Head

The Head input plugin reads events from the head of a file. Its behavior is similar to the head command.

Configuration parameters

The plugin supports the following configuration parameters:

Key	Description	Default
add_path	If enabled, the path is appended to each record.	false
buf_size	Buffer size to read the file.	256
file	Absolute path to the target file. For example: /proc/uptime.	none
interval_nsec	Polling interval (nanoseconds).	0
interval_sec	Polling interval (seconds).	1
key	Rename a key.	head
lines	Line number to read. If the number N is set, in_head reads first N lines like head(1) -n.	0
split_line	If enabled, in_head generates key-value pair per line.	false
threaded	Indicates whether to run this input in its own thread.	false

Getting started

To read the head of a file, you can run the plugin from the command line or through the configuration file.

Command line

The following example will read events from the /proc/uptime file, tag the records with the uptime name and flush them back to the stdout plugin:

fluent-bit -i head -t uptime -p File=/proc/uptime -o stdout -m '*'

The output will look similar to:

Fluent Bit v1.x.x
* Copyright (C) 2019-2020 The Fluent Bit Authors
* Copyright (C) 2015-2018 Treasure Data
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2016/05/17 21:53:54] [ info] starting engine
[0] uptime: [1463543634, {"head"=>"133517.70 194870.97"}]
[1] uptime: [1463543635, {"head"=>"133518.70 194872.85"}]
[2] uptime: [1463543636, {"head"=>"133519.70 194876.63"}]
[3] uptime: [1463543637, {"head"=>"133520.70 194879.72"}]

Configuration file

In your main configuration file append the following:

fluent-bit.yaml

pipeline:
  inputs:
    - name: head
      tag: uptime
      file: /proc/uptime
      buf_size: 256
      interval_sec: 1
      interval_nsec: 0

  outputs:
    - name: stdout
      match: '*'

fluent-bit.conf

The interval is calculated like this:

Total interval (sec) = interval_sec + (interval_nsec / 1000000000).

For example: 1.5s = 1s + 500000000ns.

Split line mode

Use this mode to get a specific line. The following example gets CPU frequency from /proc/cpuinfo.

/proc/cpuinfo is a special file to get CPU information.

processor    : 0
vendor_id    : GenuineIntel
cpu family   : 6
model        : 42
model name   : Intel(R) Core(TM) i7-2640M CPU @ 2.80GHz
stepping     : 7
microcode    : 41
cpu MHz      : 2791.009
cache size   : 4096 KB
physical id  : 0
siblings     : 1

The CPU frequency is cpu MHz : 2791.009. The following configuration file gets the needed line:

fluent-bit.yaml

pipeline:
  inputs:
    - name: head
      tag: head.cpu
      file: /proc/cpuinfo
      lines: 8
      split_line: true

  filters:
    - name: record_modifier
      match: '*'
      whitelist_key: line7

  outputs:
    - name: stdout
      match: '*'

fluent-bit.conf

If you run the following command:

fluent-bit -c head.conf

The output is something similar to;

Fluent Bit v1.x.x
* Copyright (C) 2019-2020 The Fluent Bit Authors
* Copyright (C) 2015-2018 Treasure Data
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2017/06/26 22:38:24] [ info] [engine] started
[0] head.cpu: [1498484305.000279805, {"line7"=>"cpu MHz        : 2791.009"}]
[1] head.cpu: [1498484306.011680137, {"line7"=>"cpu MHz        : 2791.009"}]
[2] head.cpu: [1498484307.010042482, {"line7"=>"cpu MHz        : 2791.009"}]
[3] head.cpu: [1498484308.008447978, {"line7"=>"cpu MHz        : 2791.009"}]