hosts

#!/bin/bash

help() {
   echo 'usage: hosts allow|block|status FQDN'
   echo '       hosts --help'
   echo
   echo '   adds or comments out an entry in /etc/hosts like this:'
   echo
   echo '       $ grep FQDN /etc/hosts'
   echo '       127.0.0.2 FQDN'
   echo
   echo '   thereby blockin or allowing access to the FQDN'
   echo
   echo '   ATTENTION: no checks are made on well-formed-ness of the'
   echo '              provided FQDN'
   exit 1
}

[ "$1" == "--help" ] && help

set -e          # stop on error
#set -u          # stop on undefined variable
set -o pipefail # stop part of pipeline failing

enforce_FQDN_arg() {
  # "$1" must be set
  #
  if [ "$1" == "" ]; then
    echo 'ERROR: you need to pass a FQDN hostname as parameter' >&2
    exit 1
  fi
  # if "$2" is given then the status of "$1" must be equal to "$2"
  #
  if [ "$2" != "" ]; then
    if [ "$2" != "$( hosts status "$1" )" ]; then
      echo "ERROR: $1 was not in state '$2' as it should have been"
      exit 1
    fi
  fi
}

case "$1" in
  allow)
    enforce_FQDN_arg "$2" 'blocked'
    sed --in-place \
        "s/^127.0.0.2 $2$/#127.0.0.2 $2/" \
        /etc/hosts
    ;;
  block)
    enforce_FQDN_arg "$2" 'allowed'
    sed --in-place \
        "s/^#127.0.0.2 $2$/127.0.0.2 $2/" \
        /etc/hosts
    ;;
  status)
    enforce_FQDN_arg "$2"
    if grep -q -E "^#127.0.0.2 $2$" /etc/hosts; then
      echo 'allowed'
    elif grep -q -E "^127.0.0.2 $2$" /etc/hosts; then
      echo 'blocked'
    else
      echo 'unknown'
    fi
    ;;
  *)
    echo "ERROR: you need to pass either 'block' or 'allow' as an argument" >&2
    exit 1
    ;;
esac