| blob | 6c9c8afa918a55e503846682cee666ad1d157157 |
1 jw 21.10.93
2 05.05.94
4 screen: frequently asked questions -- known problems -- unimplemented bugs
5 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
8 Q: Why is it impossible to download a file with Kermit/sz/rz when
9 screen is running? Do I need to set some special variables?
11 A: Screen always interprets control-sequences sent by the
12 applications and translates/optimizes them for the current
13 terminal type. Screen always parses the user input for its
14 escape character (CTRL-A). Both are basic screen features and
15 cannot be switched off. Even if it were possible to switch
16 screen into a completely transparent mode, you could never switch
17 between windows, while kermit/sz/rz is downloading a file. You
18 must wait til the end as kermit/sz/rz will not transmit your
19 input during a file transfer and as kermit/sz/rz would be very
20 confused if screen switched away the window containing the
21 other kermit/sz/rz. Simply detach your screen session for each
22 file transfer and start the transfer program only from the shell
23 where you started screen.
25 Q: I am using screen with a YYY terminal, which supports the XXX
26 graphic language. I am very happy with it, except one thing: I
27 cannot render graphics into screen windows.
29 A: You are out of luck there. Screen provides a fixed set of escape
30 sequences in order to make it possible to switch terminal types.
31 Screen has to know exactly what the escape sequences do to the
32 terminal because it must hold an image in memory. Otherwise
33 screen could not restore the image if you switch to another
34 window. Because of this you have to change screens escape
35 sequence parser (ansi.c) to pass the XXX graphics sequences to
36 the terminal. Of course the graphics will be lost if you switch
37 to another window. Screen will only honour graphics sequences
38 that are demanded by an overwhelming majority.
40 Q: For some unknown reason, the fifo in /tmp/screens/S-myname is
41 gone, and i can't resume my screen session. Is there a way to
42 recreate the fifo?
44 A: Screen checks the fifo/socket whenever it receives a SIGCHLD
45 signal. If missing, the fifo/socket is recreated then. If screen
46 is running non set-uid the user can issue a 'kill -CHLD
47 screenpid' directly (it is -CHILD on some systems). Screenpid is
48 the process-id of the screen process found in a 'ps -x' listing.
49 But usually this won't work, as screen should be installed set-
50 uid root. In this case you will not be able to send it a signal,
51 but the kernel will. It does so, whenever a child of screen
52 changes its state. Find the process-id (shellpid below) of the
53 "least important" shell running inside screen. The try 'kill
54 -STOP shellpid'. If the fifo/socket does not reappear, destroy
55 the shell process. You sacrify one shell to save the rest. If
56 nothing works, please do not forget to remove all processes
57 running in the lost screen session.
59 Q: When you start "screen" a page of text comes up to start you
60 off. Is there a way to get rid of this text as a command line
61 argument or by using a switch of some sort.
63 A: Just put the following line in your ~/.screenrc:
64 startup_message off
65 Many peole ask this, although it is in the man page, too :-)
67 Q: Start "screen emacs" and run emacs function suspend-emacs
68 (ctrl-z). The window containing emacs vanishes.
70 A: This is a known bug. Unfortunatly there is no easy fix
71 because this is specified in the POSIX standard. When a new
72 window is created Screen opens up a new session because the
73 window has to get the pty as a controlling terminal (a
74 session can only have one controlling terminal). With the
75 setsid() call the process also creates a new process
76 group. This process group is orphaned, because there is no
77 process in the session which is not in the process
78 group. Now if the process group leader (i.e. your program)
79 gets a TTIN/TTOU/TSTP, POSIX states that the kernel must
80 send a KILL signal to the process group because there is no
81 one left to continue the process. Even if screen would
82 try to restart the program, that would be after it received the
83 KILL signal which cannot be caught or ignored.
85 tromey@klab.caltech.edu (Tom Tromey): I've noticed this exact
86 same problem. I put this in my .emacs file. It seems to work:
88 ;; If running under screen, disable C-z.
