From 81f2dbc142bc5bedf19fa1f301ca486d10d127c3 Mon Sep 17 00:00:00 2001
From: Callum Andrew <contact@candrew.net>
Date: Sat, 7 Jun 2025 14:20:39 +1000
Subject: [PATCH 1/2] fix(scripts/chsh.in): allow passing absolute path to
 shell

chsh first checks for a shell using its absolute path (if it starts with
`/`), before checking in `$PREFIX/bin/`
---
 scripts/chsh.in | 63 +++++++++++++++++++++++++++++++++++++------------
 1 file changed, 48 insertions(+), 15 deletions(-)

diff --git a/scripts/chsh.in b/scripts/chsh.in
index b74f58e..32702b0 100644
--- a/scripts/chsh.in
+++ b/scripts/chsh.in
@@ -1,24 +1,58 @@
 #!/bin/sh
+# shellcheck shell=dash
 
-set -e -u
+set -eu
 
-show_usage () {
+is_executable_file() {
+	test -f "$1" -a -x "$1"
+}
+
+show_usage() {
 	echo "usage: chsh [-s shell]"
 	echo "Change the login shell."
+	echo
+	if [ "${1:-}" = "login" ]; then
+		echo "The shell value must be one of following and cannot be 'login':"
+	else
+		echo "The shell value must be one of following:"
+	fi
+	echo " - Empty value to restore default shell."
+	echo " - Absolute path to shell starting with a '/'."
+	echo " - Relative path to shell not starting with a '/' relative to '@TERMUX_PREFIX@/bin'."
 }
 
-set_shell () {
-	if [ "$1" = login ]; then
-		echo "login is not a valid shell"
+set_shell() {
+	if [ -z "${1:-}" ]; then
+		echo "Restoring default shell"
+		rm -f "$HOME/.termux/shell"
+		exit 0
+	fi
+
+	if [ "$1" = "login" ]; then
+		show_usage "login"
 		exit 1
 	fi
-	mkdir -p $HOME/.termux
-	NEW_SHELL=@TERMUX_PREFIX@/bin/$1
-	if test -x $NEW_SHELL -a ! -d $NEW_SHELL; then
-		ln -f -s $NEW_SHELL $HOME/.termux/shell
-	else
-		echo "$NEW_SHELL is not an executable file!"
+	mkdir -p "$HOME/.termux"
+
+	unset NEW_SHELL
+	case "$1" in
+		"/"*) NEW_SHELL="$1" ;;
+		*) NEW_SHELL="@TERMUX_PREFIX@/bin/$1" ;;
+	esac
+
+	SHELL_TARGET="$(realpath -s "$NEW_SHELL")"
+
+	if ! is_executable_file "$SHELL_TARGET"; then
+		echo "The shell file '$SHELL_TARGET' is not an executable file." 1>&2
+		exit 1
 	fi
+
+	if [ "$SHELL_TARGET" -ef "@TERMUX_PREFIX@/bin/login" ]; then
+		echo "The shell file '$SHELL_TARGET' must not point to the '@TERMUX_PREFIX@/bin/login' script." 1>&2
+		exit 1
+	fi
+
+	ln -sf "$SHELL_TARGET" "$HOME/.termux/shell"
 }
 
 O=`getopt -l help -- hs: "$@"`
@@ -26,19 +60,18 @@ eval set -- "$O"
 while true; do
 	case "$1" in
 		-h|--help) show_usage; exit 0;;
-		-s) set_shell $2; exit 0;;
+		-s) set_shell "$2"; exit 0;;
 		--)	shift; break;;
 		*)	echo Error; show_usage; exit 1;;
 	esac
 done
 
 DEFAULT_SHELL=bash
-if [ ! -x @TERMUX_PREFIX@/bin/$DEFAULT_SHELL ]; then DEFAULT_SHELL=sh; fi
+if ! is_executable_file "@TERMUX_PREFIX@/bin/$DEFAULT_SHELL"; then DEFAULT_SHELL=sh; fi
 
 echo Changing the login shell
 echo Enter the new value, or press ENTER for the default
 printf "        Login Shell [$DEFAULT_SHELL]: "
 read shell
 
-if [ -z "$shell" ]; then shell=$DEFAULT_SHELL; fi
-set_shell $shell
+set_shell "$shell"

From 50528670dae323b49d76e3ff3e71b68f6701eb11 Mon Sep 17 00:00:00 2001
From: Callum Andrew <contact@candrew.net>
Date: Tue, 10 Jun 2025 12:15:17 +1000
Subject: [PATCH 2/2] fix(scripts/login.in): don't canonicalize symlink value

also allow shells owned by different effective group IDs
---
 scripts/login.in | 39 ++++++++++++++++++++++++++++++---------
 1 file changed, 30 insertions(+), 9 deletions(-)

diff --git a/scripts/login.in b/scripts/login.in
index 2e7aa79..e862089 100644
--- a/scripts/login.in
+++ b/scripts/login.in
@@ -1,4 +1,18 @@
 #!/bin/sh
+# shellcheck shell=dash
+
+is_executable_file() {
+	test -f "$1" -a -x "$1"
+}
+
+set_default_shell() {
+	for file in "@TERMUX_PREFIX@/bin/bash" "@TERMUX_PREFIX@/bin/sh" "/system/bin/sh"; do
+		if is_executable_file "$file"; then
+			SHELL="$file"
+			break
+		fi
+	done
+}
 
 if tty >/dev/null 2>&1 && [ $# = 0 ] && [ ! -f ~/.hushlogin ] && [ -z "$TERMUX_HUSHLOGIN" ]; then
 	# Use user defined dynamic motd file if it exists
@@ -19,17 +33,24 @@ if tty >/dev/null 2>&1 && [ $# = 0 ] && [ -f @TERMUX_PREFIX@/etc/motd-playstore
 	printf '\033[0;31m'; cat @TERMUX_PREFIX@/etc/motd-playstore; printf '\033[0m'
 fi
 
-if [ -G ~/.termux/shell ]; then
-	export SHELL="`realpath ~/.termux/shell`"
-else
-	for file in @TERMUX_PREFIX@/bin/bash @TERMUX_PREFIX@/bin/sh /system/bin/sh; do
-		if [ -x $file ]; then
-			export SHELL=$file
-			break
-		fi
-	done
+unset SHELL
+SHELL_FILE=~/.termux/shell
+
+if [ -L "$SHELL_FILE" ]; then
+	# Set the symlink's value as the shell target.
+	SHELL_TARGET="$(readlink "$SHELL_FILE")"
+	if is_executable_file "$SHELL_TARGET" &&
+		[ ! "$SHELL_TARGET" -ef "@TERMUX_PREFIX@/bin/login" ]; then
+		SHELL="$SHELL_TARGET"
+	fi
 fi
 
+if [ -z "${SHELL:-}" ]; then
+	set_default_shell
+fi
+
+export SHELL
+
 # TERMUX_APP_PACKAGE_MANAGER should be exported by termux-app v0.119.0+ itself
 if [ -z "${TERMUX_APP_PACKAGE_MANAGER-}" ]; then
 	if { [ -n "$(command -v dpkg)" ] && dpkg --compare-versions "$TERMUX_VERSION" lt 0.119.0; } ||   # apt