diff --git a/.gitignore b/.gitignore
index a205373..af04dc8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,6 @@
 *.pyc
 *.tmp
 /redo-sh
+*.did
+.do_built
+.do_built.dir
diff --git a/minimal/default.zz.do b/minimal/default.zz.do
new file mode 100644
index 0000000..397db75
--- /dev/null
+++ b/minimal/default.zz.do
@@ -0,0 +1 @@
+:
diff --git a/minimal/do b/minimal/do
index 1794b48..61889d9 100755
--- a/minimal/do
+++ b/minimal/do
@@ -28,22 +28,24 @@ if [ -n "$TERM" -a "$TERM" != "dumb" ] && tty <&2 >/dev/null 2>&1; then
 	plain="$(printf '\033[m')"
 fi
 
+# Split $1 into a dir part ($_dirsplit_dir) and base filename ($_dirsplit_base)
 _dirsplit()
 {
-	base=${1##*/}
-	dir=${1%$base}
+	_dirsplit_base=${1##*/}
+	_dirsplit_dir=${1%$_dirsplit_base}
 }
 
+# Like /usr/bin/dirname, but avoids a fork and uses _dirsplit semantics.
 dirname()
 (
 	_dirsplit "$1"
-	dir=${dir%/}
+	dir=${_dirsplit_dir%/}
 	echo "${dir:-.}"
 )
 
 _dirsplit "$0"
-export REDO=$(cd "${dir:-.}" && echo "$PWD/$base")
-if [ "$base" = "redo-ifchange" ]; then ifchange=1; else ifchange=; fi
+export REDO=$(cd "${_dirsplit_dir:-.}" && echo "$PWD/$_dirsplit_base")
+_cmd=$_dirsplit_base
 
 DO_TOP=
 if [ -z "$DO_BUILT" ]; then
@@ -68,7 +70,7 @@ _debug() {
 	[ -z "$_do_opt_debug" ] || echo "$@" >&2
 }
 
-if [ -z "$DO_BUILT" ]; then
+if [ -z "$DO_BUILT" -a "$_cmd" != "redo-whichdo" ]; then
 	DO_TOP=1
 	[ "$#" -gt 0 ] || set all  # only toplevel redo has a default target
 	export DO_BUILT=$PWD/.do_built
@@ -84,7 +86,7 @@ if [ -z "$DO_BUILT" ]; then
 	export PATH=$DO_PATH:$PATH
 	rm -rf "$DO_PATH"
 	mkdir "$DO_PATH"
-	for d in redo redo-ifchange; do
+	for d in redo redo-ifchange redo-whichdo; do
 		ln -s "$REDO" "$DO_PATH/$d"
 	done
 	[ -e /bin/true ] && TRUE=/bin/true || TRUE=/usr/bin/true
@@ -94,35 +96,148 @@ if [ -z "$DO_BUILT" ]; then
 fi
 
 
-_find_dofile_pwd()
+# Chop the "file" part off a /path/to/file pathname.
+# Note that if the filename already ends in a /, we just remove the slash.
+_updir()
 {
-	dofile=default.$1.do
+	local v="${1%/*}"
+	[ "$v" != "$1" ] && echo "$v"
+	# else "empty" which means we went past the root
+}
+
+
+# Returns true if $1 starts with $2.
+_startswith()
+{
+	[ "${1#"$2"}" != "$1" ]
+}
+
+
+# Returns true if $1 ends with $2.
+_endswith()
+{
+	[ "${1%"$2"}" != "$1" ]
+}
+
+
+# Prints $1 as a path relative to $PWD (not starting with /).
+# If it already doesn't start with a /, doesn't change the string.
+_relpath()
+{
+	local here="$(command pwd)" there="$1" out= hadslash=
+	#echo "RP start '$there' hs='$hadslash'" >&2
+	_startswith "$there" "/" || { echo "$there" && return; }
+	[ "$there" != "/" ] && _endswith "$there" "/" && hadslash=/
+	here=${here%/}/
+	while [ -n "$here" ]; do
+		#echo "RP out='$out' here='$here' there='$there'" >&2
+		[ "${here%/}" = "${there%/}" ] && there= && break;
+		[ "${there#$here}" != "$there" ] && break
+		out=../$out
+		_dirsplit "${here%/}"
+		here=$_dirsplit_dir
+	done
+	there=${there#$here}
+	if [ -n "$there" ]; then
+		echo "$out${there%/}$hadslash"
+	else
+		echo "${out%/}$hadslash"
+	fi
+}
+
+
+# Prints a "normalized relative" path, with ".." resolved where possible.
+# For example, a/b/../c will be reduced to just a/c.
+_normpath()
+(
+	local path="$1" out= isabs=
+	#echo "NP start '$path'" >&2
+	if _startswith "$path" "/"; then
+		isabs=1
+	else
+		path="${PWD%/}/$path"
+	fi
+	set -f
+	IFS=/
+	for d in $path; do
+		#echo "NP out='$out' d='$d'" >&2
+		if [ "$d" = ".." ]; then
+			out=$(_updir "${out%/}")/
+		else
+			out=$out$d/
+		fi
+	done
+	#echo "NP out='$out' (done)" >&2
+	out=${out%/}
+	if [ -n "$isabs" ]; then
+		echo "${out:-/}"
+	else
+		_relpath "${out:-/}"
+	fi
+)
+
+
+# List the possible names for default*.do files in dir $1 matching the target
+# pattern in $2.  We stop searching when we find the first one that exists.
+_find_dofiles_pwd()
+{
+	local dodir="$1" dofile="$2"
+	_startswith "$dofile" "default." || dofile=${dofile#*.}
 	while :; do
 		dofile=default.${dofile#default.*.}
-		[ -e "$dofile" -o "$dofile" = default.do ] && break
+		echo "$dodir$dofile"
+		[ -e "$dodir$dofile" ] && return 0
+		[ "$dofile" = default.do ] && break
 	done
-	ext=${dofile#default}
-	ext=${ext%.do}
-	base=${1%$ext}
+	return 1
 }
 
