Dynamic Screen Utility (dscreen) Tips

Which keys perform the preceding functions depends on the terminal and the terminal description in the dsinfo file.

NOTES:

1. Do not switch screens when the current screen is being written to; an escape sequence may be truncated and may leave the terminal in an unknown state.
2. Some terminal displays may save the cursor position for individual screens but may not save other states such as insert mode, inverse video, and so on. If this is the case, users should avoid these modes while switching screens.

When a new virtual screen is created, it is assigned a Select key. Pressing the Select key causes the following actions:

• A switch occurs from the physical terminal to the video page associated with the particular virtual screen.
• Input and output is directed appropriately between the physical terminal and the virtual screen.

Once all of the Select keys defined in the dsinfo file have virtual screens assigned to them, no more screens may be created. Individual screen sessions end when the original shell process exits. This frees the associated Select key for use with another virtual screen. The dscreen utility is ended when there are no more active screens.

Block keys are used to stop output in a fashion similar to Ctrl-S when using IXON flow control. The purpose of these keys is to allow for transparently setting up terminal sessions on two computers using a terminal that has two serial ports.

When the New screen key is pressed, dscreen creates a new logical screen and assigns it to one of the Select keys. Each new screen requires:

• a Select key as defined in the dsinfo file
• a dscreen pseudo-terminal device
• memory enough for the various structures used in screen tracking
• a process from which to run the shell

If any of these are not available, the new screen operation will fail and print a message indicating the reason for the failure.

When the End key is pressed, dscreen will do the following:

• send a SIGHUP signal to all the screen sessions
• clean up
• exit with a status of zero (0)

When the Quit key is pressed, dscreen will perform the same actions but will exit with status of 1.

When the Previous key is pressed, dscreen will switch the terminal to the screen that was last displayed.

When the List key is pressed, dscreen will display a list of keys and their actions on the terminal display. Only those keys recognized by dscreen will be shown. When a new screen is created using dscreen, the message Press <KEY> for help is displayed on the terminal where KEY is the name of the List key. The message is only displayed if there is a List key defined.

The terminal description entry in the dsinfo file will have the same number of screen selection keys as the terminal has physical screen pages. If more screen selection keys are defined than the number of physical screen pages, dscreen will dynamically assign physical screen pages to virtual screens.

When a virtual screen is selected that does not have an associated page of screen memory, dscreen assigns the least recently used physical screen to the virtual screen. Depending on the specifications maintained in the dsinfo description file, an indication that the physical screen is connected to a different virtual screen may be noticeable. For example, the screen may be cleared.

dsinfo is a database of terminal descriptions used by the dscreen multiple screen utility. The file contains the following information:

• dscreen keys and the functions they perform
• the number of screen memory pages for the terminal
• the code sequences sent or received to use the preceding features

Entries in the dsinfo file consist of comma-separated fields. The first field is a list of alternate names for the terminal. Each name is separated by a pipe (|) character. The remaining fields are string types and value strings, in the format string_type=value_string, describing the capabilities of the terminal to the dscreen utility. Within these strings, the following escape codes are recognized:

   \E,\e     escape character 
   ,\l       newline (or linefeed) character
             carriage return 
             tab character 
            backspace character
            formfeed character 
   \s        space character 
   nn        character with octal value nnn 
   ^x        control-x for any appropriate x value 

Any other character preceded by a backslash will yield the character itself.

The string types used in the dsinfo file are as follows:

dskx

A string type that starts with dsk describes a key. The type must be four letters long, and the fourth letter indicates what action is taken when the key is received. The key types are:

   TYPE       ACTION 
___________________________________________ 
   dsks       Switch Screens 
   dskb       Block Input and Output 
   dske       End dscreen 
   dskq       Quit dscreen (exit status=1) 
   dskc       Create New Screen 
   dskp       Switch to Previous Screen 
   dskl       List Keys and Actions 

Any other key type, for example, a string type dskx where x is not s, b, e, q, c, p, or l) will cause no internal dscreen action but will show up in the key listing and will be recognized and acted upon. A type of dskn (n for No Operation) should be used when no internal dscreen action is desired.

The value string to the right of the equal sign (=) for each key is divided into three substrings, which are separated by pipe (|) characters.

NOTE: Use \| to include the | character in one of the substrings.

• The first substring is the sequence of characters that the terminal sends to dscreen when the key is pressed.
• The second substring is a label for the key that is printed when a list of keys is displayed.
• The third substring is a sequence of characters that dscreen sends to the terminal when this key is pressed before performing the action this key requests.

dsp

A string type of dsp describes a physical screen in the terminal. One dsp string should be present for each physical screen in the terminal.

The value string for each physical screen has two substrings, which are separated by a pipe (|) character.

