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View Code? Open in Web Editor NEWA simple program that emulates the detach feature of screen
License: GNU General Public License v2.0
A simple program that emulates the detach feature of screen
License: GNU General Public License v2.0
1. INTRODUCTION dtach is a program written in C that emulates the detach feature of screen, which allows a program to be executed in an environment that is protected from the controlling terminal. For instance, the program under the control of dtach would not be affected by the terminal being disconnected for some reason. dtach was written because screen did not adequately meet my needs; I did not need screen's extra features, such as support for multiple terminals or terminal emulation support. screen was also too big, bulky, and had source code that was difficult to understand. screen also interfered with my use of full-screen applications such as emacs and ircII, due to its excessive interpretation of the stream between the program and the attached terminals. dtach does not have a terminal emulation layer, and passes the raw output stream of the program to the attached terminals. The only input processing that dtach does perform is scanning for the detach character (which signals dtach to detach from the program) and processing the suspend key (which tells dtach to temporarily suspend itself without affecting the running program), and both of these can both be disabled if desired. Contrary to screen, dtach has minimal features, and is extremely tiny. This allows dtach to be more easily audited for bugs and security holes, and makes it accessible in environments where space is limited, such as on rescue disks. dtach has only been tested on the Linux/x86 platform, however it should be easily portable to other variants of Unix. It currently assumes that the host system uses POSIX termios, and has a working forkpty function available. dtach may need access to various devices in the filesystem depending on what forkpty does. For example, dtach on Linux usually needs access to /dev/ptmx and /dev/pts. 2. QUICK START Compiling dtach should be simple, as it uses autoconf: $ ./configure $ make If all goes well, a dtach binary should be built for your system. You can then copy it to the appropriate place on your system. dtach uses Unix-domain sockets to represent sessions; these are network sockets that are stored in the filesystem. You specify the name of the socket that dtach should use when creating or attaching to dtach sessions. For example, let's create a new session that is running ircII. We will use /tmp/foozle as the session's socket: $ dtach -A /tmp/foozle irc RuneB irc.freenode.net Here, -A tells dtach to either create a new session or attach to the existing session. If the session at /tmp/foozle does not exist yet, the program will be executed. If it does exist, then dtach will attach to the existing session. dtach has another attach mode, which is specified by using -a. The -a mode attaches to an already existing session, but will not create a new session. Each attaching process can have a separate detach character, suspend behavior, and redraw method, which are explained in the following sections. dtach is able to attach to the same session multiple times, though you will likely encounter problems if your terminals have different window sizes. Pressing ^L (Ctrl-L) will reset the window size of the program to match the current terminal. dtach also has a mode that copies the contents of standard input to a session. For example: $ echo -ne 'cd /var/log\nls -l\n' | dtach -p /tmp/foozle The contents are sent verbatim including any embedded control characters (e.g. the newline characters in the above example), and dtach will not scan the input for a detach character. 3. DETACHING FROM THE SESSION By default, dtach scans the keyboard input looking for the detach character. When the detach character is pressed, dtach will detach from the current session and exit, leaving the program running in the background. You can then re-attach to the program by running dtach again with -A or -a. The default detach character is ^\ (Ctrl-\). This can be changed by supplying the -e option to dtach when attaching. For example: $ dtach -a /tmp/foozle -e '^A' That command would attach to the existing session at /tmp/foozle and use ^A (Ctrl-A) as the detach character, instead of the default ^\. You can disable processing of the detach character by supplying the -E option to dtach when attaching. 4. SUSPENDING DTACH By default, dtach also processes the suspend key (^Z or Ctrl-Z) itself, instead of passing it to the program. Thus, pressing suspend only suspends the attaching process, instead of the running program. This can be very useful for applications such as ircII, where you may not necessarily want the program to be suspended. Processing of the suspend key can be disabled by supplying the -z option to dtach when attaching. 5. REDRAW METHOD When attaching, dtach can use one of three methods to redraw the screen (none, ctrl_l, or winch). By default, dtach uses the ctrl_l method, which simply sends a ^L (Ctrl-L) character to the program if the terminal is in character-at-a-time and no-echo mode. The winch method forces a WINCH signal to be sent to the program, and the none method disables redrawing completely. For example, this command tells dtach to attach to a session at /tmp/foozle and use the winch redraw method: $ dtach -a /tmp/foozle -r winch When creating a new session (with the -c or -A modes), the specified method is used as the default redraw method for the session. 