Alexander Gromnitsky's Blog

XTGETTCAP

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When the authors of a new terminal emulator (TE) decide which capabilities to support, they inevitably end up with a subset of XTerm's features. Then, much like a non-mainstream web browser hiding its true identity behind a fake User-Agent request header, the new TE often claims to be a variant of xterm. This frustrates the current XTerm maintainer & the maintainers of the terminfo database, who insist that every TE should have its own entry in terminfo, and that it's always better to esse quam videri. So far, TE authors have largely ignored this prudent advice.

Sometimes, the ability to deceive programs that rely on the TERM environment variable can be useful. Suppose you want to display an infobox to a user via dialog(1), but you want it to use a monochrome palette. The dialog program doesn't have a flag for this, but we can trick it into thinking it's running in a terminal that doesn't support colours:

$ TERM=vt100 dialog --infobox 'ну шо ти малá' 0 0

This works because the dialog(1) program uses the ncurses library, which in turn relies on the terminfo database. The latter includes an entry for the vt100 terminal, that, according to terminfo, is fairly unsophisticated:

$ TERM=vt100 tput colors
-1

To get a full list of such simpletons, run:

$ find /usr/share/terminfo -type f |
    xargs -n1 basename |
    while read line; do [ `tput -T $line colors` = -1 ] && echo $line; done

In general, CLI programs that use terminal features beyond simply deleting the previous character fall into 2 categories--those that:

  1. consult terminfo for supported capabilities;
  2. include their own little database of compatible TE.

Both rely on $TERM, hence, both can be easily lead astray.

There is a 3rd way: ask the terminal emulator (TE) to report its capabilities using escape sequences. This mechanism, called XTGETTCAP, is slowly gaining popularity among TEs. Obviously, it "works over ssh" & doesn't require terminfo to be installed on the remote machine. XTGETTCAP was first introduced by XTerm1 and is now supported by Kitty and iTerm2. With this mechanism, the value of $TERM becomes less relevant:

$ echo $TERM
xterm-256color
$ TERM=vt100 ./XTGETTCAP TN
xterm-256color
$ ./XTGETTCAP colors
256

Alas, there is no widely known CLI utility named XTGETTCAP--in the example above, it's just a ~small shell script. Even more unfortunate is that doing a proper XTGETTCAP query & reading the response is a tricky business. You need to:

  1. Construct a query using an escape sequence & hexify the capability name, e.g., colors becomes 636f6c6f7273.

  2. Switch the TE from canonical to raw mode.

  3. Handle raw mode correctly. In this mode, the TE doesn't assemble characters into lines, so we can't rely on it to return everything up to one line at a time. Instead, we must either request a response of a known size (we don't know the size!) or read an unknown-length response with a timeout. If you royally screw this up, your read operation may block ∞.

  4. Restore canonical mode.

  5. Unpack the reply.

#!/bin/sh

set -e

eh() { echo "Error: $*" 1>&2; trap - 0; exit 1; }
text2hex() { od -A n -t x1 | tr -d ' \n'; }
hex2text() { sed 's/../0x& /g' | xargs printf '\\\\%03o\n' | xargs printf %b; }
stty_orig=`stty -g`
stty_restore() { stty "$stty_orig"; }

[ -n "$1" ] || eh Usage: XTGETTCAP capability

capablity=`printf '%s' "$1" | text2hex`
trap stty_restore 0

stty -echo raw time 1 min 0
printf '\033P+q%s\033'\\ "$capablity" > /dev/tty
buf=`dd status=none count=1`
stty_restore

[ -n "$buf" ] || eh unsupported terminal
[ "$V" ] && printf %s "$buf" | xxd

buf=`printf %s "$buf" | sed 's/[^a-zA-Z0-9+=]//g'`
[ P1 = "${buf%+*}" ] || eh unknown capabilitah
printf %s "${buf#*=}" | hex2text | xargs

A couple of notes:

  • the script doesn't work under screen/tmux, even if an underline TE supports the XTGETTCAP mechanism.

  • if you decide to fiddle with time and min settings, think twice; here's a matrix from APUE, ch. 18:

  • hex2text() may look silly, but it works even under FreeBSD (unless the source contains encoded null bytes) & doesn't rely on non-standard utilities like xxd;

  • in principle, dd ... call can be replaced with a simple cat.

$ ./XTGETTCAP TN
xterm-kitty
$ VERBOSE=1 ./XTGETTCAP colors
00000000: 1b50 312b 7236 3336 6636 6336 6637 3237  .P1+r636f6c6f727
00000010: 333d 3332 3335 3336 1b5c                 3=323536.\
256
$ ./XTGETTCAP шо?
Error: unknown capabilitah
  1. Requires allowTcapOps resource to be true. By default, it's off on all IBM distros due to security concerns (e.g., see CVE-2022-45063).

Tags: ойті
Authors: ag