root@plum:/etc/fluent# fluentd -c /etc/fluent/fluent.conf 
2020-07-06 20:48:08 +0900 [info]: parsing config file is succeeded path="/etc/fluent/fluent.conf"
2020-07-06 20:48:08 +0900 [info]: gem 'fluentd' version '1.11.1'
2020-07-06 20:48:08 +0900 [warn]: 'type' is deprecated parameter name. use '@type' instead.
2020-07-06 20:48:08 +0900 [warn]: 'type' is deprecated parameter name. use '@type' instead.
2020-07-06 20:48:08 +0900 [info]: using configuration file: <ROOT>
  <source>
    type forward
  </source>
  <match debug.*>
    type file
    path "/tmp/fluent_debug.txt"
    <buffer time>
      path "/tmp/fluent_debug.txt"
    </buffer>
  </match>
</ROOT>
2020-07-06 20:48:08 +0900 [info]: starting fluentd-1.11.1 pid=1542 ruby="2.5.1"
2020-07-06 20:48:09 +0900 [info]: spawn command to main:  cmdline=["/usr/bin/ruby2.5", "-Eascii-8bit:ascii-8bit", "/usr/bin/fluentd", "-c", "/etc/fluent/fluent.conf", "--under-supervisor"]
2020-07-06 20:48:13 +0900 [warn]: #0 'type' is deprecated parameter name. use '@type' instead.
2020-07-06 20:48:13 +0900 [info]: adding match pattern="debug.*" type="file"
2020-07-06 20:48:13 +0900 [warn]: #0 'type' is deprecated parameter name. use '@type' instead.
2020-07-06 20:48:13 +0900 [info]: adding source type="forward"
2020-07-06 20:48:13 +0900 [info]: #0 starting fluentd worker pid=1551 ppid=1542 worker=0
2020-07-06 20:48:13 +0900 [info]: #0 listening port port=24224 bind="0.0.0.0"
2020-07-06 20:48:13 +0900 [info]: #0 fluentd worker is now running worker=0
^C2020-07-06 20:48:37 +0900 [info]: Received graceful stop
2020-07-06 20:48:38 +0900 [info]: #0 fluentd worker is now stopping worker=0
2020-07-06 20:48:38 +0900 [info]: #0 shutting down fluentd worker worker=0
2020-07-06 20:48:38 +0900 [info]: #0 shutting down input plugin type=:forward plugin_id="object:1288ef0"
2020-07-06 20:48:38 +0900 [info]: #0 shutting down output plugin type=:file plugin_id="object:109af04"
2020-07-06 20:48:39 +0900 [info]: Worker 0 finished with status 0

root@plum:~# echo '{"key" : "value"}' | fluent-cat debug.test

root@plum:~# cat /tmp/fluent_debug.txt/buffer.b5a9c4738f597488ed4d4450559b17a02.log
2020-07-06T20:48:21+09:00	debug.test	{"key":"value"}