Cannot obtain elasticsearch version from docker container as one-liner

Question

I tried to get Elasticsearch version by one-liner command:

$ docker run --rm -ti --memory=512m --cpus=1 docker.elastic.co/elasticsearch/elasticsearch:8.0.0 elasticsearch --version

It starts Elasticsearch, show logs and did not return version. In case, I run Elasticsearch with bash, and ask for Elasticsearch version, it works fine:

$ docker run --rm -ti --memory=512m --cpus=1 docker.elastic.co/elasticsearch/elasticsearch:8.0.0 /bin/bash
$ elasticsearch --version
Version: 8.0.0, Build: default/docker/1b6a7ece17463df5ff54a3e1302d825889aa1161/2022-02-03T16:47:57.507843096Z, JVM: 17.0.1

When I test it with date command, it works fine:

$ docker run --rm -ti --memory=512m --cpus=1 docker.elastic.co/elasticsearch/elasticsearch:8.0.0 /bin/date                                                                                                                                            
WARNING: Your kernel does not support swap limit capabilities or the cgroup is not mounted. Memory limited without swap.
Fri Feb 18 14:25:08 UTC 2022

Why I cannot obtain Elasticsearch version from command line as one-liner but it works with date command?

Answer 1

To make sure only the given command is run you should set the entrypoint to empty string while executing the docker run.

docker run --rm --entrypoint '' docker.elastic.co/elasticsearch/elasticsearch:8.0.0 elasticsearch --version

In docker run, if an entrypoint is defined in an image, the command is passed as an argument to this entrypoint. And there is a logic in the current entrypoint that executes the passed command unless it is elasticsearch.

A related code in the entrypoint that caused the observed behaviour:

# docker run --rm docker.elastic.co/elasticsearch/elasticsearch:8.0.0 cat /usr/local/bin/docker-entrypoint.sh

#!/bin/bash
set -e

# Files created by Elasticsearch should always be group writable too
umask 0002

# Allow user specify custom CMD, maybe bin/elasticsearch itself
# for example to directly specify `-E` style parameters for elasticsearch on k8s
# or simply to run /bin/bash to check the image
if [[ "$1" == "eswrapper" || $(basename "$1") == "elasticsearch" ]]; then
  # Rewrite CMD args to remove the explicit command,
  # so that we are backwards compatible with the docs
  # from the previous Elasticsearch versions < 6
  # and configuration option:
  # https://www.elastic.co/guide/en/elasticsearch/reference/5.6/docker.html#_d_override_the_image_8217_s_default_ulink_url_https_docs_docker_com_engine_reference_run_cmd_default_command_or_options_cmd_ulink
  # Without this, user could specify `elasticsearch -E x.y=z` but
  # `bin/elasticsearch -E x.y=z` would not work. In any case,
  # we want to continue through this script, and not exec early.
  set -- "${@:2}"
else
  # Run whatever command the user wanted
  exec "$@"
fi
...