withHosts.sh

withHosts.sh

#!/usr/bin/env bash

# Runs a command with a modified /etc/hosts file.
# The modified file is created by adding the contents of the WITH_HOSTS, which
# is in the form of "name dest" pairs, separated by newlines.
#
# Usage:
# 
#   WITH_HOSTS=$'some.host localhost' \
#     withHosts curl -v https://some.host/
#
#   WITH_HOSTS=$'some.host localhost \n some.other.host google.com' \
#     withHosts curl -v https://some.other.host/

set -e
set -u
set -o pipefail

readonly HOSTS_FILE='/etc/hosts'

main() {
  local fd
  exec {fd}< <(genHostsFile "${HOSTS_FILE}")
  exec bwrap --dev-bind / / --ro-bind-data "${fd}" "${HOSTS_FILE}" "$@"
}

genHostsFile() {
  local name dest ip

  if [[ -n "${WITH_HOSTS:-}" ]]; then
    echo '## > withHosts: additional hosts'

    while read -r name dest ; do
      if [[ -z "${name}" ]]; then
        echo >&2 "# withHosts: skipping host entry for '${name}' -> '${dest}'"
        continue
      fi

      if [[ -z "${dest}" ]]; then
        echo >&2 "# withHosts: skipping host entry for '${name}' -> '${dest}'"
        continue
      fi

      ip="$(getent ahosts "${dest}" | awk '{print $1; exit;}' || true)"
      if [[ -z "${ip}" ]]; then
        echo >&2 "# withHosts: skipping host entry for '${name}' -> '${dest}' (${ip:-?})"
        continue
      fi

      echo >&2 "# withHosts: adding host entry for '${name}' -> '${dest}' (${ip})"
      echo "${ip} ${name}"
    done <<< "${WITH_HOSTS}"

    echo '## < withHosts'
    echo
  fi

  # cat the original /etc/hosts file
  cat "$@"
}

main "$@"