diff --git a/src/all.bash b/src/all.bash
index 67c19cd43fd..00c1ca74dd9 100755
--- a/src/all.bash
+++ b/src/all.bash
@@ -5,4 +5,4 @@
 
 set -e
 bash make.bash
-bash run.bash
+bash run.bash --no-rebuild
diff --git a/src/env.bash b/src/env.bash
new file mode 100644
index 00000000000..6ab491ae439
--- /dev/null
+++ b/src/env.bash
@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# Copyright 2009 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+if test -z "$GOBIN"; then
+	if ! test -d "$HOME"/bin; then
+		echo '$GOBIN is not set and $HOME/bin is not a directory or does not exist.' 1>&2
+		echo 'mkdir $HOME/bin or set $GOBIN to a directory where binaries should' 1>&2
+		echo 'be installed.' 1>&2
+		exit 1
+	fi
+	GOBIN="$HOME/bin"
+elif ! test -d "$GOBIN"; then
+	echo '$GOBIN is not a directory or does not exist' 1>&2
+	echo 'create it or set $GOBIN differently' 1>&2
+	exit 1
+fi
+
+GOROOT=${GOROOT:-$(cd ..; pwd)}
+if ! test -f "$GOROOT"/include/u.h
+then
+	echo '$GOROOT is not set correctly or not exported' 1>&2
+	exit 1
+fi
+
+# Double-check that we're in $GOROOT, for people with multiple Go trees.
+# Various aspects of the build cd into $GOROOT-rooted paths,
+# making it easy to jump to a different tree and get confused.
+DIR1=$(cd ..; pwd)
+DIR2=$(cd $GOROOT; pwd)
+if [ "$DIR1" != "$DIR2" ]; then
+	echo 'Suspicious $GOROOT: does not match current directory.' 1>&2
+	exit 1
+fi
+
+GOARCH=${GOARCH:-$(uname -m | sed 's/^..86$/386/; s/^.86$/386/; s/x86_64/amd64/')}
+case "$GOARCH" in
+amd64 | 386 | arm)
+	;;
+*)
+	echo '$GOARCH is set to <'$GOARCH'>, must be amd64, 386, or arm' 1>&2
+	exit 1
+esac
+
+GOOS=${GOOS:-$(uname | tr A-Z a-z)}
+case "$GOOS" in
+darwin | freebsd | linux | mingw | nacl)
+	;;
+*)
+	echo '$GOOS is set to <'$GOOS'>, must be darwin, freebsd, linux, mingw, or nacl' 1>&2
+	exit 1
+esac
+
+export GOBIN GOROOT GOARCH GOOS
diff --git a/src/make.bash b/src/make.bash
index c2a350af7f1..d8638145eab 100755
--- a/src/make.bash
+++ b/src/make.bash
@@ -4,48 +4,11 @@
 # license that can be found in the LICENSE file.
 
 set -e
+. ./env.bash
 
-if test -z "$GOBIN"; then
-	if ! test -d "$HOME"/bin; then
-		echo '$GOBIN is not set and $HOME/bin is not a directory or does not exist.' 1>&2
-		echo 'mkdir $HOME/bin or set $GOBIN to a directory where binaries should' 1>&2
-		echo 'be installed.' 1>&2
-		exit 1
-	fi
-	GOBIN="$HOME/bin"
-elif ! test -d "$GOBIN"; then
-	echo '$GOBIN is not a directory or does not exist' 1>&2
-	echo 'create it or set $GOBIN differently' 1>&2
-	exit 1
-fi
-
-GOBIN="${GOBIN:-$HOME/bin}"
 export MAKEFLAGS=-j4
-
 unset CDPATH	# in case user has it set
 
-if ! test -f "$GOROOT"/include/u.h
-then
-	echo '$GOROOT is not set correctly or not exported' 1>&2
-	exit 1
-fi
-
-case "$GOARCH" in
-amd64 | 386 | arm)
-	;;
-*)
-	echo '$GOARCH is set to <'$GOARCH'>, must be amd64, 386, or arm' 1>&2
-	exit 1
-esac
-
-case "$GOOS" in
-darwin | freebsd | linux | mingw | nacl)
-	;;
-*)
-	echo '$GOOS is set to <'$GOOS'>, must be darwin, freebsd, linux, mingw, or nacl' 1>&2
-	exit 1
-esac
-
 rm -f "$GOBIN"/quietgcc
 CC=${CC:-gcc}
 sed -e "s|@CC@|$CC|" < "$GOROOT"/src/quietgcc.bash > "$GOBIN"/quietgcc
diff --git a/src/run.bash b/src/run.bash
index 35d499f9569..85dd59a55e7 100755
--- a/src/run.bash
+++ b/src/run.bash
@@ -4,12 +4,21 @@
 # license that can be found in the LICENSE file.
 
 set -e
+. ./env.bash
 
-GOBIN="${GOBIN:-$HOME/bin}"
+export MAKEFLAGS=-j4
+unset CDPATH	# in case user has it set
 
 # no core files, please
 ulimit -c 0
 
+# allow make.bash to avoid double-build of everything
+rebuild=true
+if [ "$1" = "--no-rebuild" ]; then
+	rebuild=false
+	shift
+fi
+		
 xcd() {
 	echo
 	echo --- cd $1
@@ -21,9 +30,11 @@ maketest() {
 	do
 		(
 			xcd $i
-			"$GOBIN"/gomake clean
-			time "$GOBIN"/gomake
-			"$GOBIN"/gomake install
+			if $rebuild; then
+				"$GOBIN"/gomake clean
+				time "$GOBIN"/gomake
+				"$GOBIN"/gomake install
+			fi
 			"$GOBIN"/gomake test
 		) || exit $?
 	done
@@ -36,20 +47,26 @@ maketest \
 # from what maketest does.
 
 (xcd pkg/sync;
-"$GOBIN"/gomake clean;
-time "$GOBIN"/gomake
+if $rebuild; then
+	"$GOBIN"/gomake clean;
+	time "$GOBIN"/gomake
+fi
 GOMAXPROCS=10 "$GOBIN"/gomake test
 ) || exit $?
 
 (xcd cmd/gofmt
-"$GOBIN"/gomake clean
-time "$GOBIN"/gomake
+if $rebuild; then
+	"$GOBIN"/gomake clean;
+	time "$GOBIN"/gomake
+fi
 time "$GOBIN"/gomake smoketest
 ) || exit $?
 
 (xcd cmd/ebnflint
-"$GOBIN"/gomake clean
-time "$GOBIN"/gomake
+if $rebuild; then
+	"$GOBIN"/gomake clean;
+	time "$GOBIN"/gomake
+fi
 time "$GOBIN"/gomake test
 ) || exit $?