 
+# List the possible names for default*.do files in $PWD matching the target
+# pattern in $1.  We stop searching when we find the first name that works.
+# If there are no matches in $PWD, we'll search in .., and so on, to the root.
+_find_dofiles()
+{
+	local target="$1" dodir= dofile= newdir=
+	_debug "find_dofile: '$PWD' '$target'"
+	dofile="$target.do"
+	echo "$dofile"
+	[ -e "$dofile" ] && return 0
+
+	# Try default.*.do files, walking up the tree
+	_dirsplit "$dofile"
+	dodir=$_dirsplit_dir
+	dofile=$_dirsplit_base
+	for i in $(seq 100); do
+		[ -n "$dodir" ] && dodir=${dodir%/}/
+		#echo "_find_dofiles: '$dodir' '$dofile'" >&2
+		[ -e "$dodir$dofile" ] && return 0
+		_find_dofiles_pwd "$dodir" "$dofile" && return 0
+		newdir=$(_normpath "${dodir}..")
+		[ "$newdir" = "$dodir" ] && break
+		dodir=$newdir
+	done
+	return 1
+}
+
+
+# Print the last .do file returned by _find_dofiles.
+# If that file exists, returns 0, else 1.
 _find_dofile()
 {
-	local prefix=
-	while :; do
-		_find_dofile_pwd "$1"
-		[ -e "$dofile" ] && break
-		[ "$PWD" = "/" ] && break
-		target=${PWD##*/}/$target
-		tmp=${PWD##*/}/$tmp
-		prefix=${PWD##*/}/$prefix
-		cd ..
-	done
-	base=$prefix$base
+	local files="$(_find_dofiles "$1")"
+	rv=$?
+	#echo "files='$files'" >&2
+	[ "$rv" -ne 0 ] && return $rv
+	echo "$files" | {
+		while read -r linex; do line=$linex; done
+		printf "%s\n" "$line"
+	}
 }
 
 
+# Actually run the given $dofile with the arguments in $@.
+# Note: you should always run this in a subshell.
 _run_dofile()
 {
 	export DO_DEPTH="$DO_DEPTH  "
@@ -133,7 +248,7 @@ _run_dofile()
 	cmd=${line1#"#!/"}
 	if [ "$cmd" != "$line1" ]; then
 		set -$_do_opt_verbose$_do_opt_exec
-		/$cmd "$PWD/$dofile" "$@" >"$tmp.tmp2"
+		exec /$cmd "$PWD/$dofile" "$@" >"$tmp.tmp2"
 	else
 		set -$_do_opt_verbose$_do_opt_exec
 		:; . "$PWD/$dofile" >"$tmp.tmp2"
@@ -141,23 +256,37 @@ _run_dofile()
 }
 
