advertise_service

#!/bin/bash
# usage:
#  advertise_service <service_name>: adds the service in /etc/hosts
#  advertise_service -r <service_name>: removes the service from /etc/hosts
#  advertise_service -i <local_ip> [-r] <service_name>: adds or remove the
#                         service from value of local_ip (instead of 127.0.0.1)

# resolves the hostname of a Docker service and adds it in local host resolution
_advertise_service(){
  local _host_file="/etc/hosts"
  local _container_id
  local _hostname
  local _loop=0
  local _retries=12
  local _remove_only=0
  if [[ "x$1" = "xremove" ]]; then _remove_only=1; _retries=1; shift ; fi
  local _line_pattern=$1
  local _service=$2
  if [[ -z "$_service" ]]; then echo "ERROR: $(basename $0) should be called with the name of a service"; return 1 ; fi
  if [[ ! -w "$_host_file" ]]; then echo "WARN - $_host_file is not writable, won't change it for $_service"; return 0; fi
  while [[ -z $_container_id ]]; do
    _container_id=$(docker ps --filter "name=${_service}\." --format '{{ .ID }}' 2>/dev/null | tail -1)
    if [[ ${PIPESTATUS[0]} -ne 0 ]]; then
      echo "WARN - caught a Docker error while advertising $_service"
      return 0
    fi
    ((_loop++))
    if [[ $_loop -gt $_retries ]]; then
      if [[ $_remove_only -eq 0 ]]; then echo "WARN - cannot find service $_service to advertise"; fi
      return 0
    fi
    sleep 1
  done
  if [[ $(echo "$_container_id" | wc -w) -ne 1 ]]; then
    if [[ $_remove_only -eq 0 ]]; then
      echo "ERROR - unable to advertise service $_service"
      return 1
    else
      return 0
    fi
  fi
  cat "$_host_file" | grep "$_line_pattern" | grep -q "$_container_id"
  if [[ $? -eq 0 ]]; then
    sed -i.bak  "s/ $_container_id//" "$_host_file"
    echo "$_container_id has been removed from $_host_file"
  fi
  # check for any other hostname that looks like a Docker host
  _hostname=$(grep "$_line_pattern" "$_host_file" | sed 's/^.*\([a-h0-9]\{12\}\).*$/\1/')
  #if no match, _hostname will be the full line
  echo "$_hostname" | grep -q "$_line_pattern"
  if [[ $? -ne 0 && -n "$_hostname" ]]; then
    sed -i.bak  "s/^\($_line_pattern.*\)$_hostname\(.*\)$/\1\2/" "$_host_file"
    echo "WARN - $_hostname looks like a Docker hostname and has been removed from $_host_file"
  fi
  egrep -q "^$_line_pattern[ 	]*$" $_host_file
  if [[ $? -eq 0 ]]; then
    sed -i.bak "s/^$_line_pattern[ 	]*$//" "$_host_file"
    echo "$_line_pattern removed from $_host_file because empty"
  fi
  
  if [[ $_remove_only -eq 1 ]]; then return 0; fi
  grep -q "$_line_pattern" "$_host_file"
  if [[ $? -ne 0 ]]; then
    echo "$_line_pattern $_container_id" >> "$_host_file"
  else
    sed -i.bak "s/^\(${_line_pattern}.*\)$/\1 $_container_id/" $_host_file
  fi
  echo "$_container_id has been added to $_host_file"
}
 
ip=127.0.0.1
services=""
remove=""
while getopts ":i:r" opt; do
  case $opt in
    r)
      remove=remove
      shift
      ;;
    i)
      ip=$OPTARG
      shift 2
      ;;
    \?)
      echo "Invalid option: -$OPTARG" >&2
      ;;
  esac
done
services=$@
_advertise_service $remove "$ip" $services