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"screen" is a window manager that allows you to handle several independent
screens (UNIX ttys) on a single physical terminal; each screen has its own
set of processes connected to it (typically interactive shells).  Each
virtual terminal created by "screen" emulates a DEC VT100 plus several ANSI
X3.64 and ISO 2022 functions (including DEC VT102 features such as line and
character deletion and insertion).

Since "screen" uses pseudo-ttys, the select system call, and UNIX-domain
sockets, it will not run under a system that does not include these
features of 4.2 and 4.3 BSD UNIX.

If you want to get a quick idea how "screen" works but don't want to read
the entire manual, do the following:
 
     -  call "screen" without arguments
     -  wait for the shell prompt; execute some commands
     -  type ^A ^C (Control-A followed by Control-C)
     -  wait for the shell prompt; do something in the new window
     -  type ^A ^A repeatedly to switch between the two windows
     -  terminate the first shell ("screen" switches to the other window)
     -  terminate the second shell

If you have got "vttest" (the VT100 test program from mod.sources) you
may want to run it from within "screen" to verify that it correctly
emulates a VT100 on your terminal (except for 132 column mode and
double width/height characters, of course).

By the way, "screen" can be used to compensate for certain bugs of "real"
VT100 terminals.  For instance, our 4.2 BSD version of mille(6) garbles
the display on terminals of the VT100 family, but it works quite fine
when it is invoked from within "screen".  In addition, "screen" enables
you to use EMACS on terminals that are unable to generate Control-S and
Control-Q from the keyboard or that require flow control using Control-S
and Control-Q.  This is the reason why I have an alias like
     alias emacs "screen emacs"
in my .cshrc file.


Major changes between this and the previous release are:

*  "screen" allows you to `detach' the "screen" session from the physical
   terminal and resume it at a later point in time (possibly on a
   different terminal or in a different login session).

  To get an impression of this functionality do the following:

     - call "screen" and create a couple of windows
     - type Control-A Control-D (screen terminates; you are back
       in the shell)
     - call "screen -r" to resume the detached "screen"

*  "screen" supports multiple character sets and the ISO 2022 control
   functions to designate and switch between character sets.
   This allows you, for instance, to make use of the VT100 graphics
   character set or national character sets


Before typing "make", you should have a look into the Makefile.
If your system maintains a 4.3-BSD-style load average, add -DLOADAV to
the compiler options.  If the load average maintained by your system
is in the Sun format (three longs; not three doubles), set -DSUNLOADAV
in addition to -DLOADAV.

In addition, you must set -DGETTTYENT if your system has the new format
/etc/ttys and the getttyent(3) routines.

If the bcopy() from your system's C library supports overlapping
source and destination addresses, add -DUSEBCOPY.  Otherwise screen
uses its own (possibly slower) version of bcopy().  If you are in
doubt or don't know what bcopy() is, leave the option out.

"screen" should be granted read and write access to /etc/utmp and, if
-DLOADAV has been specified, read access to /vmunix and /dev/kmem.
"screen" should be installed with set-uid and owner root to enable it
to correctly change the owner of newly allocated virtual terminals.
Failing to do this (e.g. if you fear a trojan horse) doesn't have any
major disadvantages, except that w(1) and some other utilities may have
some problems with the tty files of your virtual terminals.


Regards,
    Oliver Laumann
    Technical University of Berlin,
    Communications and Operating Systems Research Group.

    net@tub.BITNET     US: pyramid!tub!net     Europe: unido!tub!net
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