resizable serial console window?

Question

When using the serial console of my system I always end up with $COLUMNS=80 and $LINES=24.

While I can change these variables manually it is somewhat annoying to do this any time when the client side terminal window has been resized.

Usually I'm connecting to the console using screen /dev/mytty baudrate.

Changing the $TERM environment variable to "screen" or "xterm" does not help.

Will I need to call getty with some of those instead of vt100?

Needless to say that all of this works fine, when I connect to the same machine using ssh.

Answer 1

Like the commentators before me mentioned there is no alternative to calling resize after every command, if you don't have this command and you don't want to install a package where it's in (xterm), here are two POSIX shell script that do the same using ANSI terminal escape codes:

res() {

  old=$(stty -g)
  stty raw -echo min 0 time 5

  printf '\0337\033[r\033[999;999H\033[6n\0338' > /dev/tty
  IFS='[;R' read -r _ rows cols _ < /dev/tty

  stty "$old"

  # echo "cols:$cols"
  # echo "rows:$rows"
  stty cols "$cols" rows "$rows"
}

res2() {

  old=$(stty -g)
  stty raw -echo min 0 time 5

  printf '\033[18t' > /dev/tty
  IFS=';t' read -r _ rows cols _ < /dev/tty

  stty "$old"

  # echo "cols:$cols"
  # echo "rows:$rows"
  stty cols "$cols" rows "$rows"
}

• res is based on the solution presented at https://wiki.archlinux.org/index.php/working_with_the_serial_console#Resizing_a_terminal. It works as follows:

  1. The cursor position is saved (\0337, DECSC).
  2. The cursors is moved to the most bottom-right point possible (\033[r\033[999;999H, DECSTBM? + CUP).
  3. The cursor position is reported (\033[6n, DSR).
  4. The cursor is moved back to the previously saved position (\0338, DECRC).

• res2 is influenced by resize.sh from xterm (see https://github.com/ThomasDickey/xterm-snapshots/blob/master/vttests/resize.sh). It uses a specific xterm code for getting the information we want (implemented in many terminal emulators), see: http://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h2-Functions-using-CSI-_-ordered-by-the-final-character_s_ ("Report the size of the text area in characters.").

BTW, in my .profile file you will find the following: [ $(tty) = /dev/ttyS0 ] && res so that the the terminal size is determined on every login over the serial line (the one I use for management), e.g. after you reboot the device.
See also the idea by rsaw in the comments to have the line [ $(tty) = /dev/ttyS0 ] && trap res2 DEBUG there instead so the resizing runs after every command (note that AFAIK it's not or not always possible on busybox though).

Answer 2

Just for the record, here is the answer to this Problem (Usenet won):

Console Applications running inside virtual terminal applications (xterm, rxvt and friends) will receive SIGWINCH after a resize operation has taken place. Thus the application will be able to redraw the window etc. in the corresponding signal handler.

Unfortunately when using a serial console, there is no such mechanism.

It is however possible for the application to actively ask for the current console Window size. So the second best thing is to do this every time a command prompt is printed by the shell.

In bash this can be achieved using the following in bashrc:

if [ $(tty) == '/dev/ttyS0' ]; then
  trap resize DEBUG
fi

The actual sourcecode of the resize command can be found here.

Of course this will not change the console size settings in a console application during runtime.

Answer 3

"Resizable" terminals as such are a result of NAWS (Negotiate About Window Size from RFC 1073 Telnet Window Size Option).

If you are connected directly to the computer using a serial port, there is no negotiation involved, and the computer has no direct knowledge of your terminal's screen-size.

If a terminal can negotiate the size, the computer will send SIGWINCH to applications running in the terminal, telling them to update their notion of the screensize.

When the computer does not know the screensize, it typically sets the size shown by stty -a (rows and columns) to zero. For interactive use, this is a little unfriendly, and some systems use environment variables LINES and COLUMNS to help. The values assigned may be derived from the terminal description; more often they are simply hardcoded. The convention for these variables requires that they take effect unless explicitly suppressed, e.g., in curses applications use_env function. On the positive side, those variables can be useful when no reliable information is available. On the negative side, there is no convenient method for altering those variables.

The resize program (a utility provided with xterm) can use the VT100-style cursor position report escape sequence for determining the screen size. This can be run from the command-line; there is (again) no convenient way to do it automatically. As a side-effect, resize updates the information on rows/columns seen by stty. Its use for providing updated environment variables is mainly useful for cases such as this, where LINES and COLUMNS are set, and should be updated.

Answer 4

Here is another solution that worked great for me on my embedded Linux system (Overo running Angstrom). I just ran it from my .bashrc file. I didn't want to use resize because that requires installing some X packages, and I didn't want that.

Telling your Raspberry Pi that your terminal is bigger than 24 lines | Shallow Thoughts Blog

Answer 5

In case you could use FreeBSD instead, there's the resizewin(1) command, which does exactly what you want.

Answer 6

When running a shell session over a serial line it is sufficient to call the resize command inside that session - after establishing the connection and after each terminal geometry change.

The resize command is part of xterm but doesn't depend on X11. For example, on Fedora it's separately packaged as xterm-resize.

How it works: the resize command measures the height/width via some cursor movements and then sends those values to the terminal via escape sequences.

With a shell like zsh this also automagically updates the LINES and COLUMNS variables (alternatively, one can evaluate the export statements the commands prints to stdout).

Why this is necessary: with a local or ssh session the terminal is able to signal the session about geometry changes (cf. SIGWINCH). This mechanism doesn't work over a serial connection.