Chapter 7
Displays and Emulations
Today hundreds of types of displays and terminals are in the marketplace.
Every one of them one is different and every one has unique features. They range
from basic ASCII terminals to video terminals that can run both ASCII and X.
There are smart terminals and dumb terminals, and in many cases the dumb terminals
are smarter than the smart terminals.
No matter what kind of terminal you plan to use, you should be
able to find a compatible emulator or proper settings that support the various
features and sequences that programs use to manipulate the screen. Depending
on what you plan to do when you log in to a system, you might not need to worry
about how the terminal reacts if the commands you are issuing don't require
any specific terminal emulator.
These commands include programs such as who,
cat, uname, tail,
head, ls, and others
that need only a simple linefeed that can endlessly scroll down the screen,
similar to a teletype machine that can print on a endless roll of paper. Almost
any serial device that displays or prints can be considered a terminal. In the
'70s and early '80s, teletype machines worked great as terminals.
Although they were slow and used a lot of paper, you could still finish your
work.
By now you might be wondering how UNIX can manipulate so many
different types of terminals with different features and different control commands.
Or in more basic terms, why does the clear command
work on one terminal and not on another?
There are two parts to UNIX that make terminals emulate properly:
a database and a subroutine library. The database describes the capabilities
that are supported on the terminal, and the subroutine library is used to query
the database to use the capabilities that are stored there. When you put these
together, they make what you know today as the termcap file (named
for terminal capabilities ). Over
the years the termcap file has grown extremely large. In the AT&T flavors
of UNIX, termcap was turned into a database called terminfo. Instead
of one large file that holds everything, each terminal description is compiled
and stored in a separate file. When programs make use of the terminfo files,
they reference the environment variable TERM to
figure out what the terminal type is. After this is known, they can go to the
database and get the proper database definition for the terminal that you are
using.
7.1 Terminal Types
7.1.1 Description
Administrators need to be able to set the correct terminal type
and know which one to use.
Example One: Working With /etc/termcap
Flavor: BSD
The
BSD version uses the termcap file that is stored in /etc/termcap. A typical
termcap entry has the name of the terminal and any alias names for it. The following
lines indicate the special features and the settings for it:
d0|vt100|vt100-am|vt100am|dec vt100:\
:do=^J:co#80:li#24:cl=50\E[;H\E[2J:sf=5\ED:\
:le=^H:bs:am:cm=5\E[%i%d;%dH:nd=2\E[C:up=2\E[A:\
:ce=3\E[K:cd=50\E[J:so=2\E[7m:se=2\E[m:us=2\E[4m:ue=2\E[m:\
:md=2\E[1m:mr=2\E[7m:mb=2\E[5m:me=2\E[m:is=\E[1;24r\E[24;1H:\
:rf=/usr/share/lib/tabset/vt100:\
:rs=\E>\E[?3l\E[?4l\E[?5l\E[?7h\E[?8h:ks=\E[?1h\E=:ke=\E[?1l\E>:\
:ku=\EOA:kd=\EOB:kr=\EOC:kl=\EOD:kb=^H:\
:ho=\E[H:k1=\EOP:k2=\EOQ:k3=\EOR:k4=\EOS:pt:sr=5\EM:vt#3:xn:\
:sc=\E7:rc=\E8:cs=\E[%i%d;%dr:
If this entry was not already in the termcap file, it needs only
to be appended to the bottom of the file. There are no daemons to restart--a
query is sent to the file whenever the environment variable TERM
is referenced.
Here is a partial list of definitions in the vt100 entry. On BSD
systems, consult the termcap (5) man pages for
a more complete listing.
do moves the cursor
down n lines
co sets the number of
columns
li sets the number of
lines
le controls how a backspace
will be handled
rf sends the next input
character
up moves the cursor up
n lines
If you want to know which terminals your termcap currently supports,
you can use the following command to display all the available entries in your
/etc/termcap file:
rocket 1% cat termcap | egrep -v '(#|=)' | cut -d"|" -f2 | more
The command processes ( cat) through
a termcap file, stripping out ( egrep -v ) anything
with a # character and a =
sign in it. It then displays the second field ( cut)
that is delimited by a vertical bar ( |). The first
field of the entry is usually an abbreviated name, whereas the second field
is generally always more descriptive. Because the file holds many entries, pipe
it to more so you can easily read all the entries.
