diff --git a/build/jam/FileRules b/build/jam/FileRules
index 04c7020933..65cf50917e 100644
--- a/build/jam/FileRules
+++ b/build/jam/FileRules
@@ -335,67 +335,8 @@ actions CopySetHaikuRevision1
 {
 	$(HOST_ADD_BUILD_COMPATIBILITY_LIB_DIR)
 
-	originalDir=`pwd`
-	cd $(HAIKU_TOP)
-	export LC_ALL=C
-	if [ -d .svn ]; then
-		revision=`svn info 2>/dev/null | grep Revision | awk '{printf $2}'`
-	elif [ -d .git/svn ]; then
-		revision=`git svn info 2>/dev/null | grep Revision | awk '{printf $2}'`
-	elif [ -d .git ]; then
-		revision=`cat $(HAIKU_BUILD_OUTPUT_DIR)/haiku-revision 2>/dev/null`
-		lastBuiltRevision=`cat $(HAIKU_BUILD_OUTPUT_DIR)/last-built-revision \
-			2>/dev/null`
-		localRev=`git rev-list -n1 HEAD`
-
-		# only determine the haiku-revision if anything has changed from
-		# last build
-		if [ -z "$revision" -o "$lastBuiltRevision" != "$localRev" ]; then
-			currentBranch=`git branch --contains HEAD | awk '{printf $2}'`
-			haikuTip=`git rev-list -n1 haiku/${currentBranch}.tip`
-
-			# make sure that haiku-tip matches the remote tracking branch,
-			# otherwise the tag hasn't been fetched/updated and we should
-			# print a warning
-			trackingBranch=`git branch -r --contains $haikuTip \
-				| grep -v -- '->' | tr -d ' '`
-			haikuTipLag=`git rev-list --count ${haikuTip}..$trackingBranch`
-			if [ "$haikuTipLag" != "0" ]; then
-				echo "*********************************************************"
-				echo "!!! haiku/${currentBranch}.tip is $haikuTipLag behind \
-					remote tracking branch"
-				echo "!!! you may want to 'git fetch --tags' to update the tag"
-				echo "*********************************************************"
-			fi
-
-			# the haiku revision the HEAD is based on is the nearest common ancestor
-			# of these two (i.e. the merge base)
-			haikuBaseRev=`git merge-base $localRev $haikuTip`
-
-			# the revision we use is the description of HEAD with
-			# respect to the root of the current branch
-			revision=`git describe --dirty --long --match=haiku/$currentBranch`
-			if [ "$localRev" != "$haikuBaseRev" ]; then
-				# HEAD is not a changeset from Haiku's central repo, so we add
-				# a description of the changeset in Haiku's repo HEAD is based on
-				haikuBaseRevDescr=`git describe --long --match=haiku/$currentBranch $haikuBaseRev`
-				revision="$revision [$haikuBaseRevDescr]"
-			fi
-			echo $localRev >$(HAIKU_BUILD_OUTPUT_DIR)/last-built-revision
-		fi
-	elif [ -d .hg ]; then
-		# Try searching hg log for last svn commit
-		# Extract from "(svn r12345) ..." line
-		revision=`(cd $(HAIKU_TOP) &&
-			hg log --no-merges --template "{desc|firstline}\n") 2> /dev/null |
-			grep --max-count=1 "(svn r" |
-			sed -n -e 's,(svn r\(.*\)).*,\1,p'`
-	fi
-	if [ "$revision" = "" ]; then
-		revision=0
-	fi
-	echo $revision >$(HAIKU_BUILD_OUTPUT_DIR)/haiku-revision
-	cd $originalDir
+	. $(HAIKU_TOP)/build/scripts/determine_haiku_revision $(HAIKU_TOP) \
+		$(HAIKU_BUILD_OUTPUT_DIR)
 	$(2[1]) --data $(2[3]) $(1) &&
 	$(2[2]) $(1) ${revision}
 }
diff --git a/build/scripts/determine_haiku_revision b/build/scripts/determine_haiku_revision
new file mode 100644
index 0000000000..ae1e035e22
--- /dev/null
+++ b/build/scripts/determine_haiku_revision
@@ -0,0 +1,97 @@
+#!/bin/sh
+
+# get and check the parameters
