diff --git a/hdf5.changes b/hdf5.changes
index 05ccb41..abed9cc 100644
--- a/hdf5.changes
+++ b/hdf5.changes
@@ -1,3 +1,8 @@
+-------------------------------------------------------------------
+Fri Jan 29 22:05:44 UTC 2021 - Egbert Eich <eich@suse.com>
+
+- Fix update_so_version.sh, write so versions to file being included.
+
 -------------------------------------------------------------------
 Fri Nov  6 10:41:02 UTC 2020 - Ana Guerrero Lopez <aguerrero@suse.com>
 
diff --git a/hdf5.spec b/hdf5.spec
index 2dbe751..1fc6a27 100644
--- a/hdf5.spec
+++ b/hdf5.spec
@@ -1,7 +1,7 @@
 #
 # spec file for package hdf5
 #
-# Copyright (c) 2020 SUSE LLC
+# Copyright (c) 2021 SUSE LLC
 #
 # All modifications and additions to the file contributed by third parties
 # remain the property of their copyright owners, unless otherwise agreed
@@ -412,14 +412,8 @@ ExclusiveArch:  do_not_build
  %define vname %{pname}
 %endif
 
-# TODO: The so numbers autodetected by update_so_version.sh
-# do not match the so numbers that are actually built.
-%define sonum 103
-%define sonum_CXX 103
-%define sonum_F 102
-%define sonum_HL 100
-%define sonum_HL_CXX 100
-%define sonum_HL_F 100
+# Run 'sh ./update_so_version.sh' when updating hdf5!
+%include %{_sourcedir}/so_versions
 
 Name:           %{package_name}
 Version:        %vers
@@ -429,7 +423,7 @@ License:        BSD-3-Clause
 Group:          Productivity/Scientific/Other
 URL:            https://www.hdfgroup.org/HDF5/
 Source0:        https://www.hdfgroup.org/ftp/HDF5/releases/%{pname}-%{short_ver}/%{pname}-%{src_ver}/src/%{pname}-%{src_ver}.tar.bz2
-Source100:      _multibuild
+Source100:      so_versions
 Source1000:     update_so_version.sh
 Patch0:         hdf5-LD_LIBRARY_PATH.patch
 # not really needed but we want to get noticed if hdf5 doesn' t know our host
diff --git a/so_versions b/so_versions
new file mode 100644
index 0000000..7717fb4
--- /dev/null
+++ b/so_versions
@@ -0,0 +1,6 @@
+%define sonum 103
+%define sonum_CXX 103
+%define sonum_F 102
+%define sonum_HL 100
+%define sonum_HL_CXX 100
+%define sonum_HL_F 100
diff --git a/update_so_version.sh b/update_so_version.sh
index 4d94ae5..3148129 100644
--- a/update_so_version.sh
+++ b/update_so_version.sh
@@ -6,15 +6,10 @@ exit_error()
 	exit 1
 }
 
-dbg()
-{
-	echo "dbg: $@" >&2
-}
-
 cleanup()
 {
-	rm -f lt_vers.sh sonum_spec.sed
-	rm -f hdf5.spec.tmp
+    rm -f ${spectmp}
+    rm -f ${shtmp}
 }
 
 ## We are going to parse these variables from tarball:
@@ -24,47 +19,46 @@ cleanup()
 # LT_HL_VERS_INTERFACE
 # LT_HL_CXX_VERS_INTERFACE
 # LT_HL_F_VERS_INTERFACE
-# LT_TOOLS_VERS_INTERFACE
 
 cleanup
 
-VER="$(grep -m1 "^Version:" hdf5.spec)" || exit_error "can't grep version"
+spectmp=$(mktemp hdf-XXXX.spec)
+shtmp=$(mktemp lt-XXXX.sh)
+outtmp=$(mktemp so-XXXX)
+sed -e "s/@BUILD_FLAVOR@/standard/" hdf5.spec > ${spectmp}
+VER="$(rpmspec --define "_sourcedir $(pwd)" -P ${spectmp} | grep -m1 "^Version:")" || \
+    exit_error "can't grep version"
 read x VER <<<$VER
 
-SRC="$(grep -m1 "^Source0:" hdf5.spec)" || exit_error "can't grep source"
+SRC="$(rpmspec --define "_sourcedir $(pwd)" -P ${spectmp} | grep -m1 "^Source0:")" || \
+    exit_error "can't grep source"
 read x SRC <<<$SRC
 SRC=$(basename "$SRC" | sed "s/%{version}/$VER/")
 
 test -f "$SRC" || exit_error "tarball '$SRC' does not exist"
-dbg "inspecting $SRC"
 
 tar --wildcards -x -O -f "$SRC" "*/config/lt_vers.am" \
 	| tr -d ' \t' \
-	| grep "^LT_.*INTERFACE=" \
-	> lt_vers.sh \
+	| grep -E "^LT_.*INTERFACE=|^LT_.*AGE=" \
+	> ${shtmp} \
 	|| exit_error "can't grep interface versions"
 
-# we expect exactly 7 variables
-. lt_vers.sh || exit_error "can't source lt_vers.sh"
-test $(wc -l < lt_vers.sh) -le "7" \
-	|| exit_error "more than 7 vars found, update this script!"
+source ./${shtmp} || exit_error "can't source ${shtmp}"
 
-for infix in "" _CXX _F _HL _HL_CXX _HL_F _TOOLS; do
-	var="LT${infix}_VERS_INTERFACE"
-	test "${!var}" -gt 0 || exit_error "$var='${!var}' bad or undefined"
-	dbg "update $var=${!var}"
-	def_sonum="sonum${infix}"
-	# create sed scripts for spec file
-	echo "s/^%define $def_sonum .*/%define $def_sonum ${!var}/" >> sonum_spec.sed
+for infix in "" _CXX _F _HL _HL_CXX _HL_F; do
+	current="LT${infix}_VERS_INTERFACE"
+	age="LT${infix}_VERS_AGE"
+	currv=$(eval "echo \$$current")
+	agev=$(eval "echo \$$age")
+	if [ -n "$currv" -a -n "$agev" ]; then
+	    echo %define sonum${infix} $((currv - agev)) >> ${outtmp}
+	else
+	    rm -f ${outtmp}
+	    cleanup
+	    exit 1
+	fi
 done
 
-# update spec file if needed
-sed -f sonum_spec.sed hdf5.spec > hdf5.spec.tmp
-if diff -q hdf5.spec.tmp hdf5.spec &>/dev/null; then
-	dbg "hdf5.spec was up-to-date"
-else
-	mv hdf5.spec.tmp hdf5.spec
-	echo "hdf5.spec updated" >&2
-fi
-
+[ -e ${outtmp} ] && mv ${outtmp} so_versions
+  
 cleanup