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
File	Absolute path to the target file. For example: /proc/uptime.
Buf_Size	Buffer size to read the file.
Interval_Sec	Polling interval (seconds).
Interval_NSec	Polling interval (nanoseconds).
Add_Path	If enabled, the path is appended to each records. Default: false.
Key	Rename a key. Default: head.
Lines	Line number to read. If the number N is set, in_head reads first N lines like head(1) -n.
Split_line	If enabled, in_head generates key-value pair per line.
Threaded	Indicates whether to run this input in its own thread. Default: false.

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"}]

Get 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.