Create Signing Table, Key Table, Trusted Hosts File and test DKIM key.

add-opendkim-key.sh

#!/bin/bash

# Which domain?
get_domain(){
        printf "\n"
        read -p "Domain: " DOMAIN
}

# Is domain set?
domain_is_set(){
        if [ -z "$DOMAIN" ]; then
                return 1
        fi
}

# Is domain valid?
domain_is_valid(){
        if echo $DOMAIN | grep -P '(?=^.{4,253}$)(^(?:[a-zA-Z0-9](?:(?:[a-zA-Z0-9\-]){0,61}[a-zA-Z0-9])?\.)+([a-zA-Z]{2,}|xn--[a-zA-Z0-9][a-zA-Z0-9\-]*[a-zA-Z0-9])$)' >/dev/null 2>&1; then
                return 0
        else
                return 1
        fi
}

# Prompt try again
try_again() {
        printf "\n"
        read -p "Try again? (Y|N)" -n 1

        if [[ $REPLY =~ ^[Yy]$ ]]
        then
                return 0
        else
                return 1
        fi
}

# Sign DKIM in table
dkim_add_signing_table() {
        DKIM_SIGN_STRING="*@$DOMAIN\t\tdefault._domainkey.$DOMAIN"
        if grep -Fxq "$DOMAIN" /etc/opendkim/signing.table; then
                echo "DKIM already signed in /etc/opendkim/signing.table"
        else
                echo -e $DKIM_SIGN_STRING >> /etc/opendkim/signing.table
                echo "Added $DKIM_SIGN_STRING to /etc/opendkim/signing.table"
        fi
}

# Add key to table
dkim_add_key_table(){
        DKIM_KEY_STRING="default._domainkey.$DOMAIN\t$DOMAIN:default:/etc/opendkim/keys/$DOMAIN/default.private"
         if grep -Fxq "$DOMAIN" /etc/opendkim/key.table; then
                echo "DKIM key already in /etc/opendkim/key.table"
        else
                echo -e $DKIM_KEY_STRING >> /etc/opendkim/key.table
                echo "Added $DKIM_KEY_STRING to /etc/opendkim/key.table"
        fi
}

# Add domain to trusted hosts
add_trusted_host(){
        TRUSTED_HOST_STRING="*.$DOMAIN"
        if grep -Fxq "$DOMAIN" /etc/opendkim/trusted.hosts; then
                echo "Domain already in /etc/opendkim/trusted.hosts"
        else
                echo -e $TRUSTED_HOST_STRING >> /etc/opendkim/trusted.hosts
                echo "Added $TRUSTED_HOST_STRING to /etc/opendkim/trusted.hosts"
        fi
}

mkdirs(){
        mkdir /etc/opendkim/keys/$DOMAIN
}

mkkeys(){
        opendkim-genkey -b 2048 -d $DOMAIN -D /etc/opendkim/keys/$DOMAIN -s default -v
}

own(){
        chown opendkim:opendkim /etc/opendkim/keys/$DOMAIN/default.private
        chmod 600 /etc/opendkim/keys/$DOMAIN/default.private
}

while true; do
        get_domain
        if ! domain_is_set || ! domain_is_valid; then
                echo -ne "Please provide valid domain name (ex. domain.com)."
                if ! try_again; then
                        break
                fi
        else
                dkim_add_signing_table
                dkim_add_key_table
                add_trusted_host
                mkdirs
                mkkeys
                own

                printf "\n"
                echo "You can now add these DNS records as follows:"
                echo "---------------------------------------------"
                echo "   TXT  @   v=spf1 mx ~all"
                # Print DKIM
                cat /etc/opendkim/keys/$DOMAIN/default.txt

                break
        fi
done

exit 0

test-dkim-key.sh

#!/bin/bash

printf "\n"
read -p "Domain: " DOMAIN

opendkim-testkey -d $DOMAIN -s default -vvv

Shout-out to @linuxbabe - for all his effort into creating in-depth #sysadmin tutorials about everything gnu linux. 🙏