6. CHANGES The changes in version 0.9 are: - Added AIX support. - Added dtach -N, a mode similar to dtach -n, except dtach will stay in the foreground instead of daemonizing. - Added dtach -p, which copies the contents of standard input to a session. - dtach will no longer send 255 bytes of garbage to the program when read() returns an error. - The executable bit is now set on the socket if clients are attached, and cleared when all clients have detached. - The initial state of signals such as SIGPIPE are now preserved when executing the program, instead of having the program start with some signals ignored. - A buffer overflow no longer occurs when a long socket path name is used, and dtach will now try to use chdir to get around the length limitation if necessary. The changes in version 0.8 are: - When using dtach -A or dtach -c, the master will now wait until the client attaches before trying to read from the program being executed. This avoids a race condition when the program prints something and exits before the client can attach itself. - Instead of exiting quietly, dtach will now report any errors that occur while trying to execute the program. - dtach -n can now be used without a terminal. - dtach -A will now try to detect and remove stale sockets. - Removed a Linux-specific escape sequence from the code that restores the original terminal settings. - Changed dtach.1 to use \- for the dashes in command line options, and fix an ambiguous backslash. - Use non-blocking mode in the master process, and avoid data loss by ensuring that at least one attaching client succesfully completes a write. - Fix -e ^<char> to work with lowercase characters. The changes in version 0.7 are: - The redraw method can now be explicitly specified on the command line (either no redraw at all, the old ^L character method, and the new WINCH signal method), since many programs only handle one or the other properly. - Changed the default redraw method back to the old ^L character method. - Changed the attach code to check the return value of select more carefully. - Changed the SIGWINCH handler to reinstall itself, to handle systems that always reset the handler. - Added more proper process group handling. The changes in version 0.6 are: - Redraws are now handled by sending the child process a WINCH signal instead of by sending a ^L character. This should help prevent line-oriented programs (such as bash) from clearing the screen excessively. - Flow control is now disabled when setting raw mode on the terminal. - Switched to using select instead of poll. - Changed some exits to exit succesfully instead of non-sucessfully. - Updated my email address. - Updated to Autoconf 2.59, renaming some files in the process. The changes in version 0.5 are: - Fix fd leakage. - Prevent atexit from being called twice on dtach -A. The changes in version 0.4 are: - Slightly improved README and dtach.1 - Portability updates thanks to sourceforge's compile farm. dtach should now work on: FreeBSD, Debian/alpha, Debian/sparc, Debian/PPC, and Solaris. The changes in version 0.3 are: - Fixed a typo in dtach.1 - Changed the attach code so that it tells the master when a suspend occurs. - Decreased the client <-> master packet size. - Changed the master to send a stream of text to attaching clients instead of sending a huge packet all the time. - Use getrlimit and dynamically allocate the data structures, if possible. - Added some more autoconf checks. - Initial sourceforge release. 7. AUTHOR dtach is (C)Copyright 2004-2016 Ned T. Crigler, and is under the GNU General Public License. Comments and suggestions about dtach are welcome, and can be sent to the author at: <[email protected]>.
Thank you for dtach, I love it. mobile-shell/mosh#980 says the above is a dtach issue. Would you agree/have any ideas for a solution ? I forgot to mention that pressing the escape key once before each arrow key press seems to work around this somehow, allowing movement.
Hello,
Here, when trying to compile dtach, gcc conplains that -traditional
no longer works without -E
.
GCC version used is 4.3.2.
Here is the complete error message:
gcc -traditional -L/opt/Python-2.7/lib -L/opt/Python-2.7/lib -W -Wall -I. -c -o attach.o attach.c
gcc: GNU C no longer supports -traditional without -E
make: *** [attach.o] Error 1
Adding the option in Makefile
solve this issue though.
But, then it won't compile with the following error:
gcc -traditional -E -L/opt/Python-2.7/lib -L/opt/Python-2.7/lib -W -Wall -I. -c -o attach.o attach.c
In file included from /usr/include/features.h:348,
from /usr/include/errno.h:30,
from dtach.h:25,
from attach.c:20:
/usr/include/sys/cdefs.h:32: error: #error "You need a ISO C conforming compiler to use the glibc headers"
In file included from dtach.h:26,
from attach.c:20:
/usr/include/fcntl.h:183: error: #else after #else
/usr/include/fcntl.h:175: error: the conditional began here
/usr/include/fcntl.h:204: error: #else after #else
/usr/include/fcntl.h:197: error: the conditional began here
/usr/include/fcntl.h:211: error: #endif without #if
/usr/include/fcntl.h:222: error: #endif without #if
attach.c:61: error: detected recursion whilst expanding macro "SOCK_STREAM"
make: *** [attach.o] Error 1
I'm unsure why this is failing though.
Type some commands to terminal. Then start dtach. Then exit from dtach. You will see all previous activity (i. e. before starting dtach) is disappeared (as opposed to screen and tmux).
$ dtach --version
dtach - version 0.9, compiled on May 21 2016 at 08:04:42.
