Why must default-terminal be ‘screen’, ‘tmux’ or a derivative of them? Is this still true today?

[its-kostya]

tmux(1) states

default-terminal terminal
       Set the default terminal for new windows created in this session -
       the default value of the TERM environment variable.  For tmux to
       work correctly, this must be set to ‘screen’, ‘tmux’ or a deriva‐
       tive of them.

It is my understanding that $TERM informs applications about the capabilities of the terminal which can affect various behaviors such as color support, control sequences, function keys and mouse support, and other terminal features.

terminfo is a database and a set of libraries to describe the capabilities and characteristics of various terminal types. Applications lookup $TERM in terminfo db and can dynamically adjust their output based on the terminal's capabilities.

I'd like to better understand, say there is a program call foo. When foo starts, it will use the env variable $TERM to determine the terminal and then look up the terminal's terminfo. Here is an strace from foo:

newfstatat(AT_FDCWD, "/home/user/.terminfo", 0x58d4fbc6a020, 0) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "/etc/terminfo", {st_mode=S_IFDIR|0755, st_size=4096, ...}, 0) = 0
newfstatat(AT_FDCWD, "/lib/terminfo", {st_mode=S_IFDIR|0755, st_size=4096, ...}, 0) = 0
newfstatat(AT_FDCWD, "/usr/share/terminfo", {st_mode=S_IFDIR|0755, st_size=4096, ...}, 0) = 0
access("/etc/terminfo/a/alacritty", R_OK) = 0
openat(AT_FDCWD, "/etc/terminfo/a/alacritty", O_RDONLY) = 3
newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=3652, ...}, AT_EMPTY_PATH) = 0
read(3, "\32\1&\0&\0\17\0\235\1\n\6alacritty|alacritty "..., 32768) = 3652
read(3, "", 28672)                      = 0
close(3)

If I am using a terminal like alacritty, foo wants alacritty's terminfo. But if I launch tmux from my alacritty terminal, doesn't tmux make more instances of alacritty? Shouldn't foo want alacritty's terminfo, or tmux's?

Or, does foo read tmux-256color terminfo so it knows how to output into tmux and tmux reads alacritty's terminfo so that it can output to alacritty? So an output from foo will be foo > tmux > alacritty.

The capabilities between tmux-256color and alacritty and pretty significant and I am wondering if I cannot set default-terminal to be alacritty if I will be giving up some capabilities.

user ~$ infocmp -d tmux-256color alacritty
comparing tmux-256color to alacritty.
    comparing booleans.
        bce: F:T.
        ccc: F:T.
        mc5i: F:T.
        npc: F:T.
    comparing numbers.
        pairs: 65536, 32767.
    comparing strings.
        acsc: '++\,\,--..00``aaffgghhiijjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~', '``aaffggiijjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~'.
        blink: '\E[5m', NULL.
        clear: '\E[H\E[J', '\E[H\E[2J'.
        cnorm: '\E[34h\E[?25h', '\E[?12l\E[?25h'.
        cuu1: '\EM', '\E[A'.
        cvvis: '\E[34l', '\E[?12;25h'.
        dsl: '\E]0;\007', '\E]2;\007'.
        ech: NULL, '\E[%p1%dX'.
        enacs: '\E(B\E)0', NULL.
        flash: '\Eg', '\E[?5h$<100/>\E[?5l'.
        initc: NULL, '\E]4;%p1%d;rgb:%p2%{255}%*%{1000}%/%2.2X/%p3%{255}%*%{1000}%/%2.2X/%p4%{255}%*%{1000}%/%2.2X\E\\'.
        is2: '\E)0', '\E[!p\E[?3;4l\E[4l\E>'.
        kb2: NULL, '\EOE'.
        kend: '\E[4~', '\EOF'.
        kent: NULL, '\EOM'.
        khome: '\E[1~', '\EOH'.
        mc0: NULL, '\E[i'.
        mc4: NULL, '\E[4i'.
        mc5: NULL, '\E[5i'.
        nel: '\EE', NULL.
        oc: NULL, '\E]104\007'.
        rep: NULL, '%p1%c\E[%p2%{1}%-%db'.
        rin: NULL, '\E[%p1%dT'.
        rmacs: '^O', '\E(B'.
        rmam: NULL, '\E[?7l'.
        rmcup: '\E[?1049l', '\E[?1049l\E[23;0;0t'.
        rmm: NULL, '\E[?1034l'.
        rs1: NULL, '\Ec\E]104\007'.
        rs2: '\Ec\E[?1000l\E[?25h', '\E[!p\E[?3;4l\E[4l\E>'.
        sgr: '\E[0%?%p6%t;1%;%?%p2%t;4%;%?%p1%p3%|%t;7%;%?%p4%t;5%;%?%p5%t;2%;%?%p7%t;8%;m%?%p9%t\016%e\017%;', '%?%p9%t\E(0%e\E(B%;\E[0%?%p6%t;1%;%?%p5%t;2%;%?%p2%t;4%;%?%p1%p3%|%t;7%;%?%p4%t;5%;%?%p7%t;8%;m'.
        sgr0: '\E[m\017', '\E(B\E[m'.
        smacs: '^N', '\E(0'.
        smam: NULL, '\E[?7h'.
        smcup: '\E[?1049h', '\E[?1049h\E[22;0;0t'.
        smm: NULL, '\E[?1034h'.
        tsl: '\E]0;', '\E]2;'.
        u8: '\E[?1;2c', '\E[?%[;0123456789]c'.

Perhaps I am over complicating things, but I am trying to understand how things work. I'd like to know why I need the following in my tmux.conf

set -g default-terminal "tmux-256color"
set -ag terminal-overrides ",$TERM:RGB"

instead of

set -g default-terminal "$TERM"
set -ag terminal-overrides ",$TERM:RGB"

[nicm]

I'm not sure how you can write your first two paragraphs and not know why TERM needs to be set correctly. If TERM tells the application to clear the terminal by sending foo but the terminal expects to receive bar to clear, obviously the application will not be able to clear the terminal and may in fact be asking the terminal to do something entirely different when it tries.

Setting TERM to alacritty inside tmux is likely to mostly work because tmux and alacritty have many commonalties and are both based on the protocol originally designed for the DEC VT100 and later DEC terminals. But you may find that some features of some applications do not work, or work incorrectly.

[its-kostya]

Well, I'm still learning and I have a knowledge gap which I am trying to close.

What trips me up is the introduction of another layer of abstraction between what is displayed and the running application (say a text editor). If the application wants to flash the screen and tmux expects foo but alacritty expects bar, what does the application send? I expect the application sends foo which is interpreted by tmux and then tmux send bar to alacritty?

[nicm]

Right. tmux is a terminal that happens to run inside another terminal.

[its-kostya]

Awesome, I think I have enough of a mental model of how this works - thanks!

I've also dug up that default-terminal screen and tmux are acceptable because tmux is derived from screen : https://invisible-island.net/ncurses/terminfo.src.html#t20150502 ; cool!

Kinda of crazy that tmux's terminal description has been part of the terminal database since only 2015. I thought it was way more OG; certainly feels that way due to its ubiquity.