89 (if (and (getenv "STY") (not window-system))
90 (global-unset-key "\C-z"))
92 Q: Screen gets the terminal size wrong and messes up.
94 A: Before you start screen: Check with 'stty -a' what the terminal
95 driver thinks about rows and columns. Check the environment
96 variables LINES and COLUMNS. Then from within screen check with
97 the info command (CTRL-A i) what size screen thinks your terminal
98 is. If correcting tty driver setting and environment variables
99 does not help, look up the terminal capability definition. First
100 the TERMCAP environment variable. If this is not set, look up the
101 terminals name as defined in the environment variable TERM in
102 /etc/termcap or in the terminfo database with untic or infocmp.
103 There may be :li=...: and :co=...: or even :ll=...: entries
104 (cols#... and lines#... when it's terminfo) defined incorrectly.
105 Either construct your own TERMCAP environment variables with
106 correct settings, use screens terminfo/termcap command in your
107 .screenrc file or have the database corrected by the system
108 administrator.
110 Q: Screen messes up the terminal output when I use my favourite ap-
111 plication. Setting the terminal size does not help.
113 A: Probably you got the termcap/terminfo entries wrong. Fixing this
114 is a three stage procedure. First, find out if terminfo or
115 termcap is used. If your system only has /etc/termcap,
116 but not /usr/lib/terminfo/... then you are using termcap.
117 Easy. But if your system has both, then it depends how the appli-
118 cation and how screen were linked. Beware, if your applica-
119 tion runs on another host via rlogin, telnet or the like, you
120 should check the terminfo/termcap databases there. If you cannot
121 tell if terminfo or termcap is used (or you just want to be
122 save), the do all steps in stage 3 in parallel for both
123 systems (on all envolved hosts). Second: Understand the basic
124 rules how screen does its terminal emulation. When screen is
125 started or reattached, it relies on the TERM environment variable
126 to correctly reflect the terminal type you have physically
127 in front of you. And the entry should either exist in the system
128 terminfo/termcap database or be specified via the TERMCAP en-
129 vironment variable (if screen is using the termcap system). On
130 the other end, screen understands one set of control codes. It
131 relies on the application using these codes. This means applica-
132 tions that run under screen must be able to adapt their con-
133 trol codes to screen. The application should use the TERM vari-
134 able and termcap or terminfo library to find out how to drive
135 its terminal. When running under screen, the terminal is virtual
136 and is only defined by the set of control codes that screen
137 understands. The TERM variable is automatically set to
138 "screen" and the "screen"-entries should exist in the data-
139 bases. If your application uses hardcoded control codes rather
140 than a database, you are on your own. Hint: The codes under-
141 stood by screen are a superset of the very common definition
142 named "vt100". Look at the documentation of screen. The
143 codes are listed there. Third: Have the entry "screen" in-
144 stalled on all hosts or make sure you can live with "vt100".
145 Check the codes sent by your application, when the TERM variable
146 is set to "screen". Do not try to set the TERM variable inside
147 screen to anything other than "screen" or "vt100" or compati-
148 ble. Thus your application can drive screen correctly. Also take
149 care that a good entry is installed for your physical terminal
150 that screen has to drive. Even if the entry was good enough
151 for your application to drive the terminal directly, screen may
152 find flaws, as it tries to use other capabilities while op-
153 timizing the screen output. The screenrc commands
154 "termcap" and/or "terminfo" may help to fine-tune capabilities
155 without calling the supervisor to change the database.
157 Q: I cannot configure screen. Sed does not work.
159 A: The regular expressions used in our configure scrip are too
160 complicated for GNU sed version 2.03. In this regard it is bug
161 compatible with Ultrix 3.1 "sed": GNU sed version 2.03 dumps
162 core with our configure script. Try an older release. E.g. from
163 ftp.uni-erlangen.de:/pub/utilities/screen/sed-2.02b.tar.gz
165 Q: When reattaching a session from a different Workstation, the
166 DISPLAY environment variable should be updated. Even ``CTLR-A
167 : setenv DISPLAY newhost:0'' does not work as expected.
