantity of software uses Linux or Linux tools.

Here is the absolute bare minimum to get you started (I wrote these up for someone once..).

Quick Linux Guide

Once logged into the Linux machine via Telnet, you will see a command prompt. In Linux, the software that is running that displays the prompt and allows you to type content is called the ‘shell’. You can leave the shell at any time by typing ‘exit’.

Command Prompt

Commands typically (but not always) have the syntax

command -option -option argument

An example would be

ls –l *.txt

The command ‘ls’ performs a directory (i.e. folder) listing. The argument ‘-l’ displays in a ‘lengthy’ format. The ‘*.txt’ argument defines what files to list.

Redirection

You can force the output into a file, using ‘>’.

For example,

ls > listing.txt

would place the directory listing output into a file called listing.txt.

History

You can see earlier commands that you have typed by using the history command. It is very useful.

Commands

Useful commands are shown here.

The most useful GNU/Linux command is "man", which displays a short user guide ("manual page") for the command.  For example, "man ls" displays a guide for the "ls" command.

To get started with Raspberry a lot of people recommend The MagPi: http://www.themagpi.com/.

To get started with GNU/Linux, there are lots of books out there but I'll leave recommendations to others.  You might look at your local library for hard copy.  GNU/Linux evolves very slowly at the user and application developer level, so older books are a fine way to get started.  I mostly just use the Unix commands I learned back in the late 1970s and learn the latest options -- which do evolve -- using "man".

Agreed that #man is THE most usefull command.  Try #man man and you will soon discover the power of #man -k .