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]>.