While playing with dtach
in a Docker container, I found that dtach -N <some socket> <some command>
seems to exit silently when it's invoked in an environment which has no terminal (which is Docker's default) but works fine if I make Docker allocate a terminal. That would make sense for the -c
mode, but I'd imagine that -N
could theoretically work just fine when dtach
is started in a "terminal-less" environment since it creates its own PTY for the command and never attaches to it. Is this just a side effect of how dtach
currently handles terminals, or is there a fundamental limitation in the Linux APIs that would prevent dtach
from functioning correctly in a terminal-less environment without daemonizing?
It's an easy enough fix in my own project to just allocate a terminal, but I figured I'd mention it in case you thought it was worth looking at eventually.
it would be nice if detach would allow for a way to figure out if a session is attached or not. screen uses the execute bit on the socket to document this: when a session is attached, it's executable, when it's not, it's not.
irssi can use that to automatically mark users as away for example.
Hi, I couldn't find any way to attach to a session and execute a command from a script. I tried searching for help or trying out stuff on my own, but nothing work, is there no way to do this? I found that screen has the -dm switches that serve for this purpose, so it'd be nice to have something similar.
It happens after reattaching.
I've been trying to set up a dtach session for rtorrent. dtach -n socket rtorrent; dtach -a socket
works perfectly, but wierdly enough, when there's more than 1 second between the first command and the second, the up, down, right and left arrows are remapped to A, B, C and D respectively, and the terminal cursor is actually visible in the bottom left corner of rtorrent (it's normally invisible).
This means that dtach -n socket rtorrent; sleep 1; dtach -a socket
isn't buggy, but dtach -n socket rtorrent; sleep 2; dtach -a socket
is.
Every other program that I tried didn't get the bug, except vim, which would exit insert mode and throw an "E388: Couldn't find definition" error when any arrow key was pressed, but I'm not sure which key press that would be. I also tried xev, but the arrow key codes looked normal there.
I'm using st with $TERM set to xterm-256color. Also tried urxvt and xterm, and setting TERM to screen-256color. According to vim, $TERM doesn't change inside the dtach session though.
dtach version 0.9 (pacman version) on an up-to-date Arch Linux install.
I was wondering if it's possible to capture the STDOUT from dtach socket to a log file. The idea is to capture all the session into a log file that I can analyse later.
I have tried playing with socat
but could not find the correct incantation. Is that even possible?
When ever dtach exits because the command started by dtach finishes it always seems to exit with -1 when started in dtach -n mode.
For example,
strace -s 8192 -ff ./dtach -n /tmp/f /bin/sh -c "sleep 1"
Produces the following snipped output,
<... select resumed> ) = 1 (in [4])
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=14569, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
rt_sigreturn({mask=[]}) = 1
read(4, 0x7ffdb1138830, 4096) = -1 EIO (Input/output error)
unlink("/tmp/f") = 0
exit_group(1) = ?
+++ exited with 1 +++
Hello @crigler,
I'm noticing extremely high CPU usage for essentially idle dtach sessions.
# ps aux | head -n 1 ; ps aux | grep dtach
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1592 0.0 0.0 14220 992 pts/6 S+ 23:22 0:00 grep --color=auto dtach
user 4111 42.7 0.0 14868 704 ? Rs May05 2177:36 dtach -A /home/user/save/irc zsh
user 4133 39.6 0.0 6288 388 pts/4 R+ May05 2015:43 dtach -A /home/user/save/irc zsh
user 4201 0.0 0.0 14868 700 ? Ss May05 0:01 dtach -A /home/user/save/irc zsh
# uptime
23:23:30 up 45 days, 12:05, 3 users, load average: 1.95, 1.69, 1.66
Here is what strace shows
select, read, write in a tight loop
select(4, [0 3], NULL, NULL, NULL) = 1 (in [0])
read(0, "\3\rxe\f\f\3", 8) = 7
write(3, "\0\7\3\rxe\f\f\3\0", 10) = 10
select(4, [0 3], NULL, NULL, NULL) = 1 (in [0])
read(0, "\3\rxe\f\f\3", 8) = 7
write(3, "\0\7\3\rxe\f\f\3\0", 10) = 10
select(4, [0 3], NULL, NULL, NULL) = 1 (in [0])
read(0, "\3\rxe\f\f\3", 8) = 7
write(3, "\0\7\3\rxe\f\f\3\0", 10) = 10
Here is the process tree
systemd─┬─
├─dtach───zsh───dtach
├─dtach───zsh───irssi───{gmain}
Here is lsof for the two dtach sessoins
/home/user
/
/usr/bin/dtach
