From a4bfa48cdda924f7a77a81e8d8e93e326a01b8a0 Mon Sep 17 00:00:00 2001
From: Kris Yotam <75515498+krisyotam@users.noreply.github.com>
Date: Thu, 2 Jul 2026 00:11:17 -0500
Subject: [PATCH] mksh: reliable history hook via HISTFILE growth + binary
 record parse

PS1 command substitution runs in a subshell so hook state must be
persisted to disk (per-PID file); fc there is lagged one command and
drops each session's last command. Detect new commands by HISTFILE
growth and parse the last record from mksh's on-disk format.
---
 .mkshrc | 37 ++++++++++++++++++++++++++-----------
 1 file changed, 26 insertions(+), 11 deletions(-)

diff --git a/.mkshrc b/.mkshrc
index 4e9069bfda14ef010b12eadbf07602f632247c75..6b9e9984f75f2552838a6d67979c2305348b1d89 100644
--- a/.mkshrc
+++ b/.mkshrc
@@ -45,18 +45,33 @@ if [ -n "$BASH_VERSION" ]; then
 	trap '_kh_last_cmd=$(fc -ln -1 2>/dev/null | sed "s/^[[:space:]]*//")' DEBUG
 	PROMPT_COMMAND='_kh_log "$_kh_last_cmd" "$?"'
 else
-	# mksh: no DEBUG trap / PROMPT_COMMAND. Log from PS1 each prompt.
-	# The leading $(...) captures $? first, so the exit code is accurate,
-	# and _kh_prompt emits nothing so the visible prompt is unchanged.
-	# Skip the first render: at shell start fc -ln -1 is a stale HISTFILE
-	# entry, not a command run this session.
-	_kh_first_prompt=1
+	# mksh: no DEBUG trap / PROMPT_COMMAND, and PS1 command substitution
+	# runs in a subshell, so hook state cannot live in shell variables --
+	# it is persisted to a per-PID file instead. We detect that a new
+	# command ran by watching HISTFILE grow, then read the last entry
+	# straight from the file (fc in the PS1 subshell is lagged by one and
+	# would drop the final command of every session).
+	#
+	# mksh HISTFILE record layout: 0xFF, 4-byte big-endian counter,
+	# command bytes, NUL. So split on 0xFF, take the last record, drop the
+	# 4 counter bytes (cut -b5-), strip the trailing NUL.
+	[ -d "$HOME/.cache" ] || mkdir -p "$HOME/.cache"
+	_kh_state="$HOME/.cache/kh_mksh_$$"
 	_kh_prompt() {
-		if [ -n "$_kh_first_prompt" ]; then
-			unset _kh_first_prompt
-			return
-		fi
-		_kh_log "$(fc -ln -1 2>/dev/null | sed 's/^[[:space:]]*//')" "$1"
+		typeset ec="$1" sz prev cmd
+		[ -f "$HISTFILE" ] || return
+		sz=$(wc -c < "$HISTFILE" 2>/dev/null)
+		prev=$(cat "$_kh_state" 2>/dev/null)
+		print -r -- "$sz" > "$_kh_state"
+		# first prompt of the session: record baseline, do not log
+		[ -z "$prev" ] && return
+		# only log when a new command was appended to history
+		[ "$sz" -gt "$prev" ] 2>/dev/null || return
+		cmd=$(tail -c 4096 "$HISTFILE" | tr "\377" "\n" | tail -n1 | cut -b5- | tr -d "\0")
+		_kh_log "$cmd" "$ec"
 	}
+	# leading $(...) captures $? first so the exit code is accurate; the
+	# hook prints nothing so the visible prompt is unchanged
 	PS1='$(_kh_prompt "$?")'"$PS1"
+	trap 'rm -f "$_kh_state"' EXIT
 fi