For example:

   dsp=\E pA|\EH\EJ,   # Sequence for screen 1 of a 
                       #   3151 Terminal 

• The first substring is the sequence of characters to send to the terminal to display and output to the physical page on the terminal.
• The second substring is sent to the terminal when the page is used for something new. This second substring is often set to the clear screen sequence. It is sent under the following two conditions:
  1. when a new virtual terminal session is being created
  2. when there are more virtual terminals than there are physical screens; if a virtual terminal is selected which requires dscreen to reuse one of the physical screens, it will send this sequence to the screen to indicate that the screen contents do not match the output of the virtual terminal connected

     NOTE: Running with more virtual terminals than physical screens can be confusing and is not recommended; it can be avoided by defining no more screen selection keys (dsks=) than physical screens (dsp=) in the dsinfo entry.

dst

A string with a type of dst adjusts dscreen's input timeout. The value of the string is a decimal number. The timeout value is in tenths of seconds and has a maximum value of 255 (default=1 [or .1 seconds]).

When dscreen recognizes a prefix of an input key sequence but does not have all the characters of the sequence, it will wait for more characters to be sent until it is recognizable. If the timeout occurs before more characters are received, the characters are sent on to the virtual screen and dscreen will not consider these characters as part of an input key sequence.

It may be necessary to raise this value if one or more of the keys dscreen is to trigger on is actually a number of keystrokes; that is, assigning Ctrl-Z 1, Ctrl-Z 2, Ctrl-Z 3, and so on for screen selection, Ctrl-Z N for new screen and so on.

The following example dsinfo entry is for a Wyse 60 with three screen sessions:

wy60|wyse60|wyse model 60, 
        dsks=^A` 
|Shift-F1|, 
        dsks=^Aa 
|Shift-F2|, 
        dsks=^Ab 
|Shift-F3|, 
        dskc=\200|Ctrl-F1|, 
        dske=\201|Ctrl-F2|\Ew0\E+, 
        dskl=\202|Ctrl-F3|, 
        dsp=\Ew0|\E+, 
        dsp=\Ew1|\E+, 
        dsp=\Ew2|\E+, 

In the preceding example:

• Shift-F1 through Shift-F3 are used for selecting screens one through three.
• Ctrl-F1 creates a new screen.
• Ctrl-F2 sends Esc w 0 Esc + to the screen (switching to window 0 and clearing the screen) and then ends dscreen.
• Ctrl-F3 will list the keys and their functions.

Each time a physical screen is used for a new screen, the sequence Esc + will be sent to the terminal, which will clear the screen.

The following example is for a Wyse 60 with three screen sessions but one of the screens is on a second computer communicating through the second serial port on the terminal:

   wy60-1|wyse60-1|wyse model 60 - first serial port 
             dsks=^A`^M|Shift-F1|, 
             dsks=^Aa^M|Shift-F2|, 
             dskb=^Ab^M|Shift-F3|\Ed#^Ab 
   ^T\Ee9, 
             dskc=\200|Ctrl-F1|, 
             dske=\201|Ctrl-F2|\Ed#\201^T\Ew0\E+, 
             dskl=\202|Ctrl-F3|, 
             dsp=\Ew0|\E+,dsp=\Ew1|\E+, 
   wy60-2|wyse60-2|wyse model 60 - second serial port 
             dsks=^A`^M|Shift-F1|\Ed#^A` 
  ^T\Ee8, 
             dsks=^Aa^M|Shift-F2|\Ed#^Aa 
  ^T\Ee8, 
             dsks=^Ab^M|Shift-F3|, 
             dskc=\200|Ctrl-F1|, 
             dske=\201|Ctrl-F2|\Ed#\201^T\Ew0\E+, 
             dskl=\202|Ctrl-F3|, 
             dsp=\Ew2|\E+, 

dscreen must be run on both computers, with terminal type wy60-1 on the first computer and terminal type wy60-2 on the second computer (using the -t option for dscreen).

In the wy60-1 entry, the first two key entries are unchanged from the original wy60 entry. The third key, however, has type dskb , which means block both input and output. When this key is pressed, the following sequence is sent to the terminal:

     <Esc> d #  <Ctrl-A>  b  <CR>  <Ctrl-T>  <Esc> e 9 

The sequence Esc d # puts the terminal in Transparent Print Mode, which echoes all characters up to a Ctrl-T out through the other serial port.

The characters Ctrl-A b <CR> are sent out the other serial port, informing the dscreen process on the other computer that it should activate the window associated with the Shift-F3 key.

The Ctrl-T exits the Transparent Print Mode. Esc e 9 tells the terminal to switch to the other AUX serial port for data communications.

At this point, the other computer takes over and sends an Esc w 2 to switch to the third physical screen and then resumes normal communication.

The wy60-2 entry follows the same general pattern for keys Shift-F1 and Shift-F2:

• contact
• switch to transparent print mode
• send function key string to other computer
• switch transparent print off
• switch to the other serial port

The end key Ctrl-F2 works the same for both computers; it sends the end key sequence to the other computer through the transparent print mechanism, switches the terminal to window 0, clears the screen, and then exits.

[ Doc Ref: 90605201114636     Publish Date: May. 05, 2000     4FAX Ref: 1399 ]