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Chapter 3. Logging in |  |
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Chapter 3. Logging in | |
"Basic Concepts About Termcap and Terminfo quoted from www.die.net
The file /etc/termcap is a text file that lists the terminal capabilities. Several applications use the termcap information to move the cursor around the screen and do other screen-oriented tasks. tcsh, bash, vi and all the curses-based applications use the termcap database.
The database describes several terminal types. The TERM environment variable selects the right behaviour at run-time, by naming a termcap entry to be used by applications.
Within the database, each capability of the terminal appears as a two-letter code and a representation of the actual escape sequence used to get the desired effect. The separator character between different capabilities is the colon (":"). As an example, the audible bell, with code "bl", usually appears as "bl=^G". This sequence tells that the bell sound is obtained by printing the control-G character, the ASCII BEL.
In addition to the bl capability, the vb capability is recognized. It is used to represent the "visible bell". vb is usually missing in the linux entry of the termcap file.
Most modern applications and libraries use the terminfo database instead of termcap. This database uses one file per terminal-type and lives in /usr/lib/terminfo; to avoid using huge directories, the description of each terminal type is stored in a directory named after its first letter; the linux entry, therefore, is /usr/lib/terminfo/l/linux.
To build a terminfo entry you'll ``compile'' the termcap description; refer to the tic program and its manual page.
Terminfo (formerly Termcap) is a database of terminal capabilities and more.
For every (well almost) model of terminal it tells application programs what the terminal is capable of doing. It tells what escape sequences (or control characters) to send to the terminal in order to do things such as move the cursor to a new location, erase part of the screen, scroll the screen, change modes, change appearance (colours, brightness, blinking, underlining, reverse video etc.). After about 1980, many terminals supported over a hundred different commands (some of which take numeric parameters).
One way in which terminfo gives the its information to an application program is via the "ncurses" functions that a programmer may put into a C program. Some programs get info directly from a terminfo files without using ncurses.
Included in the terminfo are often a couple of initialisation strings, which may change the appearance of the screen, change what mode the terminal is in, and/or make the terminal emulate another terminal.
However this is not done automatically, one might expect that the getty program should do this but if it did, one could make a change to the set-up at the terminal and this change wouldn't be happen because the init string would automatically cancel it.
To force an intialisation you will use commands given on the command line (or in a shell script such as /etc/profile) to send the init strings - "tset", "tput init", or "setterm -initialise".
Sometimes there is no need to send the init strings since the terminal may set itself up correctly when it is powered on (using options/preferences one has set up and saved in the non-volatile memory of the terminal). Most dumb terminals have an entire setup sequence sort of like the CMOS of a PC, and it is here that you can set the hardware side of the handshake.
For the Debian Distribution of Linux, several commonly used terminals (including the monitor-console) are in the ncurses-term package. These are put into /etc/terminfo/. All of the terminals in the database are in the ncurses-bin package and go into /usr/share/terminfo/.
See the man pages: terminfo(5) or termcap(5) for the format required to create (or modify) the source files.
The data in the source files is compiled with the "tic" program (Also capable of converting between termcap source format and terminfo format).
The installation program which was used to install Linux probably installed the compiled files on your hard disk so you don't need to compile anything unless you modify /etc/termcap (or terminfo.src ).
"tic" will automatically install the resulting compiled files into a terminfo directory ready to be used by application programs.
In order to save disk space, one may delete all of the terminfo database except for the terminals types that you have (or might need in the future). Don't delete any of the termcaps for a "Linux terminal" (the console) or the xterm ones if you use X Window. The terminal type "dumb" may be needed when an application program can't figure out what type of terminal you are using. It would save disk space if install programs only installed the terminfo for the terminals that you have and if you could get a termcap for a newly installed terminal over the Internet in a few seconds.
We have discussed the environment variables prior to this course. The Environment variable TERM should be set to the name of terminal you are using.
This name must be in the Terminfo data base.
Type "set" at the command line to see what TERM is set to (or type: tset -q).
At a console (monitor) TERM is set to "linux" which is the PC monitor emulating a fictitious terminal model named "linux". Since "linux" is close to a vt100 terminal and many text terminals are also, the "linux" designation will sometimes work as a temporary expedient with a text terminal.
If more than one type of terminal may be connected to the same port (/dev/tty...) then TERM needs to be set each time someone connects to the serial port. There is often a query escape sequence so that the computer may ask the terminal what type it is. Another way is to ask the user to type in (or select) the type of terminal s/he is using.
You may need to use tset for this or write a short shell script to handle this.
The .profile login script is executed and contains within it the following statement: eval `tset -s ?vt100`. The user is then asked if they are using a vt100 and either responds yes or types in the actual terminal type they are using. Then tset sends the init string and sets TERM to this terminal name (type).
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Terminal emulation and the gettydefs file layout always remind me how flexible a Unix derived system actually is. You can change anything you want or need to change to make things work and these sections are a good portrayal of that fact. |
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There are warnings as well, no system this powerful can be changed with no thought as to how the change may affect other sections of the operating system, as you can see they are linked and intricately so.
There are 6 virtual terminals in Linux, available by using Alt-F1 through Alt-F6.
There are normally 6 terminals available in X also, F7 through F12. If an X session is started from F1 and you also have an active session on F2, you can type Ctrl-Alt-F2 to go from the X session to the virtual console on F2. Also to get back to your X session, you can type Ctrl-Alt-F7.
The above paragraph assumes that your terminals are set up in the standard manner with 6 virtual terminals available, all that spawn the getty program.
Check the /etc/inittab file for the following lines:
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:2345:respawn:/sbin/mingetty tty3
4:2345:respawn:/sbin/mingetty tty4
5:2345:respawn:/sbin/mingetty tty5
6:2345:respawn:/sbin/mingetty tty6
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mingetty and fbgetty are used most often on the consoles for Linux (as long as you are not using a real text terminal). (See fbgetty). |
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Each virtual terminal uses approx 8KB of kernel memory. As per performance tuning examples allocate fewer virtual terminals if this amount of kernel memory is affecting the performance. (see table below, "off" instead of "respawn")
1:2345:respawn:/sbin/mingetty tty1
2:2345:off:/sbin/mingetty tty2
3:2345:off:/sbin/mingetty tty3
4:2345:off:/sbin/mingetty tty4
5:2345:off:/sbin/mingetty tty5
6:2345:off:/sbin/mingetty tty6
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When the screen goes "insane" say after you read a binary file with the command "cat" e.g. /etc/wtmp.
$ reset
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As you type the word reset you may not be able to see it on your screen, just continue typing though and enter. (We used to use "J stty sane ^J" for serial terminals in the old days.)
"The screen program allows you to run multiple virtual terminals, each with its own interactive shell, on a single physical terminal or terminal emulation window. Even if you use Linux virtual consoles or multiple xterm windows, it is worth exploring screen for its rich feature set, which includes:
scrollback history,
copy-and-paste,
output logging,
digraph entry, and
the ability to detach an entire screen session from your terminal and reattach it later.
If you frequently log on to a Linux machine from a remote terminal or using a VT100 terminal program, screen will make your life much easier with the detach feature.
You are logged in via a dialup connection, and are running a complex screen session with editors and other programs open in several windows.
Suddenly you need to leave your terminal, but you don't want to lose your work by hanging up.
Simply type ^A d to detach the session, then log out. (Or, even quicker, type ^A DD to have screen detach and log you out itself.)
When you log on again later, enter the command screen -r, and screen will magically reattach all the windows you had open. "
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| init |  |
Chapter 4. The Kernel versus Process Management |