Command Line Option

This article describes the command-line tools and its options in fluentd project.


Following are the fluentd command-line options (fluentd -h):

Usage: fluentd [options]
    -s, --setup [DIR=/etc/fluent]    install sample configuration file to the directory
    -c, --config PATH                config file path (default: /etc/fluent/fluent.conf)
        --dry-run                    Check fluentd setup is correct or not
        --show-plugin-config=PLUGIN  [DEPRECATED] Show PLUGIN configuration and exit(ex: input:dummy)
    -p, --plugin DIR                 add plugin directory
    -I PATH                          add library path
    -r NAME                          load library
    -d, --daemon PIDFILE             daemonize fluent process
        --under-supervisor           run fluent worker under supervisor (this option is NOT for users)
        --no-supervisor              run fluent worker without supervisor
        --workers NUM                specify the number of workers under supervisor
        --user USER                  change user
        --group GROUP                change group
        --umask UMASK                change umask
    -o, --log PATH                   log file path
        --log-rotate-age AGE         generations to keep rotated log files
        --log-rotate-size BYTES      sets the byte size to rotate log files
        --log-event-verbose          enable log events during process startup/shutdown
    -i CONFIG_STRING,                inline config which is appended to the config file on-the-fly
        --emit-error-log-interval SECONDS
                                     suppress interval seconds of emit error logs
        --suppress-repeated-stacktrace [VALUE]
                                     suppress repeated stacktrace
        --without-source             invoke a fluentd without input plugins
        --use-v1-config              Use v1 configuration format (default)
        --use-v0-config              Use v0 configuration format
        --strict-config-value        Parse config values strictly
        --enable-input-metrics       Enable input plugin metrics on fluentd
        --enable-size-metrics        Enable plugin record size metrics on fluentd
    -v, --verbose                    increase verbose level (-v: debug, -vv: trace)
    -q, --quiet                      decrease verbose level (-q: warn, -qq: error)
        --suppress-config-dump       suppress config dumping when fluentd starts
    -g, --gemfile GEMFILE            Gemfile path
    -G, --gem-path GEM_INSTALL_PATH  Gemfile install path (default: $(dirname $gemfile)/vendor/bundle)
        --conf-encoding ENCODING     specify configuration file encoding
        --disable-shared-socket      Don't open shared socket for multiple workers

Important Options

  • -g, --gemfile: Fluentd starts with bundler-managed dependent plugins.

  • --suppress-config-dump: Fluentd starts without configuration dump. If you do not want to show the configuration in fluentd logs, e.g. it contains private keys, then this option is useful.

  • --suppress-repeated-stacktrace: If true, suppresses the stacktrace in fluentd logs. Since v0.12, this option is true by default.

  • --without-source: Fluentd starts without input plugins. This option is useful for flushing buffers with no new incoming events.

  • -i, --inline-config: If fluentd is used on XaaS which does not support persistent disks, this option is useful.

  • --no-supervisor: If you want to use your supervisor tools, this option avoids double supervisor.

Set via Configuration File

Some options can be set via <system> directive via configuration file. See configuration file article for more on <system> directive.


The fluent-cat command sends an event to fluentd in_forward/in_unix plugin. This is particularly useful for testing.

Here is its usage (fluent-cat --help):

Usage: fluent-cat [options] <tag>
    -p, --port PORT                  fluent tcp port (default: 24224)
    -h, --host HOST                  fluent host (default:
    -u, --unix                       use unix socket instead of tcp
    -s, --socket PATH                unix socket path (default: /var/run/fluent/fluent.sock)
    -f, --format FORMAT              input format (default: json)
        --json                       same as: -f json
        --msgpack                    same as: -f msgpack
        --none                       same as: -f none
        --message-key KEY            key field for none format (default: message)
        --time-as-integer            Send time as integer for v0.12 or earlier
        --retry-limit N              Specify the number of retry limit (default: 5)
        --event-time TIME_STRING     Specify the time expression string (default: nil)


Send JSON message with debug.log tag to the local fluentd instance:

echo '{"message":"hello"}' | fluent-cat debug.log

Send JSON message to an instance of fluentd on another machine on the network:

echo '{"message":"hello"}' | fluent-cat debug.log --host testserver --port 24225


It generates the formatted configuration document with the specified format for a plugin.

Here is its usage (fluent-plugin-config-format -h):

Usage: fluent-plugin-config-format [options] <type> <name>

Output plugin config definitions

        type: input,output,filter,buffer,parser,formatter,storage,service_discovery
        name: registered plugin name

        --verbose                    Be verbose
    -c, --compact                    Compact output
    -f, --format=FORMAT              Specify format. (markdown,txt,json)
    -I PATH                          Add PATH to $LOAD_PATH
    -r NAME                          Load library
    -p, --plugin=DIR                 Add plugin directory
    -t, --table                      Use table syntax to dump parameters


Generate a README style document from the plugin's config parameters:

fluent-plugin-config-format output null

Generate an old-style output from the plugin's config parameters:

fluent-plugin-config-format -f txt output null

Generate a markdown table style output from the plugin's config parameters:

fluent-plugin-config-format -f markdown --table output null


It generates the Fluentd plugin project template. It is good for starting to Fluentd plugin development for using the new API plugin. For more details, refer to the Generating plugin project skeleton section.

Here is its usage (fluent-plugin-generate -h);

Usage: fluent-plugin-generate [options] <type> <name>

Generate a project skeleton for creating a Fluentd plugin

        type: input,output,filter,parser,formatter,storage
        name: Your plugin name (fluent-plugin- prefix will be added to <name>)

        --[no-]license=NAME          Specify license name (default: Apache-2.0)


Control fluentd process using Signals or Windows Event.

Usage: fluent-ctl COMMAND [PID_OR_SVCNAME]



You can specify the process id of the supervisor process.

fluent-ctl shutdown 11111

If you run Fluentd as a Windows service, then you can omit PID_OR_SVCNAME when the service name is the default value fluentdwinsvc.

fluent-ctl dump

About dump

This function is mainly for Windows.

On Windows, this makes all Fluentd processes (including all worker processes) dump their internal status to the system temp directory (C:\\Windows\\Temp). This is the same behavior as sending SIGCONT to all processes on non-Windows.

Since this uses SIGDUMP, you can change the output path by specifying SIGDUMP_PATH environment variable. Note that the path has to be a file path.

$ $env:SIGDUMP_PATH="/sigdump/sigdump.log" # The directory `sidgump` has to exist.
$ fuentd -c ...
$ fluent-ctl dump # At another shell.

Then Fluentd dumps files as following. Each process id is automatically added to the path.

... [info]: fluent/log.rb:330:info: dump to /sigdump/sigdump-41544.log.
... [info]: #0 fluent/log.rb:330:info: dump to /sigdump/sigdump-21152.log.
... [info]: #1 fluent/log.rb:330:info: dump to /sigdump/sigdump-15656.log.

As for non-Windows, you don't have to use this function because you can manually send SIGCONT to each process. Although you can use this function on non-Windows, this just sends SIGCONT to the supervisor process, so you can get only the status of the supervisor process.


Control Linux Capability for Fluentd. See Linux Capability article.

If this article is incorrect or outdated, or omits critical information, please let us know. Fluentd is an open-source project under Cloud Native Computing Foundation (CNCF). All components are available under the Apache 2 License.

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