+if [ $# -lt 2 ]; then
+	echo Usage: $0 '<haiku top> <haiku build output dir>' >&2
+	exit 1
+fi
+
+HAIKU_TOP=$1
+HAIKU_BUILD_OUTPUT_DIR=$2
+
+
+determineGitRevision()
+{
+	revision=`cat ${HAIKU_BUILD_OUTPUT_DIR}/haiku-revision 2>/dev/null`
+	lastBuiltRevision=`cat ${HAIKU_BUILD_OUTPUT_DIR}/last-built-revision \
+		2>/dev/null`
+	localRev=`git rev-list -n1 HEAD`
+
+	# only determine the haiku-revision if anything has changed from
+	# last build
+	if [ -z "$revision" -o "$lastBuiltRevision" != "$localRev" ]; then
+		haikuBranch=`git describe --abbrev=0 --match 'haiku/*.base' \
+			| sed -re 's/haiku\/(.*)\.base/\1/'`
+		if [ -z "$haikuBranch" ]; then
+			echo "*** unable to find haiku branch the build is based on"
+			exit 1;
+		fi
+		haikuTip=`git rev-list -n1 haiku/${haikuBranch}.tip`
+		if [ -z "$haikuTip" ]; then
+			echo "*** unable to find tip of haiku branch the build is based on"
+			exit 1;
+		fi
+
+		# make sure that haiku-tip matches the remote tracking branch,
+		# otherwise the tag hasn't been fetched/updated and we should
+		# print a warning
+		trackingBranch=`git branch -r --contains $haikuTip \
+			| grep -v -- '->' | tr -d ' '`
+		if [ -z "$trackingBranch" ]; then
+			echo "*** unable to find tracking branch the build is based on"
+			exit 1;
+		fi
+		haikuTipLag=`git rev-list --count ${haikuTip}..$trackingBranch`
+		if [ "$haikuTipLag" != "0" ]; then
+			echo "*********************************************************"
+			echo "!!! haiku/${haikuBranch}.tip is $haikuTipLag behind \
+				remote tracking branch"
+			echo "!!! you may want to 'git fetch --tags' to update the tag"
+			echo "*********************************************************"
+		fi
+
+		# the haiku revision the HEAD is based on is the nearest common ancestor
+		# of these two (i.e. the merge base)
+		haikuBaseRev=`git merge-base $localRev $haikuTip`
+		if [ -z "$haikuTip" ]; then
+			echo "*** unable to find merge-base for haiku tip and build"
+			exit 1;
+		fi
+
+		# the revision we use is the description of HEAD with
+		# respect to the root of the current branch
+		revision=`git describe --dirty --long --match=haiku/$haikuBranch`
+		if [ "$localRev" != "$haikuBaseRev" ]; then
+			# HEAD is not a changeset from Haiku's central repo, so we add
+			# a description of the changeset in Haiku's repo HEAD is based on
+			haikuBaseRevDescr=`git describe --long --match=haiku/$haikuBranch \
+				$haikuBaseRev`
+			revision="$revision [$haikuBaseRevDescr]"
+		fi
+		echo $localRev >${HAIKU_BUILD_OUTPUT_DIR}/last-built-revision
+	fi
+}
+
+
+originalDir=`pwd`
+cd ${HAIKU_TOP}
+export LC_ALL=C
+if [ -d .svn ]; then
+	revision=`svn info 2>/dev/null | grep Revision | awk '{printf $2}'`
+elif [ -d .git/svn ]; then
+	revision=`git svn info 2>/dev/null | grep Revision | awk '{printf $2}'`
+elif [ -d .git ]; then
+	determineGitRevision
+elif [ -d .hg ]; then
+	# Try searching hg log for last svn commit
+	# Extract from "(svn r12345) ..." line
+	revision=`(cd ${HAIKU_TOP} &&
+		hg log --no-merges --template "{desc|firstline}\n") 2> /dev/null |
+		grep --max-count=1 "(svn r" |
+		sed -n -e 's,(svn r\(.*\)).*,\1,p'`
+fi
+if [ "$revision" = "" ]; then
+	revision=0
+fi
+echo $revision >${HAIKU_BUILD_OUTPUT_DIR}/haiku-revision
+cd $originalDir