===== The Virtual Terminal =====

Each window in a ''screen'' session emulates a VT100 terminal, with some extra functions added.  The VT100 emulator is hardcoded; 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 overstrike).

Also, you can customize the $TERMCAP value used by ''screen'' by using the "[[commands:termcap]]" .screenrc command, or by defining the variable $SCREENCAP prior to startup.  When the is 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 "[[commands: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 ''[[commands: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 |
|                             || 7   |     | Visible |
| ''ESC =''                        ||| (V) | Application Keypad Mode |
| ''ESC >''                        ||| (V) | Numeric Keypad Mode |
| ''ESC # 8''                      ||| (V) | Fill Screen with Es |
| ''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 ''[[commands:aclchg]] :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 |
|                             || 1   |     | From Beginning of Screen to Cursor |
|                             || 2   |     | Entire Screen |
| ''ESC [ //Pn// K''               |||     | Erase in Line |
|               | ''//Pn//'' = | None or 0 | | From Cursor to End of Line |
|                             || 1   |     | From Beginning of Line to Cursor |
|                             || 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 |
|                             || 1   |     | Bold |
|                             || 2   | (A) | Faint |
|                             || 3   | (A) | Standout Mode (ANSI: Italicized) |
|                             || 4   |     | Underlined |
|                             || 5   |     | Blinking |
|                             || 7   |     | Negative Image |
|                             || 22  | (A) | Normal Intensity |
|                             || 23  | (A) | Standout Mode off (ANSI: Italicized off) |
|                             || 24  | (A) | Not Underlined |
|                             || 25  | (A) | Not Blinking |
|                             || 27  | (A) | Positive Image |
|                             || 30  | (A) | Foreground Black |
|                             || 31  | (A) | Foreground Red |
|                             || 32  | (A) | Foreground Green |
|                             || 33  | (A) | Foreground Yellow |
|                             || 34  | (A) | Foreground Blue |
|                             || 35  | (A) | Foreground Magenta |
|                             || 36  | (A) | Foreground Cyan |
|                             || 37  | (A) | Foreground White |
|                             || 39  | (A) | Foreground Default |
|                             || 40  | (A) | Background Black |
|                             || ... |     | |
|                             || 49  | (A) | Background Default |
| ''ESC [ //Pn// g''               |||     | Tab Clear |
|               | ''//Pn//'' = | None or 0 | | Clear Tab at Current Position |
|                             || 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 |
|                             || 20  | (A) | Automatic Linefeed Mode |
|                             || 34  |     | Normal Cursor Visibility |
|                             || ?1  | (V) | Application Cursor Keys |
|                             || ?3  | (V) | Change Terminal Width to 132 columns |
|                             || ?5  | (V) | Reverse Video |
|                             || ?6  | (V) | Origin Mode |
|                             || ?7  | (V) | Wrap Mode |
|                             || ?9  |     | X10 mouse tracking |
|                             || ?25 | (V) | Visible Cursor |
|                             || ?47 |     | Alternate Screen (old xterm code) |
|                             || ?1000 | (V) | VT200 mouse tracking |
|                             || ?1047 |   | Alternate Screen (new xterm code) |
|                             || ?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 |