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jw 21.10.93
05.05.94
screen: frequently asked questions -- known problems -- unimplemented bugs
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Q: Why is it impossible to download a file with Kermit/sz/rz when
screen is running? Do I need to set some special variables?
A: Screen always interprets control-sequences sent by the
applications and translates/optimizes them for the current
terminal type. Screen always parses the user input for its
escape character (CTRL-A). Both are basic screen features and
cannot be switched off. Even if it were possible to switch
screen into a completely transparent mode, you could never switch
between windows, while kermit/sz/rz is downloading a file. You
must wait til the end as kermit/sz/rz will not transmit your
input during a file transfer and as kermit/sz/rz would be very
confused if screen switched away the window containing the
other kermit/sz/rz. Simply detach your screen session for each
file transfer and start the transfer program only from the shell
where you started screen.
Q: I am using screen with a YYY terminal, which supports the XXX
graphic language. I am very happy with it, except one thing: I
cannot render graphics into screen windows.
A: You are out of luck there. Screen provides a fixed set of escape
sequences in order to make it possible to switch terminal types.
Screen has to know exactly what the escape sequences do to the
terminal because it must hold an image in memory. Otherwise
screen could not restore the image if you switch to another
window. Because of this you have to change screens escape
sequence parser (ansi.c) to pass the XXX graphics sequences to
the terminal. Of course the graphics will be lost if you switch
to another window. Screen will only honour graphics sequences
that are demanded by an overwhelming majority.
Q: For some unknown reason, the fifo in /tmp/screens/S-myname is
gone, and i can't resume my screen session. Is there a way to
recreate the fifo?
A: Screen checks the fifo/socket whenever it receives a SIGCHLD
signal. If missing, the fifo/socket is recreated then. If screen
is running non set-uid the user can issue a 'kill -CHLD
screenpid' directly (it is -CHILD on some systems). Screenpid is
the process-id of the screen process found in a 'ps -x' listing.
But usually this won't work, as screen should be installed set-
uid root. In this case you will not be able to send it a signal,
but the kernel will. It does so, whenever a child of screen
changes its state. Find the process-id (shellpid below) of the
"least important" shell running inside screen. The try 'kill
-STOP shellpid'. If the fifo/socket does not reappear, destroy
the shell process. You sacrify one shell to save the rest. If
nothing works, please do not forget to remove all processes
running in the lost screen session.
Q: When you start "screen" a page of text comes up to start you
off. Is there a way to get rid of this text as a command line
argument or by using a switch of some sort.
A: Just put the following line in your ~/.screenrc:
startup_message off
Many peole ask this, although it is in the man page, too :-)
Q: Start "screen emacs" and run emacs function suspend-emacs
(ctrl-z). The window containing emacs vanishes.
A: This is a known bug. Unfortunatly there is no easy fix
because this is specified in the POSIX standard. When a new
window is created Screen opens up a new session because the
window has to get the pty as a controlling terminal (a
session can only have one controlling terminal). With the
setsid() call the process also creates a new process
group. This process group is orphaned, because there is no
process in the session which is not in the process
group. Now if the process group leader (i.e. your program)
gets a TTIN/TTOU/TSTP, POSIX states that the kernel must
send a KILL signal to the process group because there is no
one left to continue the process. Even if screen would
try to restart the program, that would be after it received the
KILL signal which cannot be caught or ignored.
tromey@klab.caltech.edu (Tom Tromey): I've noticed this exact
same problem. I put this in my .emacs file. It seems to work:
;; If running under screen, disable C-z.
(if (and (getenv "STY") (not window-system))
(global-unset-key "\C-z"))
Q: Screen gets the terminal size wrong and messes up.
A: Before you start screen: Check with 'stty -a' what the terminal
driver thinks about rows and columns. Check the environment
variables LINES and COLUMNS. Then from within screen check with
the info command (CTRL-A i) what size screen thinks your terminal
is. If correcting tty driver setting and environment variables
does not help, look up the terminal capability definition. First
the TERMCAP environment variable. If this is not set, look up the
terminals name as defined in the environment variable TERM in
/etc/termcap or in the terminfo database with untic or infocmp.
There may be :li=...: and :co=...: or even :ll=...: entries
(cols#... and lines#... when it's terminfo) defined incorrectly.
Either construct your own TERMCAP environment variables with
correct settings, use screens terminfo/termcap command in your
.screenrc file or have the database corrected by the system
administrator.
Q: Screen messes up the terminal output when I use my favourite ap-
plication. Setting the terminal size does not help.
A: Probably you got the termcap/terminfo entries wrong. Fixing this
is a three stage procedure. First, find out if terminfo or
termcap is used. If your system only has /etc/termcap,
but not /usr/lib/terminfo/... then you are using termcap.
