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What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'?

I think these terms almost refer to the same thing, when used loosely:

• terminal
• shell
• tty
• console

What exactly do each of these terms refer to?

Ans::

In unix terminology, the short answer is that

• terminal = tty = text input/output environment
• console = physical terminal
• shell = command line interpreter

a tty is a particular kind of device file which implements a number of additional commands (ioctls) beyond read and write.

terminal is synonymous with tty. Some ttys are provided by the kernel on behalf of a hardware device

Other ttys, sometimes called pseudo-ttys, are provided (through a thin kernel layer) by programs called terminal emulators, such as Xterm (running in the X Window System),

The console appears to the operating system as a (kernel-implemented) tty. On some systems, such as Linux and FreeBSD, the console appears as several ttys,the name given to each particular tty can be “console”, ”virtual console”, ”virtual terminal”, and other variations.

a shell is a special program that interacts with a user through a controlling ttyand offers

The division of labor between the terminal and the shell is not completely obvious. Here are their main tasks.

• Input: the terminal converts keys into control sequences (e.g. Left → \e[D). The shell converts control sequences into commands (e.g. \e[D → backward-char).
• Line edition, input history and completion are provided by the shell.
  • The terminal may provide its own line edition, history and completion instead, and only send a line to the shell when it's ready to be executed. The only common terminal that operates in this way is M-x shell in Emacs.
• Output: the shell emits instructions such as “display foo”, “switch the foreground color to green”, “move the cursor to the next line”, etc. The terminal acts on these instructions.
• The prompt is purely a shell concept.
• The shell never sees the output of the commands it runs (unless redirected). Output history (scrollback) is purely a terminal concept.
• Inter-application copy-paste is provided by the terminal (usually with the mouse or key sequences such as Ctrl+Shift+V or Shift+Insert). The shell may have its own internal copy-paste mechanism as well (e.g. Meta+W and Ctrl+Y).
• Job control (launching programs in the background and managing them) is mostly performed by the shell. However, it's the terminal that handles key combinations like Ctrl+C to kill the foreground job and Ctrl+Z to suspend it.