The file then displays all the entries in a format that you can easily read:
dumb
switch
ansi
sun
w50
xterm
teletec
Example Two: Add a File to terminfo
Flavor: AT&T
Syntax:
tic file
On the AT&T side, there is an entry similar to the type of
database entries found in the termcap. Each entry is a file for the terminal
that it represents. After the entry is made into the terminfo file, it is compiled
with tic. The terminfo compiler creates a binary
representation of the database description and places it in the appropriate
terminfo directory under /usr/lib/terminfo.
Here is one type of entry for an ANSI terminal that needs to be
compiled into the terminfo database:
# vi newansi
newansi|ansi 6502,
am, cols#80, it#8, lines#24,
clear=\E[H\E[2J$<50>, cub1=\E[D, cud1=\E[B, cuf1=\E[C,
cup=\E[%i%p1%d;%p2%dH, cuu1=\E[A, el=\E[K, home=\E[H,
ht=^I,
This is a short entry by some standards. Most can be as many as
20 lines of settings, depending on the complexity of the terminal. This one
is pretty basic by comparison. What follows is a short list of variables that
are used within the terminfo file. You should see some similarities with the
termcap file entries:
am terminal has automatic
margins
cols number of columns
in a line
it number of spaces
for the tab setting
lines number of lines
on a screen
clear clear the screen
and home the cursor
cup move to row #1 column
#2
home home the cursor
ht hardware tab stop
Many more variables are available and definable. Check your terminfo
man pages for a list of all the settings that are available on your system.
Now you have an
entry called newansi that can be compiled into the
terminfo database. Before doing so, run an integrity check on the entry to verify
that everything is valid and in the proper format. To do this, use the terminfo
compiler ( tic) with the -c
option.
# tic -c newansi
If there are errors in the entry, tic
attempts to figure out what the error is and where it is located. If everything
runs successfully, nothing is displayed to STDOUT. When the file checks out
clean, run tic with no options to compile it into
place.
# tic newansi
It almost appears as if nothing happened, but if you look into
the terminfo directory, you should see new entries for the file you just compiled.
The
terminfo directory is made up of subdirectories that are named after the first
letter of the terminfo files in the database. Therefore, the compiled newansi
file can be found in the directory /usr/lib/terminfo/n:
# ls -l /usr/lib/terminfo/n*
-rw-r--r-- 1 root sys 12 Jun 13 1997 net
-rw-r--r-- 1 root sys 12 Jun 13 1997 network
-rw-r--r-- 1 root daemon 982 Nov 29 12:11 newansi
# file /usr/lib/terminfo/n/newansi
/usr/lib/terminfo/n/newansi: Compiled Terminfo Entry
Reason
If you want to use advanced features to manipulate the screen,
terminals function properly only when they have an entry in the database.
Real World Experience
The newer terminals on the market today can emulate multiple types
of terminals. The Wyse60 ASCII terminal supports vt100, vt220, wyse60, and other
types. If this is the case with your terminal, set up the terminal to support
one of the entries already available in termcap or terminfo.
If you receive a new terminal from a manufacturer that is not
listed in the termcap file, check the documentation. If it is not documented
anywhere, you can do several things:
-
Email the
manufacturer. In many cases the vendor can email the entries back to you.
(In many instances, manufacturers use the address support@[manufacturer's
name].com or have an alias set up to get to technical support.)
-
nCall the manufacturer. You might be able to get the entry
faxed to you. However, finding someone at the manufacturer who knows what
a terminal entry is can be a problem from time to time. Tell them you need
to speak to someone familiar with UNIX. They don't even have to be
in technical support. If you can reach someone with some UNIX knowledge,
often they can relay what you are looking for more quickly and easily than
you can by trying to describe it to someone who is UNIX illiterate.
-
Check the
Web. If you have acquired a terminal by some other means, and it has no
documentation or the vendor is out of business, your last resort is to check
for Web sites that mention either the terminal or termcap/terminfo entries.
You can also check the Terminfo/Termcap Resource Web page at http://tuxedo.org/~esr/terminfo.
-
Posting to the Internet. If the entry still cannot be found,
try to post a message into the comp.sys.unix
Usenet newsgroup or one of the other related newsgroups. Someone might have
the entry on his or her system or know how to build one from scratch.
-
Try a standard entry. You can see
whether one of the standard entries works with your terminal. Your best
chances are probably with ANSI, vt100, vt220, wyse, or xterm.
Other Resources
Man pages:
termcap, terminfo
World Wide Web:
Terminfo/Termcap Resource-- http://tuxedo.org/~esr/terminfo
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