Modify /etc/hosts by adding or removing comments from lines by IP address.

comment-hosts.sh

#!/bin/sh
# 1.0
set -eu

HOSTS=/etc/hosts

usage() {
    cat <<EOM
usage: $(basename "$0") OPTION...
Modify /etc/hosts by adding or removing comments from lines by IP address.

Options:
   -c IP    Comment IP.
   -u IP    Uncomment IP.
   -f       Don't interactively confirm changes.

This script modifies /etc/hosts atomically. It will diff changes and prompt for
confirmation unless -f is given.

EOM
}

if [ $# -eq 0 ]; then
    usage
    exit 1
fi

tmpf="$(mktemp /etc/hosts.new-XXXXXX)" || { echo>&2 "Are you root?"; exit 2; }
trap "rm -f '$tmpf'" EXIT
cp -a "$HOSTS" "$tmpf"

comment_ip() {
    ip="$1"
    echo>&2 "Commenting: $ip"
    sed -ri "s/^$ip /# $ip /" "$tmpf"
}

uncomment_ip() {
    ip="$1"
    echo>&2 "Uncommenting: $ip"
    sed -ri "s/^# ?$ip /$ip /" "$tmpf"
}

check_ip() {
    if ! echo "$*" | grep -E '^[0-9]{1,3}(\.[0-9]{1,3}){3}$' > /dev/null; then
        echo >&2 "Invalid IP: '$*'"
        return 1
    fi
}

force=
while getopts hc:u:f OPT; do
    case $OPT in
        h)
            usage
            exit 0
            ;;
        c)
            check_ip "$OPTARG"
            comment_ip "$OPTARG"
            ;;
        u)
            check_ip "$OPTARG"
            uncomment_ip "$OPTARG"
            ;;
        f)
            force=1
            ;;
        \?)
            usage
            exit 1
            ;;
    esac
done

if diff -u "$HOSTS" "$tmpf"; then
    # no changes
    echo >&2 "Nothing to do."
    exit
fi

if [ -n "$force" ]; then
    ans=y
else
    read -p "Commit change to $HOSTS ? [y/N] " ans
fi

if [ "$ans" = "y" ]; then
    mv -v "$tmpf" "$HOSTS"
    trap - EXIT
else
    echo >&2 "Made no changes."
fi