diff --git a/releng/run_all_releng b/releng/run_all_releng
index be32b300..ad682298 100755
--- a/releng/run_all_releng
+++ b/releng/run_all_releng
@@ -28,6 +28,9 @@ HLP
 fi
 
 ########################################################
+ if [ -f "${CLOG}" ]; then
+   mv "${CLOG}" "${CLOG}".prev
+ fi
  > ${CLOG}
  DSTART=`date --utc`
  printf "\n${SELF}: Started at ${DSTART}"
@@ -60,12 +63,19 @@ fi
  printf "\n${SELF}: Stopped at ${DEND}.\n"
  
  if   [ "${1}" == "-cleanup" ]; then
-  rm -f "${CLOG}"
+   rm -f "${CLOG}"
  else
-  E2=`date '+%s'`
-  let ES="${E2} - ${E1}"
-  printf "\n${SELF}: Total time elapsed ${ES} sec."
-  printf "\n${SELF}: stdout&stderr saved in ${CLOG}.\n"
+   E2=`date '+%s'`
+   let ES="${E2} - ${E1}"
+   printf "\n${SELF}: Total time elapsed ${ES} sec."
+   printf "\n${SELF}: stdout&stderr saved in ${CLOG}.\n"
+
+# TODO: work out a less noisy diff'ing technique!
+#   if [ -f "${CLOG}".prev -a -f "${CLOG}" ]; then
+#     printf "\n${SELF}: Log diff follows:\n"
+#     diff -Naur "${CLOG}".prev "${CLOG}"
+#   fi
+
  fi
 
  exit 0