contrib: Add bash completion for new bitcoin command

diff --git a/contrib/completions/bash/bitcoin.bash b/contrib/completions/bash/bitcoin.bash
new file mode 100644
index 00000000..a2ecae03
--- /dev/null
+++ b/contrib/completions/bash/bitcoin.bash
@@ -0,0 +1,100 @@
+# bash programmable completion for bitcoin(1) wrapper
+# Copyright (c) 2026-present The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+# wrapper to delegate completion to the real function
+_bitcoin_wrap() {
+    local delegate="$1" shift_count="$2"
+    local func="_${delegate//-/_}"
+    local words cword dir file
+
+    # load completion script if function not yet defined
+    if ! declare -F $func >/dev/null; then
+        dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+        file="$(grep -l -E "complete[[:space:]]+-F[[:space:]]+${func}" "$dir"/* 2>/dev/null | head -n1)"
+        [ -z "$file" ] && file="$(grep -l -E "^[[:space:]]*${func}[[:space:]]*\(\)" "$dir"/* 2>/dev/null | head -n1)"
+        [ -n "$file" ] && source "$file" || return 0
+    fi
+
+    _get_comp_words_by_ref -n = words cword
+
+    COMP_WORDS=( "$delegate" "${words[@]:$shift_count}" )
+    COMP_CWORD=$(( cword - (shift_count - 1) ))
+    COMP_LINE="${COMP_WORDS[@]}"
+    COMP_POINT=${#COMP_LINE}
+    $func "$delegate"
+}
+
+_bitcoin() {
+    local cur prev words cword
+    local bitcoin subcmd offset
+
+    # save and use original argument to invoke bitcoin for help
+    # it might not be in $PATH
+    bitcoin="$1"
+
+    COMPREPLY=()
+    _get_comp_words_by_ref -n = cur prev words cword
+
+    case "${words[1]}" in
+        -m|-M|--multiprocess|--monolithic)
+            subcmd="${words[2]}"
+            offset=3
+            ;;
+        *)
+            subcmd="${words[1]}"
+            offset=2
+            ;;
+    esac
+
+    case "$subcmd" in
+        gui|node)
+            _bitcoin_wrap bitcoind "$offset"
+            return 0
+            ;;
+        rpc)
+            _bitcoin_wrap bitcoin-cli "$offset"
+            return 0
+            ;;
+        tx)
+            _bitcoin_wrap bitcoin-tx "$offset"
+            return 0
+            ;;
+    esac
+
+    case "$cur" in
+        -*=*)   # prevent nonsense completions
+            return 0
+            ;;
+        *)
+            local options commands
+
+            # only parse help if sensible
+            if [[ -z "$cur" || "$cur" =~ ^- ]]; then
+                options=$($bitcoin help 2>&1 | awk '{ for(i=1;i<=NF;i++) if ($i~/^--/) { sub(/=.*/, "=",$i); print $i } }' )
+            fi
+            if [[ -z "$cur" || "$cur" =~ ^[a-z] ]]; then
+                commands=$($bitcoin help 2>/dev/null | awk '$1 ~ /^[a-z]/ { print $1; }')
+            fi
+
+            COMPREPLY=( $( compgen -W "$options $commands" -- "$cur" ) )
+
+            # Prevent space if an argument is desired
+            if [[ $COMPREPLY == *= ]]; then
+                compopt -o nospace
+            fi
+            return 0
+            ;;
+    esac
+
+} &&
+complete -F _bitcoin bitcoin
+
+# Local variables:
+# mode: shell-script
+# sh-basic-offset: 4
+# sh-indent-comment: t
+# indent-tabs-mode: nil
+# End:
+# ex: ts=4 sw=4 et filetype=sh