 
+# Find and run the right .do file, starting in dir $1, for target $2, using
+# filename $3 as the temporary output file.  Renames the temp file to $2 when
+# done.
 _do()
 {
-	local dir="$1" target="$2" tmp="$3"
-	if [ -z "$ifchange" ] ||
+	local dir="$1" target="$2" tmp="$3" dopath= dodir= dofile= ext=
+	if [ "$_cmd" = "redo" ] ||
 	    ( [ ! -e "$target" -o -d "$target" ] &&
 	      [ ! -e "$target.did" ] ); then
 		printf '%sdo  %s%s%s%s\n' \
 			"$green" "$DO_DEPTH" "$bold" "$dir$target" "$plain" >&2
-		echo "$PWD/$target" >>"$DO_BUILT"
-		dofile=$target.do
-		base=$target
-		ext=
-		[ -e "$target.do" ] || _find_dofile "$target"
-		if [ ! -e "$dofile" ]; then
-			echo "do: $target: no .do file" >&2
+		dopath=$(_find_dofile "$target")
+		if [ ! -e "$dopath" ]; then
+			echo "do: $target: no .do file ($PWD)" >&2
 			return 1
 		fi
+		echo "$PWD/$target" >>"$DO_BUILT"
+		_dirsplit "$dopath"
+		dodir=$_dirsplit_dir dofile=$_dirsplit_base
+		if _startswith "$dofile" "default."; then
+			ext=${dofile#default}
+			ext=${ext%.do}
+		else
+			ext=
+		fi
+		target=$PWD/$target
+		tmp=$PWD/$tmp
+		cd "$dodir" || return 99
+		target=$(_relpath "$target") || return 98
+		tmp=$(_relpath "$tmp") || return 97
+		base=${target%$ext}
 		[ ! -e "$DO_BUILT" ] || [ ! -d "$(dirname "$target")" ] ||
 		: >>"$target.did"
 		( _run_dofile "$target" "$base" "$tmp.tmp" )
@@ -185,19 +314,20 @@ _dir_shovel()
 	xdir=$1 xbase=$2 xbasetmp=$2
 	while [ ! -d "$xdir" -a -n "$xdir" ]; do
 		_dirsplit "${xdir%/}"
-		xbasetmp=${base}__$xbase
-		xdir=$dir xbase=$base/$xbase
+		xbasetmp=${_dirsplit_base}__$xbase
+		xdir=$_dirsplit_dir xbase=$_dirsplit_base/$xbase
 		_debug "xbasetmp='$xbasetmp'" >&2
 	done
 }
 
 
+# Implementation of the "redo" command.
 _redo()
 {
 	set +e
 	for i in "$@"; do
 		_dirsplit "$i"
-		_dir_shovel "$dir" "$base"
+		_dir_shovel "$_dirsplit_dir" "$_dirsplit_base"
 		dir=$xdir base=$xbase basetmp=$xbasetmp
 		( cd "$dir" && _do "$dir" "$base" "$basetmp" )
 		[ "$?" = 0 ] || return 1
@@ -205,7 +335,19 @@ _redo()
 }
 
 
-_redo "$@"
+# Implementation of the "redo-whichdo" command.
+_whichdo()
+{
+	_find_dofiles "$1"
+}
+
+
+case $_cmd in
+	do|redo|redo-ifchange) _redo "$@" ;;
+	redo-whichdo) _whichdo "$1" ;;
+	do.test) ;;
+	*) printf "$0: '%s': unexpected redo command" "$_cmd" >&2; exit 99 ;;
+esac
 [ "$?" = 0 ] || exit 1
 