Easy. But if your system has both, then it depends how the appli-
cation and how screen were linked. Beware, if your applica-
tion runs on another host via rlogin, telnet or the like, you
should check the terminfo/termcap databases there. If you cannot
tell if terminfo or termcap is used (or you just want to be
save), the do all steps in stage 3 in parallel for both
systems (on all envolved hosts). Second: Understand the basic
rules how screen does its terminal emulation. When screen is
started or reattached, it relies on the TERM environment variable
to correctly reflect the terminal type you have physically
in front of you. And the entry should either exist in the system
terminfo/termcap database or be specified via the TERMCAP en-
vironment variable (if screen is using the termcap system). On
the other end, screen understands one set of control codes. It
relies on the application using these codes. This means applica-
tions that run under screen must be able to adapt their con-
trol codes to screen. The application should use the TERM vari-
able and termcap or terminfo library to find out how to drive
its terminal. When running under screen, the terminal is virtual
and is only defined by the set of control codes that screen
understands. The TERM variable is automatically set to
"screen" and the "screen"-entries should exist in the data-
bases. If your application uses hardcoded control codes rather
than a database, you are on your own. Hint: The codes under-
stood by screen are a superset of the very common definition
named "vt100". Look at the documentation of screen. The
codes are listed there. Third: Have the entry "screen" in-
stalled on all hosts or make sure you can live with "vt100".
Check the codes sent by your application, when the TERM variable
is set to "screen". Do not try to set the TERM variable inside
screen to anything other than "screen" or "vt100" or compati-
ble. Thus your application can drive screen correctly. Also take
care that a good entry is installed for your physical terminal
that screen has to drive. Even if the entry was good enough
for your application to drive the terminal directly, screen may
find flaws, as it tries to use other capabilities while op-
timizing the screen output. The screenrc commands
"termcap" and/or "terminfo" may help to fine-tune capabilities
without calling the supervisor to change the database.
Q: I cannot configure screen. Sed does not work.
A: The regular expressions used in our configure scrip are too
complicated for GNU sed version 2.03. In this regard it is bug
compatible with Ultrix 3.1 "sed": GNU sed version 2.03 dumps
core with our configure script. Try an older release. E.g. from
ftp.uni-erlangen.de:/pub/utilities/screen/sed-2.02b.tar.gz
Q: When reattaching a session from a different Workstation, the
DISPLAY environment variable should be updated. Even ``CTLR-A
: setenv DISPLAY newhost:0'' does not work as expected.
A: Under unix every process has its own environment. The environ-
ment of the SCREEN process can be changed with the `setenv' com-
mand. This however cannot affect the environment of the
shells or applications already running under screen. Subsequently
spawned processes will reflect the changes. One should be aware
of this problem when running applications from very old shells.
Screen is a means for keeping processes alive.
Q: About once every 5 times I ran the program, rather than getting
a "screen," I got someone elses IRC output/input.
A: What probably happened is that an IRC process was left running on
a pseudo tty in such a way that the kernel thought the tty was
available for reallocation. You can fix this behaviour by
applying the SunOS 4.1.x tty jumbo patch (100513-04).
Q: Screen compiled on SunOS 5.3 cannot reattach a detached session.
A: You are using /usr/ucb/cc, this compiler is wrong. Actually it
links with a C-library that mis-interprets dirent. Try again
with /opt/SUNWspro/bin/cc!
Q: The "talk" command does not work when Screen is active.
A: Talk and several other programs rely on entries in the Utmp-
Database (/etc/utmp). On some systems this Database is world
writable, on others it is not. If it is not, screen must be
installed with the appropriate permissions (user or group s-bit)
just like any program that uses PTYs (rlogin, xterm, ...). When
screen cannot write to utmp, you will see messages on you display
which do not belong to any screen window.
When screen can update utmp, it is not guaranteed that it does as
you expect. First this depends on the config.h file defining
UTMPOK, LOGINDEFAULT, and perhaps CAREFULUTMP. Second it depends
on the screenrc files (system wide and per user), if utmp entries
are done. Third, you can control whether windows are logged in
with screens ``login'' command.
Q: Seteuid() does not work as expected in AIX. Attempting a multi-
user-attach results in a screen-panic: "seteuid: not owner".
A: This is not a screen problem. According to Kay Nettle
(pkn@cs.utexas.edu) you need the AIX patch PTF 423674.
Q: When I type cd directory (any directory or just blank) from
within one of the windows in screen, the whole thing just freezes
up.
A: You display the current working directory in xterm's title bar,
This may be caused by hardcoded ESC-sequences in the shell prompt
or in an cd alias. In Xterm the coding is
ESC ] n ; string_to_display ^G
where n = 1, 2, 3 selects the location of the displayed string.
Screen misinterprets this as the ansi operating system comment
sequence:
ESC ] osc_string
and waits (according to ansi) for the string terminator
ESC \
Screen versions after 3.5.12 may provide a workaround.
Q: Mesg or biff cannot be turned on or off while running screen.
A: Screen failed to change the owner of the pty it uses. You need to
install screen setuid-root. See the file INSTALL for details.
Q: The cursor left key deletes the characters instead of just moving the
cursor. A redisplay (^Al) brings everything back.
A: Your terminal emulator treats the backspace as "destructive". You
can probably change this somewhere in the setup. We can't think
of a reason why anybody would want a destructive backspace, but
if you really must have it, add the lines
termcap <TERM> 'bc@:bs@'
terminfo <TERM> 'bc@:bs@'
to your ~/.screenrc (replace <TERM> with the terminal type your
emulator uses).
Q: I have an old SysV OS (like Motorola SysV68) and sometimes screen
doesn't reset the attributes correctly. A redisplay (^Al) doesn't
make things better.
A: The libcurses library has a bug if attributes are cleared with
the special ue/se capabilities. As a workaround (other than upgrading
your system) modify 'rmul' (and 'rmso'?) in screen's terminfo entry:
rmul=\E[m, rmso=\E[m
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