/lib/x86_64-linux-gnu/libnss_files-2.23.so
/lib/x86_64-linux-gnu/libnss_nis-2.23.so
/lib/x86_64-linux-gnu/libnsl-2.23.so
/lib/x86_64-linux-gnu/libnss_compat-2.23.so
/lib/x86_64-linux-gnu/libc-2.23.so
/lib/x86_64-linux-gnu/libutil-2.23.so
/lib/x86_64-linux-gnu/ld-2.23.so
/dev/null
/dev/null
/dev/null
/home/user/save/irc type=STREAM
/dev/ptmx
/home/user/save/irc type=STREAM
/home/user
/
/usr/bin/dtach
/lib/x86_64-linux-gnu/libc-2.23.so
/lib/x86_64-linux-gnu/libutil-2.23.so
/lib/x86_64-linux-gnu/ld-2.23.so
/dev/pts/4
/dev/pts/4
/dev/pts/4
type=STREAM
/home/user
/
/usr/bin/dtach
/lib/x86_64-linux-gnu/libnss_files-2.23.so
/lib/x86_64-linux-gnu/libnss_nis-2.23.so
/lib/x86_64-linux-gnu/libnsl-2.23.so
/lib/x86_64-linux-gnu/libnss_compat-2.23.so
/lib/x86_64-linux-gnu/libc-2.23.so
/lib/x86_64-linux-gnu/libutil-2.23.so
/lib/x86_64-linux-gnu/ld-2.23.so
/dev/null
/dev/null
/dev/null
/home/user/save/irc type=STREAM
/dev/ptmx
Version info...
# apt-cache policy dtach
dtach:
Installed: 0.8-2.1
Candidate: 0.8-2.1
Version table:
*** 0.8-2.1 500
500 http://us.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
100 /var/lib/dpkg/status
Currently, the escape char is ^\
which is impossible to type in with an European keyboard layout and two keys: German, French, Italian, etc. Though, one can use -e
, it is extremely exhausting to provide this every time. Therefore, I propose to either move this to a compile time option: --with-escape-char=...
or have an env var for that: DTACH_ESCAPE_CHAR
. It would make life way easier for non-English users.
If we agree on some option, I can try to work out a patch.
dtach -N /tmp/d1 echo "ASD"
exits silently.
Let me explain an example where this is definitely a requirement. Consider the systemd unit as below:
[Unit]
Description=systemd integration for rtorrent using dtach - starts/stops rtorrent instances on startup/shutdown
Wants=network-online.target
After=network-online.target
[Service]
Environment="TERM=rxvt-unicode-256color"
KillMode=none
ExecStartPre=/usr/bin/mkdir -p %t/dtach
ExecStart=/usr/bin/dtach -N %t/dtach/rtorrent /usr/bin/rtorrent -n -o import=%E/rtorrent/.rtorrent.rc
ExecStop=/usr/bin/killall -u %u -w -s INT /usr/bin/rtorrent
# Refer https://github.com/crigler/dtach/issues/12, Success is error code 1
SuccessExitStatus=1
[Install]
WantedBy=default.target
Now what happens when rtorrent fails to start, say to due a config error? It prints to stderr/stdout. So will almost every cli program ranging from a simple echo to something heavy like rtorrent.
If I am running in foreground, then it would be prudent to redirect stdout/stderr of the command in question, or atleast have an option to do so.
It looks like the google/werm project is using code from dtach, even though it is licensed under the Apache License version 2.0, while dtach is licensed under the GNU General Public License version 2.0.
As a user, I'm likely to forget what I called my socket when I created it. I'm also likely to forget that I created one, too.
An option to list the currently opened sockets, as well as highlighting the current session, would be really handy. Then I could look up the name of my socket if I forgot its name, and I could check to see if I was connected to a socket currently too.
Here's what I would imagine the output would look like:
Currently open sockets:
socketname1 - /bin/bash (username1, joe, henry)
vegetables - /usr/bin/carrot
*browser - /usr/bin/elinks (sbrl)
logs - tail -f /var/log/kern.log (logsrus)
The format I chose above was socketname - command[ (connectedusernames)
. The asterisk (*
) in the above example denotes the current session. Perhaps the current session could be made bold, too?
Hi @crigler
I have been using dtach in a program that I use for executing commands with a long execution time on an Amazon EC2 instance. Due to the instance sometimes being unresponsive, I periodically detach and reattach to the session via my script. It has been working great in general. The only issue I have is that whatever is printed on my local terminal from the redirected output from the instance gets cleared when the session reattaches. So I am not able to see all the output from the session in one place. Is there an easy way to not clear the prior output?
Thanks a lot!
rpmlint warning:
W: file-contains-date-and-time /usr/bin/dtach
Your file uses __DATE and __TIME__ this causes the package to rebuild when
not needed
consider if https://reproducible-builds.org/specs/source-date-epoch/ is a viable alternative
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