Linux – Suitables TERM variable values for a serial console

consolelinuxserial portserial-console

To enable a serial console on Linux, one uses getty (most usually, its variant agetty). This binary takes as argument, among other, the value to initialize the TERM variable with.

On Debian, with Sys V init, the default was vt100. With systemd, the default used to be vt102, and nowadays it's vt220.

After playing a bit with QEMU virtual machines and virt-viewer, as well as virsh console command, I noticed some things:

With vt100, ls --color displays colors, but vim's syntax highlighting doesn't work
with vt102 or vt220, neither of them displays colors
Only with TERM variable set to linux, do both ls and vim use colors

So I guess that independently of the actual "color support", each application looks at the TERM variable and acts accordingly, which would explain the differences noted above.

After reading the Serial Console HOWTO, I understand that the value of the TERM variable should depend on the actual model of physical terminal which would be connected to the serial port, according to its capabilities.

Note that, according to Lennart Poettering's blog, TERM should be set to linux only with real virtual terminals (as opposed to serial ones). On the other hand, Arch Linux' Wiki doesn't seem to mind (see the /etc/inittab lines it proposes).

So my questions are:

In a general case, what happens if the TERM variable is set to linux on a console connected to a less-capable terminal, like a DEC VT100, VT102 or VT220, or some RS-232 software terminal emulators like minicom or termite ?

More realistically (in my particular case), is it OK to set the TERM variable to linux in a "virtual" serial console on a QEMU VM, to which I will connect through virt-viewer or virsh console ?

Best Answer

The TERM setting tells the application program what capabilities the terminal it is communicating with has, and how to utilize these capabilities (typically via a library like ncurses). In plain English: it tells what control sequences (escape sequences) it should send to move the cursor arount the screen, to change the text color, how to erase a region of the screen, what sequences the function keys transmit, etc. Some of these capabilities may be missing, like color support.

Most of the terminal types in use today are somehow related to the "grand daddy" of "glass ttys", the DEC VT100. This is why terminal types are mostly interchangeable, so setting the wrong type typcally results in a mostly working setup, but with some glitches.

So, to answer the questions "which should I use" and "what happens if I use the wrong setting"? Some control sequences may be mismatched, i.e. the program sends cursor movement sequences that differ from the ones the terminal emulator expects. Or color support is missing. (Btw. the original VT100 did definitely not support color...) The correct setting should be provided by the terminal emulator documentation, but there is no harm in experimenting to see which setting works the best. It's OK to use "linux" if it works for you.

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Try not exporting the TERM variable at all. This should be set by the terminal itself to an appropriate value. The linux console should set this to linux, while various X based terminal programs might use the value you set.

You should only set this as an override for when the default values do not work or don't get passed as when SSH'ing to a location that does not recognize your environment.

Resizable serial console window

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).

Best Answer

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Resizable serial console window

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