169 A: Under unix every process has its own environment. The environ-
170 ment of the SCREEN process can be changed with the `setenv' com-
171 mand. This however cannot affect the environment of the
172 shells or applications already running under screen. Subsequently
173 spawned processes will reflect the changes. One should be aware
174 of this problem when running applications from very old shells.
175 Screen is a means for keeping processes alive.
177 Q: About once every 5 times I ran the program, rather than getting
178 a "screen," I got someone elses IRC output/input.
180 A: What probably happened is that an IRC process was left running on
181 a pseudo tty in such a way that the kernel thought the tty was
182 available for reallocation. You can fix this behaviour by
183 applying the SunOS 4.1.x tty jumbo patch (100513-04).
185 Q: Screen compiled on SunOS 5.3 cannot reattach a detached session.
187 A: You are using /usr/ucb/cc, this compiler is wrong. Actually it
188 links with a C-library that mis-interprets dirent. Try again
189 with /opt/SUNWspro/bin/cc!
191 Q: The "talk" command does not work when Screen is active.
193 A: Talk and several other programs rely on entries in the Utmp-
194 Database (/etc/utmp). On some systems this Database is world
195 writable, on others it is not. If it is not, screen must be
196 installed with the appropriate permissions (user or group s-bit)
197 just like any program that uses PTYs (rlogin, xterm, ...). When
198 screen cannot write to utmp, you will see messages on you display
199 which do not belong to any screen window.
200 When screen can update utmp, it is not guaranteed that it does as
201 you expect. First this depends on the config.h file defining
202 UTMPOK, LOGINDEFAULT, and perhaps CAREFULUTMP. Second it depends
203 on the screenrc files (system wide and per user), if utmp entries
204 are done. Third, you can control whether windows are logged in
205 with screens ``login'' command.
207 Q: Seteuid() does not work as expected in AIX. Attempting a multi-
208 user-attach results in a screen-panic: "seteuid: not owner".
210 A: This is not a screen problem. According to Kay Nettle
211 (pkn@cs.utexas.edu) you need the AIX patch PTF 423674.
213 Q: When I type cd directory (any directory or just blank) from
214 within one of the windows in screen, the whole thing just freezes
215 up.
217 A: You display the current working directory in xterm's title bar,
218 This may be caused by hardcoded ESC-sequences in the shell prompt
219 or in an cd alias. In Xterm the coding is
220 ESC ] n ; string_to_display ^G
221 where n = 1, 2, 3 selects the location of the displayed string.
222 Screen misinterprets this as the ansi operating system comment
223 sequence:
224 ESC ] osc_string
225 and waits (according to ansi) for the string terminator
226 ESC \
227 Screen versions after 3.5.12 may provide a workaround.
229 Q: Mesg or biff cannot be turned on or off while running screen.
231 A: Screen failed to change the owner of the pty it uses. You need to
232 install screen setuid-root. See the file INSTALL for details.
234 Q: The cursor left key deletes the characters instead of just moving the
235 cursor. A redisplay (^Al) brings everything back.
237 A: Your terminal emulator treats the backspace as "destructive". You
238 can probably change this somewhere in the setup. We can't think
239 of a reason why anybody would want a destructive backspace, but
240 if you really must have it, add the lines
241 termcap <TERM> 'bc@:bs@'
242 terminfo <TERM> 'bc@:bs@'
243 to your ~/.screenrc (replace <TERM> with the terminal type your
244 emulator uses).
246 Q: I have an old SysV OS (like Motorola SysV68) and sometimes screen
247 doesn't reset the attributes correctly. A redisplay (^Al) doesn't
248 make things better.
250 A: The libcurses library has a bug if attributes are cleared with
251 the special ue/se capabilities. As a workaround (other than upgrading
252 your system) modify 'rmul' (and 'rmso'?) in screen's terminfo entry:
253 rmul=\E[m, rmso=\E[m