Replace NEW_VALUE with the new setting. For example, configure ip6:your-ipv6-address to add IPv6 or include:spf.yourwebhoster.eu. It will only add the value to domains who do not have this record yet to prevent duplicates.

#!/bin/bash
# Copyright 2018 Yourwebhoster.eu

# CONFIGURATION
# Directory containing the .db files
named_dir='/var/named'

# String that should be in the txt record.
spf_add_value='NEW_VALUE'

# DO NOT CHANGE ANYTHING BELOW

# Change working dir
cd $named_dir

# List domains without provided string
domains=`grep -L $spf_add_value *.db`

for domainDB in $domains; do
        echo "Updating $domainDB";
        perl -pi -e "s/v=spf1 a mx/v=spf1 a mx $spf_add_value/" $domainDB
done

echo "Updating serials in named files"
echo "action=rewrite&value=named" >> /usr/local/directadmin/data/task.queue
/usr/local/directadmin/dataskq d800

echo "Restart Named"
/usr/local/directadmin/dataskq d800

echo "Done"