screen: Window Termcap
16.1 Choosing the termcap entry for a window
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Usually 'screen' tries to emulate as much of the VT100/ANSI standard as
possible. But if your terminal lacks certain capabilities the emulation
may not be complete. In these cases 'screen' has to tell the
applications that some of the features are missing. This is no problem
on machines using termcap, because 'screen' can use the '$TERMCAP'
variable to customize the standard screen termcap.
But if you do a rlogin on another machine or your machine supports
only terminfo this method fails. Because of this 'screen' offers a way
to deal with these cases. Here is how it works:
When 'screen' tries to figure out a terminal name for itself, it
first looks for an entry named 'screen.TERM', where TERM is the contents
of your '$TERM' variable. If no such entry exists, 'screen' tries
'screen' (or 'screen-w', if the terminal is wide (132 cols or more)).
If even this entry cannot be found, 'vt100' is used as a substitute.
The idea is that if you have a terminal which doesn't support an
important feature (e.g. delete char or clear to EOS) you can build a
new termcap/terminfo entry for 'screen' (named 'screen.DUMBTERM') in
which this capability has been disabled. If this entry is installed on
your machines you are able to do a rlogin and still keep the correct
termcap/terminfo entry. The terminal name is put in the '$TERM'
variable of all new windows. 'screen' also sets the '$TERMCAP' variable
reflecting the capabilities of the virtual terminal emulated.
Furthermore, the variable '$WINDOW' is set to the window number of each
window.
The actual set of capabilities supported by the virtual terminal
depends on the capabilities supported by the physical terminal. If, for
instance, the physical terminal does not support underscore mode,
'screen' does not put the 'us' and 'ue' capabilities into the window's
'$TERMCAP' variable, accordingly. However, a minimum number of
capabilities must be supported by a terminal in order to run 'screen';
namely scrolling, clear screen, and direct cursor addressing (in
addition, 'screen' does not run on hardcopy terminals or on terminals
that over-strike).
Also, you can customize the '$TERMCAP' value used by 'screen' by
using the 'termcap' command, or by defining the variable '$SCREENCAP'
prior to startup. When the latter defined, its value will be copied
verbatim into each window's '$TERMCAP' variable. This can either be the
full terminal definition, or a filename where the terminal 'screen'
(and/or 'screen-w') is defined.
Note that 'screen' honors the 'terminfo' command if the system uses
the terminfo database rather than termcap. On such machines the
'$TERMCAP' variable has no effect and you must use the 'dumptermcap'
command (
Dump Termcap) and the 'tic' program to generate
terminfo entries for 'screen' windows.
When the boolean 'G0' capability is present in the termcap entry for
the terminal on which 'screen' has been called, the terminal emulation
of 'screen' supports multiple character sets. This allows an
application to make use of, for instance, the VT100 graphics character
set or national character sets. The following control functions from
ISO 2022 are supported: 'lock shift G0' ('SI'), 'lock shift G1' ('SO'),
'lock shift G2', 'lock shift G3', 'single shift G2', and 'single shift
G3'. When a virtual terminal is created or reset, the ASCII character
set is designated as 'G0' through 'G3'. When the 'G0' capability is
present, screen evaluates the capabilities 'S0', 'E0', and 'C0' if
present. 'S0' is the sequence the terminal uses to enable and start the
graphics character set rather than 'SI'. 'E0' is the corresponding
replacement for 'SO'. 'C0' gives a character by character translation
string that is used during semi-graphics mode. This string is built
like the 'acsc' terminfo capability.
When the 'po' and 'pf' capabilities are present in the terminal's
termcap entry, applications running in a 'screen' window can send output
to the printer port of the terminal. This allows a user to have an
application in one window sending output to a printer connected to the
terminal, while all other windows are still active (the printer port is
enabled and disabled again for each chunk of output). As a side-effect,
programs running in different windows can send output to the printer
simultaneously. Data sent to the printer is not displayed in the
window. The 'info' command displays a line starting with 'PRIN' while
the printer is active.
Some capabilities are only put into the '$TERMCAP' variable of the
virtual terminal if they can be efficiently implemented by the physical
terminal. For instance, 'dl' (delete line) is only put into the
'$TERMCAP' variable if the terminal supports either delete line itself
or scrolling regions. Note that this may provoke confusion, when the
session is reattached on a different terminal, as the value of
'$TERMCAP' cannot be modified by parent processes. You can force
'screen' to include all capabilities in '$TERMCAP' with the '-a'
command-line option (Invoking Screen).
The "alternate screen" capability is not enabled by default. Set the
'altscreen' '.screenrc' command to enable it.
Info Catalog
screen: Termcap
screen: Dump Termcap