Getting to the Shell (chapter 2)

1.This chapter discusses what is required to provider a command line environment,

  and then walks you through the terminal emulation packages you may run into

  in the various Linux distributions.

2.A dumb terminal was usually nothing more than a monitor and keyboard connected

  to the Unix system via a communication cable(usually a multi-wire serial cable).

3.One way to get to a CLI is to take the Linux system out of graphical desktop

  mode and place it in text mode. This mode is called the Linux console

  because it emulates the old days of a hard-wired console terminal and is a

  direct interface to the Linux system.

4.The alternative to being in the Linux console is to use a terminal emulation

  package from within the graphical desktop environment.

5.Each terminal emulation package has the ability to emulate one or more specific

  types of dumb terminals. If you are going to work with the shell in Linux,

  unfortunately you'll need to know a little bit about terminal emulation.

6.A control code is a special code not used in the character set, which signals the

   terminal to perform a special, nonprintable operation.

7.The terminfo database is a set of files that identity the characteristics of various 

   terminals that can be used on the Linux system.

8.The Linux system stores the terminfo data for each terminal type as a separate file in

   the terminfo database directory. Some common locations are /etc/terminfo,

   /lib/terminfo, and /usr/share/terminal.

   For example: in Ubuntu, by default, ncurses will search /etc/terminfo first, then 

   /lib/terminal, then /usr/share/terminfo.

9.An individual terminfo file is a binary file that is the result of compiling a text file.

   This text files contains code words that define screen functions, associated with the

   control code required to implement the function on the terminal. we can use the 

   infocmp command to convert the binary entries into text.

digdeep@ubuntu:/lib/terminfo/l$ infocmp linux

# Reconstructed via infocmp from file: /lib/terminfo/l/linux

linux|linux console,

am, bce, ccc, eo, mir, msgr, xenl, xon,

colors#8, it#8, ncv#18, pairs#64,

... ...

10.The Linux shell uses the TERM environment variable to define which terminal 

     emulation setting in the terminfo database to use for a specific session.

     When the TERM environment variable is set to xterm, the shell knows to use the

     control codes associated with the xterm terminfo database entry for sending control

     codes to the terminal emulator. To see the TERM environment variable, you can 

     just echo if from the CLI:

digdeep@ubuntu:/lib/terminfo/x$ echo $TERM

xterm

     This example shows that the current terminal type is set to the xterm entry in the 

      terminfo database.

11.The Linux Console

     In the early days of Linux, when you booted up your system you would see a login

     prompt on your monitor, and that's all, that is called the Linux console. For example

     in Ubuntu, we can use Ctrl + Alt + F1(or F2, F3, F4, F5, F6) to enter the Linux 

     Console. And use Ctrl + Alt + F7 back to the Graphics environment.

     With modern Linux systems, when the Linux system starts it automatically creates

     several virtual consoles.

12.The xterm Terminal

     The oldest and most basic of X Window terminal emulation packages is xterm. The 

     xterm packages has been around since the original days of X Window, and is 

     included by default in most X Windows packages.