diff --git a/update-tarball.sh b/update-tarball.sh
index b61d758..e414329 100644
--- a/update-tarball.sh
+++ b/update-tarball.sh
@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 user="tianocore"
 repo="edk2"
@@ -8,21 +8,30 @@ uri="https://github.com/${user}/${repo}"
 api="${uri/github.com/api.github.com/repos}"
 tar="${uri/github.com/codeload.github.com}/legacy.tar.gz"
 
-hash=$(curl -s "${api}/git/refs/heads/${branch}" | grep '"sha"' | cut -d'"' -f4)
-if test "$hash" = ""; then
+if test $# -ge 1; then
+  hash=$1
+  short=$1
+else
+  hash=$(curl -s "${api}/git/refs/heads/${branch}" | grep '"sha"' | cut -d'"' -f4)
+  if test "$hash" = ""; then
 	echo "# failed to fetch $branch hash"
 	exit 1
+  fi
+  short=$(echo $hash | sed -e 's/^\(.......\).*/\1/')
 fi
-short=$(echo $hash | sed -e 's/^\(.......\).*/\1/')
 
-date=$(curl -s "${api}/git/commits/$hash" | awk '
-	/"committer"/	{ c=1 }
-	/"date"/	{ if (c) { print } }
-' | cut -d'"' -f4)
-date="${date%T*}"
-date="${date//-/}"
+if test $# = 2; then
+  date=$2
+else
+  date=$(curl -s "${api}/git/commits/$hash" | awk '
+	  /"committer"/	{ c=1 }
+	  /"date"/	{ if (c) { print } }
+  ' | cut -d'"' -f4)
+  date="${date%T*}"
+  date="${date//-/}"
+fi
 
-name="${repo}-${date}-${short}.tar.gz"
+name="${repo}-${date}-${short}.tar.xz"
 
 if test -f "$name"; then
 	echo "# exists: $name"
@@ -41,5 +50,5 @@ echo
 echo "# cleanup ..."
 rm -vf ${repo}-*.tar*
 echo "# fetching $name ..."
-curl "$tar/$hash" > "$name"
+curl "$tar/$hash" | zcat | xz -9e > "$name"
 exit 0