This script converts DB files created for NSS makedb command while you upgrade from RHEL6 to RHEL7 or 8.

fix_nssdb_rhel6_rhel7-8.sh

#!/bin/bash
# This script converts DB files created for NSS while you upgrade from RHEL6 to RHEL7 or RHEL8.
# It only happens when you upgrade from RHEL6
# Fixes a problem where the groups are not properly added to the file like the following:
# :john john 2001,2500,4

# First copy the old file to a new file
cp group.tdb group_test.tdb

for line in `grep ^= group.tdb | grep :$ | awk '{print $2}'`; do
  GID=`echo $line | cut -d : -f 3`
  USER=`echo $line | cut -d : -f 1`
   GIDS="$(grep $USER group.tdb  | grep : | cut -d : -f 3 | grep ^[0-9] | sort | uniq | tr '\n' , | sed 's/,$//')"
   ADD=":$USER $USER $GIDS"
   sed -i group_test.tdb -e "/=$GID $line/ a\
$ADD"
done