Check for DECDHL support (DEC double-height lines)

check-decdhl.sh

#!/usr/bin/env bash
# © David Leadbeater, 2025 <http://©.st/dgl>
# SPDX-License-Identifier: 0BSD
#
# Detect whether terminals support DECDHL (DEC double-height lines). This is
# not perfect, as terminals are a mess, to put it mildly.
#
# If you want see the output in your terminal, try: curl -i ip.wtf
#
# Usage:
#   Run directly to see the variables.
#
#   Or, in bash, or a script:
#    . check-decdhl
#    check_decdhl
#    echo $DECDHL
#
# Known issues:
# - Alacritty gets detected as supporting, when it doesn't (it seems to
#   understand that DECDHL has an impact on cursor position, but doesn't
#   render it).
# - Apple Terminal does not implement cursor position correctly, we guess
#   based on the $TERM_PROGRAM environment variable and a heuristic, but this
#   may not always work.
# - xterm before patch #380 will segfault if using TrueType fonts (e.g.
#   xterm -fa Mono; which you need to correctly render the emoji).
# - On Windows there are multiple layers to getting Unicode working correctly,
#   if you use "curl.exe -i ip.wtf" to test you may see Mojibake. Run "chcp
#   65001" first. This only works out of the box on WezTerm on Windows as
#   Windows itself does not ship flag emojis. (Windows Terminal has other
#   issues, but non-flag emojis in the BMP should work.)
#
# Terminals that do support DECDHL:
# - xterm (and so anything claiming TERM=xterm should)
# - VT100 (yes, but obviously not with emojis, etc.)
# - Windows Terminal
# - Apple Terminal
# - MinTTY (no emoji support)
# - WezTerm (with some strange behaviour where it renders \e#3, ignores #4)
#
# Terminals that don't support DECDHL:
# - Ghostty (https://github.com/ghostty-org/ghostty/discussions/3243)
# - iTerm2
# - Kitty (https://github.com/kovidgoyal/kitty/issues/6816, but see
#   https://github.com/kovidgoyal/kitty/blob/master/docs/text-sizing-protocol.rst)
# - Alacritty (but see above)
# - VTE based terminals (strong no: :( https://bugzilla.gnome.org/show_bug.cgi?id=587049)
#

getpos() {
  local r
  printf '\e[6n' > /dev/tty
  read -d R r
  echo "${r/*;}"
}

check_decdhl() {
  local pos

  if [[ -z "$COLUMNS" ]]; then
    printf '\e[10000G'
    # Note if you run bash interactively, it sets COLUMNS for you.
    # But set it ourselves if not set so this works in scripts too.
    COLUMNS=$(getpos)
    printf '\e[A'
  fi

  # Does this terminal render emoji as double width?
  printf '\e[G\e[K✅'
  pos=$(getpos)
  if [[ $pos = 3 ]]; then
    EMOJI=1
  else
    EMOJI=0
  fi

  # Does this terminal support double height fonts?
  printf '\e[G\e[2K\e#3'
  eval "printf ' %.0s' {1..$[1+$COLUMNS/2]}"
  pos=$(getpos)
  if [[ $pos = 2 ]]; then
    # Yes, xterm style reporting (line wrapped)
    DECDHL=1
    # Delete extra line
    printf '\e[M\e[F'
  elif [[ $pos = $[2+$COLUMNS/2] ]]; then
    # Correct cursor position for non-supporting, but still could be Apple
    # Terminal.
    if [[ $TERM_PROGRAM = Apple_Terminal ]]; then
      DECDHL=1
      DECDHL_EMOJI=1
      # Reset state so there is no output left.
      printf '\e[M\e[G\e[2K'
      return
    else
      # It could still be Apple Terminal, but without the environment variable.
      printf '\e[G\e[2Kx\e[2b🤗\e[2b'
      # Expect "xxx🤗🤗🤗", but Apple Terminal renders. "xxx🤗[cursor]" => 6, if REP
      # isn't supported at all the result is 4.
      # Windows Terminal does not get here, but if it did it may trip:
      # https://github.com/microsoft/terminal/issues/15390
      pos=$(getpos)
      if [[ $pos = 6 ]]; then
        # Apple terminal heuristic
        DECDHL=1
        DECDHL_EMOJI=1
        # Reset state so there is no output left.
        printf '\e[M\e[G\e[2K'
        return
      fi
      DECDHL=0
    fi
  else
    # Strange answer
    DECDHL=0
  fi

  printf '\e[G\e[2K\e#3'
  eval "printf '☑️%.0s' {1..$[1+$COLUMNS/4]}"
  pos=$(getpos)
  if [[ $pos = 3 ]]; then
    # Yes, xterm style reporting (line wrapped)
    DECDHL_EMOJI=1
    # Delete extra line
    printf '\e[M\e[F'
  elif [[ $pos = $[3+$COLUMNS/2] ]]; then
    # Correct cursor position for non-supporting, Apple was checked above.
    DECDHL_EMOJI=0
  else
    # Strange answer
    DECDHL_EMOJI=0
  fi

  # Reset state so there is no output left.
  printf '\e[M\e[G\e[2K'
}

if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
  check_decdhl
  echo "EMOJI=$EMOJI, DECDHL=$DECDHL, DECDHL_EMOJI=$DECDHL_EMOJI"
fi