Specify logstash configuration on command line with docker

Looking into the startup scripts inside the logstash docker image, one can find the following function in the logstash-core/lib/logstash/runner.rb file:

# where can I find the logstash.yml file?# 1. look for a "--path.settings path"# 2. look for a "--path.settings=path"# 3. check if the LS_SETTINGS_DIR environment variable is set# 4. return nil if not founddef fetch_settings_path(cli_args)  if i=cli_args.find_index("--path.settings")    cli_args[i+1]  elsif settings_arg = cli_args.find {|v| v.match(/--path.settings=/) }    match = settings_arg.match(/--path.settings=(.*)/)    match[1]  elsif ENV['LS_SETTINGS_DIR']    ENV['LS_SETTINGS_DIR']  else    nil  endend

So the solution seems to either pass --path.settings= (=-syntax with empty value) or set the LS_SETTINGs_DIR variable to a falsy value. And indeed:

$ docker run --rm -it docker.elastic.co/logstash/logstash-oss:6.0.0 \  --path.settings= -e 'input { stdin { } } output { stdout { } }'The stdin plugin is now waiting for input:

As Knittl stated --path.settings is consulted for logstash settings which are in a logstash.yml file that looks something like this (taken from the docker.elastic.co/logstash/logstash:6.1.2 image:

http.host: "0.0.0.0"path.config: /usr/share/logstash/pipelinexpack.monitoring.elasticsearch.url: http://elasticsearch:9200xpack.monitoring.elasticsearch.username: logstash_systemxpack.monitoring.elasticsearch.password: changeme

The problem is the path.config points to a directory containing only the default logstash.conf.

The solution with no side effects is:

$ docker run --rm -it docker.elastic.co/logstash/logstash-oss:6.0.0 \ --path.config="" -e 'input { stdin { } } output { stdout { } }'

This way the other settings in logstash.yml will persist.