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   screen    ( 1 )

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THE VIRTUAL TERMINAL

Each window in a screen session emulates a VT100 terminal, with some extra functions added. The VT100 emulator is hard-coded, no other terminal types can be emulated. 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. Notice that, however, on machines using the terminfo database this variable has no effect. 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" .screenrc command, or by defining the variable $SCREENCAP prior to startup. When the latter is 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" .screenrc command if the system uses the terminfo database rather than termcap.

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 `PRIN' while the printer is active.

Screen maintains a hardstatus line for every window. If a window gets selected, the display's hardstatus will be updated to match the window's hardstatus line. If the display has no hardstatus the line will be displayed as a standard screen message. The hardstatus line can be changed with the ANSI Application Program Command (APC): "ESC_<string>ESC\". As a convenience for xterm users the sequence "ESC]0..2;<string>^G" is also accepted.

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.

The "alternate screen" capability is not enabled by default. Set the altscreen .screenrc command to enable it.

The following is a list of control sequences recognized by screen. "(V)" and "(A)" indicate VT100-specific and ANSI- or ISO-specific functions, respectively.

ESC E Next Line

ESC D Index

ESC M Reverse Index

ESC H Horizontal Tab Set

ESC Z Send VT100 Identification String

ESC 7 (V) Save Cursor and Attributes

ESC 8 (V) Restore Cursor and Attributes

ESC [s (A) Save Cursor and Attributes

ESC [u (A) Restore Cursor and Attributes

ESC c Reset to Initial State

ESC g Visual Bell

ESC Pn p Cursor Visibility (97801)

Pn = 6 Invisible

Pn = 7 Visible

ESC = (V) Application Keypad Mode

ESC > (V) Numeric Keypad Mode

ESC # 8 (V) Fill Screen with E's

ESC \ (A) String Terminator

ESC ^ (A) Privacy Message String (Message Line)

ESC ! Global Message String (Message Line)

ESC k A.k.a. Definition String

ESC P (A) Device Control String. Outputs a string directly to the host terminal without interpretation.

ESC _ (A) Application Program Command (Hardstatus)

ESC ] 0 ; string ^G (A) Operating System Command (Hardstatus, xterm title hack)

ESC ] 83 ; cmd ^G (A) Execute screen command. This only works if multi-user support is compiled into screen. The pseudo-user ":window:" is used to check the access control list. Use "addacl :window: -rwx #?" to create a user with no rights and allow only the needed commands.

Control-N (A) Lock Shift G1 (SO)

Control-O (A) Lock Shift G0 (SI)

ESC n (A) Lock Shift G2

ESC o (A) Lock Shift G3

ESC N (A) Single Shift G2

ESC O (A) Single Shift G3

ESC ( Pcs (A) Designate character set as G0

ESC ) Pcs (A) Designate character set as G1

ESC * Pcs (A) Designate character set as G2

ESC + Pcs (A) Designate character set as G3

ESC [ Pn ; Pn H Direct Cursor Addressing

ESC [ Pn ; Pn f same as above

ESC [ Pn J Erase in Display

Pn = None or 0 From Cursor to End of Screen

Pn = 1 From Beginning of Screen to Cursor

Pn = 2 Entire Screen

ESC [ Pn K Erase in Line

Pn = None or 0 From Cursor to End of Line

Pn = 1 From Beginning of Line to Cursor

Pn = 2 Entire Line

ESC [ Pn X Erase character

ESC [ Pn A Cursor Up

ESC [ Pn B Cursor Down

ESC [ Pn C Cursor Right

ESC [ Pn D Cursor Left

ESC [ Pn E Cursor next line

ESC [ Pn F Cursor previous line

ESC [ Pn G Cursor horizontal position

ESC [ Pn ` same as above

ESC [ Pn d Cursor vertical position

ESC [ Ps ;...; Ps m Select Graphic Rendition

Ps = None or 0 Default Rendition

Ps = 1 Bold

Ps = 2 (A) Faint

Ps = 3 (A) Standout Mode (ANSI: Italicized)

Ps = 4 Underlined

Ps = 5 Blinking

Ps = 7 Negative Image

Ps = 22 (A) Normal Intensity

Ps = 23 (A) Standout Mode off (ANSI: Italicized off)

Ps = 24 (A) Not Underlined

Ps = 25 (A) Not Blinking

Ps = 27 (A) Positive Image

Ps = 30 (A) Foreground Black

Ps = 31 (A) Foreground Red

Ps = 32 (A) Foreground Green

Ps = 33 (A) Foreground Yellow

Ps = 34 (A) Foreground Blue

Ps = 35 (A) Foreground Magenta

Ps = 36 (A) Foreground Cyan

Ps = 37 (A) Foreground White

Ps = 39 (A) Foreground Default

Ps = 40 (A) Background Black

Ps = ...

Ps = 49 (A) Background Default

ESC [ Pn g Tab Clear

Pn = None or 0 Clear Tab at Current Position

Pn = 3 Clear All Tabs

ESC [ Pn ; Pn r (V) Set Scrolling Region

ESC [ Pn I (A) Horizontal Tab

ESC [ Pn Z (A) Backward Tab

ESC [ Pn L (A) Insert Line

ESC [ Pn M (A) Delete Line

ESC [ Pn @ (A) Insert Character

ESC [ Pn P (A) Delete Character

ESC [ Pn S Scroll Scrolling Region Up

ESC [ Pn T Scroll Scrolling Region Down

ESC [ Pn ^ same as above

ESC [ Ps ;...; Ps h Set Mode

ESC [ Ps ;...; Ps l Reset Mode

Ps = 4 (A) Insert Mode

Ps = 20 (A) Automatic Linefeed Mode

Ps = 34 Normal Cursor Visibility

Ps = ?1 (V) Application Cursor Keys

Ps = ?3 (V) Change Terminal Width to 132 columns

Ps = ?5 (V) Reverse Video

Ps = ?6 (V) Origin Mode

Ps = ?7 (V) Wrap Mode

Ps = ?9 X10 mouse tracking

Ps = ?25 (V) Visible Cursor

Ps = ?47 Alternate Screen (old xterm code)

Ps = ?1000 (V) VT200 mouse tracking

Ps = ?1047 Alternate Screen (new xterm code)

Ps = ?1049 Alternate Screen (new xterm code)

ESC [ 5 i (A) Start relay to printer (ANSI Media Copy)

ESC [ 4 i (A) Stop relay to printer (ANSI Media Copy)

ESC [ 8 ; Ph ; Pw t Resize the window to `Ph' lines and `Pw' columns (SunView special)

ESC [ c Send VT100 Identification String

ESC [ x Send Terminal Parameter Report

ESC [ > c Send VT220 Secondary Device Attributes String

ESC [ 6 n Send Cursor Position Report