In lots of places, depending
On virtual terminals and real terminals, the TERM environment variable is set by the program that chains to login, and is inherited all of the way along to the interactive shell that executes once one has logged on. Where, precisely, this happens varies from system to system, and according to the kind of terminal.
real terminals
Real, serial, terminals can vary in type, according to what's at the other end of the wire. So conventionally the getty program is invoked with an argument that specifies the terminal type, or is passed the TERM program from a service manager's service configuration data.
systemd's variability
The serial-getty@.service service unit file, or drop-in files that apply thereto, is where to change the terminal type for real terminals on systemd systems. Note that such a change applies to all terminal login services that employ this service unit template. (To change it for only individual terminals, one has to manually instantiate the template, or add drop-ins that only apply to instantiations.)
systemd has had at least four mechanisms during its lifetime for picking up the value of the TERM environment variable. At the time of first writing this answer, as can be seen, there was an Environment=TERM=something line in the template service unit files. At other times, the types linux and vt102 were hard-wired into the getty and serial-getty service unit files respectively. More recently, the environment variable has been inherited from process #1, which has set it in various ways.
As of 2020, the way that systemd decides what terminal type to specify in a service's TERM environment variable is quite complex, and not documented at all. The way to change it remains a drop-in configuration file with Environment=TERM=something. But where the default value originates from is quite variable. Subject to some fairly complex to explain rules that involve the TTYPath= settings of individual service units, it can be one of three values: a hardwired linux, a hardwired vt220 (no longer vt102), or the value of the TERM environment variable that process #1 inherited, usually from the kernel/bootstrap loader.
(Ironically, the getttyent() mechanism still exists in the GNU C library, and systemd could have re-used the /etc/ttys mechanism.)
kernel virtual terminals
Kernel virtual terminals, as you have noted, have a fixed type. Unlike NetBSD, which can vary the kernel virtual terminal type on the fly, Linux and the other BSDs have a single fixed terminal type implemented in the kernel's built-in terminal emulation program. On Linux, that type matches linux from the terminfo database. (FreeBSD's kernel terminal emulation since version 9 has been teken. Prior to version 9 it was cons25 OpenBSD's is pccon.)
- On systems using
mingetty or vc-get-tty (from the nosh package) the program "knows" that it can only be talking to a virtual terminal, and they hardwire the "known" virtual terminal types appropriate to the operating system that the program was compiled for.
- On systemd systems, one used to be able to see this in the
/usr/lib/systemd/system/getty@.service unit file (/lib/systemd/system/getty@.service on un-merged systems), which read Environment=TERM=linux
setting the TERM variable in the environment passed to agetty.
For kernel virtual terminals, one does not change the terminal type. The terminal emulator program in the kernel doesn't change, after all. It is incorrect to change the type. In particular, this will screw up cursor/editing key CSI sequence recognition. The linux CSI sequences sent by the Linux kernel terminal emulator are different to the xterm or vt100 CSI sequences sent by GUI terminal emulator programs in DEC VT mode. (In fact, they are highly idiosyncratic and non-standard, and different both to all real terminals that I know of, and to pretty much all other software terminal emulators apart from the one built into Linux.)
GUI terminal emulators
Your GUI terminal emulator is one of many programs, from the SSH dæmon to screen, that uses pseudo-terminals. What the terminal type is depends from what terminal emulator program is running on the master side of the pseudo-terminal, and how it is configured. Most GUI terminal emulators will start the program on the slave side with a TERM variable whose value matches their terminal emulation on the master side. Programs like the SSH server will attempt to "pass through" the terminal type that is on the client end of the connection. Usually there is some menu or configuration option to choose amongst terminal emulations.
The gripping hand
The right way to detect colour capability is not to hardwire a list of terminal types in your script. There are an awful lot of terminal types that support colour.
The right way is to look at what termcap/terminfo says about your terminal type.
colour=0
if tput Co > /dev/null 2>&1
then
test "`tput Co`" -gt 2 && colour=1
elif tput colors > /dev/null 2>&1
then
test "`tput colors`" -gt 2 && colour=1
fi
Further reading
- Jonathan de Boyne Pollard (2018).
TERM. nosh Guide. Softwares.
Best Answer
Well, first I guess I would point out that pretty much all terminals these days are "virtual" in the sense you talk about... even if the terminal is at the other end of a bona fide serial port. I mean, the days of VT-100s, Wyse terminals and other "physical", "real" terminals are pretty much gone!
That aside, let's say you want to detect what kind of Unicode support your terminal has. You can do this by writing test characters to is and seeing what happens. (You can make an effort to erase the test characters after you've written then, but the user may still see them briefly, or erasing them might not work properly in the first place.)
The idea is to ask the terminal to tell you its cursor position, output a test character, ask the terminal again to tell you its position, and compare the two positions to see how far the terminal's cursor moved.
To ask the terminal for its position, see here. Essentially:
Try outputting "é". This character takes 2 bytes in UTF-8 but displays in only one column on the screen. If you detect that outputting "é" causes the cursor to move by 2 positions, then the terminal has no UTF-8 support at all and has probably output some kind of garbage. If the cursor didn't move at all, then then terminal is probably ASCII only. If it moved by 1 position, then congratulations, it can probably display French words.
Try outputing "あ". This character takes 3 bytes in UTF-8 but displays in only two columns on the screen. If the cursor moves by 0 or 3, bad news, similar to above. If it moves by 1, then it looks like the terminal supports UTF-8 but doesn't know about wide characters (in fixed-width fonts). If it moves by 2 columns, all is good.
I'm sure there are other probe characters that you could emit which would lead to useful information. I am not aware of a tool that does this automatically.