 if [ -n "$DO_TOP" ]; then
diff --git a/minimal/do.test b/minimal/do.test
new file mode 100755
index 0000000..2c0c3c7
--- /dev/null
+++ b/minimal/do.test
@@ -0,0 +1,164 @@
+#!/bin/sh
+. ./do
+
+set -e
+
+NAME=$(basename "$0")
+TESTNUM=0
+SECT=root
+
+
+SECTION()
+{
+	SECT=$1
+	TESTNUM=0
+	printf "\nTesting \"$1\" in $0:\n"
+}
+
+
+no_nl()
+{
+	echo "$1" | {
+		out=
+		while read -r line; do
+			out="$out$line "
+		done
+		printf "%s" "${out% }"
+	}
+}
+
+
+check_s()
+{
+	local want="$1" got="$2" cmd="$3"
+	want_s=$(no_nl "$want")
+	got_s=$(no_nl "$got")
+	[ -z "$cmd" ] && cmd=$got
+	TESTNUM=$((TESTNUM + 1))
+	if [ "$want" != "$got" ]; then
+		printf "\n<<< expected:\n%s\n=== got:\n%s\n>>>\n" "$want" "$got"
+		printf "! %s:%d  '%.40s' = %s  FAILED\n" "$SECT" "$TESTNUM" "$want_s" "$cmd"
+		exit 1
+	else
+		printf "! %s:%d  '%.40s' = %s  ok\n" "$SECT" "$TESTNUM" "$want_s" "$cmd"
+	fi
+}
+
+
+check()
+{
+	local want="$1" got= rv=
+	shift
+	rv=0
+	got=$("$@") || rv=$?
+	if [ "$rv" -ne 0 ]; then
+		check_s "<returned 0>" "<returned $rv>"
+	else
+		check_s "$want" "$got" "$*"
+	fi
+}
+
+
+retval()
+{
+	set +e
+	"$@" >&2
+	local rv=$?
+	set -e
+	echo "$rv"
+	return 0
+}
+
+
+SECTION _starts_ends_with
+check "0"  retval true
+check "1"  retval false
+check "0"  retval _startswith "x.test" "x."
+check "1"  retval _startswith "x.test" "y."
+check "1"  retval _startswith "" " "
+check "1"  retval _startswith "x.test" "x*"
+check "1"  retval _startswith "do.test" "do*"
+
+check "0"  retval _endswith "x.test" ".test"
+check "1"  retval _endswith "x.test" ".best"
+check "1"  retval _endswith "" " "
+check "1"  retval _endswith "x.test" "*.test"
+check "1"  retval _endswith "do.test" "*.test"
+
+
+
+SECTION _dirsplit
+_dirsplit "/a/b/c"
+check_s "/a/b/" "$_dirsplit_dir"
+check_s "c" "$_dirsplit_base"
+
+_dirsplit "/a/b/c/"
+check_s "/a/b/c/" "$_dirsplit_dir"
+check_s "" "$_dirsplit_base"
+
+_dirsplit "/"
+check_s "/" "$_dirsplit_dir"
+check_s "" "$_dirsplit_base"
+
+
+SECTION _relpath
+check "a/b/c"  _relpath $PWD/a/b/c
+check "../a/b/c"  _relpath $PWD/../a/b/c
+check ""  _relpath "$PWD"
+(cd / && check "a/b/c"  _relpath a/b/c)
+(cd / && check "a/b/c"  _relpath /a/b/c)
+(cd / && check ""  _relpath /)
+(cd /usr/bin && check "../lib"  _relpath /usr/lib)
+(cd /usr/bin && check "../lib/"  _relpath /usr/lib/)
+(cd /usr/bin && check "../.."  _relpath /)
+(cd fakedir && check ".."  _relpath ..)
+(cd fakedir && check "../"  _relpath ../)
+(cd fakedir && check "../fakedir"  _relpath ../fakedir)
+
+
+SECTION _normpath
+check "/usr/lib"  _normpath /usr/../usr/bin/../lib
+check "/"  _normpath /a/b/c/../../..
+check "/"  _normpath /
+check "../a" _normpath ../a
+(cd fakedir && check "../a/b"  _normpath ../a/b)
+(cd fakedir && check ".." _normpath ..)
+(cd / && check "tuv"  _normpath a/b/../../tuv)
+(cd / && check ""  _normpath a/b/../..)
+check ".." _normpath ../
+check ".." _normpath ..
+
+
+SECTION _find_dofile
+check "test.do"  _find_dofiles test
+check "test.do"  _find_dofile test
+
+want="x.y.zz.do
+default.y.zz.do
+default.zz.do"
+check "$want"  _find_dofiles x.y.zz
+
+want="x.y.zz.do
+default.y.zz.do
+default.zz.do
+default.do
+../default.y.zz.do
+../default.zz.do"
+(cd fakedir && check "$want"  _find_dofiles x.y.zz)
+(cd fakedir && check "../default.zz.do"  _find_dofile x.y.zz)
+
+set +e
+got=$(_find_dofiles x.y.z)
+rv=$?
+set -e
+check_s "1" "$rv"
+top_want="x.y.z.do
+default.y.z.do
+default.z.do
+default.do
+../default.y.z.do
+../default.z.do
+../default.do"
+check_s "$top_want" "$(echo "$got" | head -n 7)" "_find_dofiles x.y.z"
+
+exit 0
diff --git a/minimal/fakedir/.empty b/minimal/fakedir/.empty
new file mode 100644
index 0000000..e69de29
diff --git a/minimal/test.do b/minimal/test.do
new file mode 100644
index 0000000..9239fdc
--- /dev/null
+++ b/minimal/test.do
@@ -0,0 +1,2 @@
+exec >&2
+./do.test
diff --git a/test.do b/test.do
index a092156..9ea2c31 100644
--- a/test.do
+++ b/test.do
@@ -1,3 +1,3 @@
 redo-ifchange _all
-redo t/all
+redo minimal/test t/all
 [ -n "$DO_BUILT" ] || echo "Don't forget to test 'minimal/do -c test'" >&2