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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 (see Write out the window’s termcap entry) 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 (see Invoking Screen).
The "alternate screen" capability is not enabled by default.
Set the altscreen .screenrc command to enable it.
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