Add more docker_path error checks

And also improve general error handling so that the user knows when the
error comes from docker or from this shim.

Dockerfile

@@ -13,6 +13,9 @@ COPY docker "${DOCKER_PATH}"
 RUN mkdir -p /test && \
   echo test | tee /test/only-inside-container
 
+# Cleanup entrypoint to make execution faster
+ENTRYPOINT []
+
 # Set default stage
 FROM felipecrs/fixdockergid:latest
 

docker

@@ -1,6 +1,29 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
-set -euo pipefail
+if [[ "${DOND_SHIM_DEBUG:-false}" == true ]]; then
+  set -o xtrace
+fi
+
+# Exit on any kind of errors
+# https://unix.stackexchange.com/questions/23026
+set -o errexit
+set -o nounset
+set -o pipefail
+set -o errtrace
+set -o functrace
+shopt -s inherit_errexit
+
+function echo_error() {
+  echo "ERROR(dond-shim):" "${@}" >&2
+}
+
+function error() {
+  echo_error "${@}"
+  exit 1
+}
+
+# Ensure the user knows that the error was originated from the shim
+trap 'echo_error "Uncaught error at line ${LINENO}"' ERR
 
 # Finds all docker options that take a value from the --help output and
 # stores them in the docker_options_with_value array.
@@ -164,13 +187,16 @@ function fix_volume_arg() {
   fi
 }
 
-if [[ "${DOND_SHIM_DEBUG:-false}" == true ]]; then
-  set -x
-fi
+script_path="$(
+  cd -- "$(dirname "$0")" >/dev/null 2>&1
+  pwd -P
+)"
+readonly script_path
 
 # Parse supported environment variables
 readonly mock_container_root_on_host="${DOND_SHIM_MOCK_CONTAINER_ROOT_ON_HOST:-}"
 readonly print_command="${DOND_SHIM_PRINT_COMMAND:-false}"
+
 if [[ -n "${DOND_SHIM_DOCKER_PATH:-}" ]]; then
   # If DOND_SHIM_DOCKER_PATH is set, then use it to call the docker
   readonly docker_path="${DOND_SHIM_DOCKER_PATH}"
@@ -184,9 +210,13 @@ else
 fi
 
 # Ensure docker_path is different from this script to avoid infinite loop
-if [[ "${0}" == "${docker_path}" ]]; then
-  echo "Error: docker_path (${docker_path}) is pointing to dond-shim (${0})"
-  exit 1
+if [[ "${script_path}" == "${docker_path}" ]]; then
+  error "docker_path (${docker_path}) points to this script (${script_path})"
+fi
+
+# Ensure docker_path actually exists
+if ! command -v "${docker_path}" >/dev/null; then
+  error "docker_path (${docker_path}) points to a non-existing file or command"
 fi
 
 # Save original arguments

test.sh

@@ -1,27 +1,36 @@
-#!/bin/bash
-
-set -euo pipefail
+#!/usr/bin/env bash
 
 if [[ "${DEBUG:-false}" == true ]]; then
-  set -x
-  export DOND_SHIM_DEBUG=true
+  set -o xtrace
 fi
 
-# Set docker versions from args or use defaults
+# Exit on any kind of errors
+# https://unix.stackexchange.com/questions/23026
+set -o errexit
+set -o nounset
+set -o pipefail
+set -o errtrace
+set -o functrace
+shopt -s inherit_errexit
+
+# Ensure CTRL+C properly aborts the script
+trap "exit 130" INT
+
+# Set docker versions from args or use default
 if [[ $# -eq 0 ]]; then
   docker_versions=("18.09" "19.03" "20.10" "23" "24" latest)
 else
   docker_versions=("$@")
 fi
+readonly docker_versions
+
+readonly docker_args=(docker run --rm --env DOND_SHIM_DEBUG --volume /var/run/docker.sock:/var/run/docker.sock)
 
 for docker_version in "${docker_versions[@]}"; do
   echo "Testing with docker version: ${docker_version}"
 
   image_id="$(docker build --target test --build-arg "DOCKER_VERSION=${docker_version}" --quiet .)"
 
-  # shellcheck disable=SC2312
-  docker_args=(docker run --rm --env DOND_SHIM_DEBUG --entrypoint= --volume /var/run/docker.sock:/var/run/docker.sock)
-
   echo "Do not change global options or after the image"
   "${docker_args[@]}" --env DOND_SHIM_PRINT_COMMAND=true --volume "${PWD}:/wd" "${image_id}" \
     docker --host test run --volume /wd:/wd alpine --volume /wd:/wd |