Refresh /etc/hosts file with unpublished IPv4 aliases.

refresh-hosts.bash

#!/usr/bin/env bash
# Refresh /etc/hosts file with unpublished IPv4 aliases.
#
# Set /etc/hosts.alias with domain/alias pairs, one pair per line:
#
#     example.com server-alias
#     example.net another
#
# https://github.com/rduplain/hosts

set -e

PROG=refresh-hosts

HOSTS_ALIAS="${HOSTS_ALIAS-/etc/hosts.alias}"
HOSTS_IN="${HOSTS_IN-/etc/hosts.base}"
HOSTS_OUT="${HOSTS_OUT-/etc/hosts}"
DELIMITER="${DELIMITER- }" # For use in newly created hosts entries.

ip_alias() {
    # Generate IPv4/alias pairs for domain/alias pairs in $HOSTS_ALIAS.

    set -- $(cat "$HOSTS_ALIAS")

    while [ "$1" != "" ]; do
        domain="$1"
        alias="$2"
        shift 2

        ip="$(dig "$domain" A +short | grep '^[.0-9]*$'; true)"
        if [ -z "$ip" ]; then
            echo "$PROG: error. unable to find IPv4 for '$domain'." >&2
            return 1
        fi

        echo "$ip" "$alias"
    done
}

main() {
    # Refresh $HOSTS_OUT with DNS result of domain/alias pairs in $HOSTS_ALIAS.

    args=(-f "$HOSTS_IN" -d "$DELIMITER")
    set -- $(ip_alias)
    while [ "$1" != "" ]; do
        ip="$1"
        alias="$2"
        shift 2

        args+=(-s "$ip$DELIMITER$alias")
    done

    exec hosts "${args[@]}" > "$HOSTS_OUT"
}

main "$@"