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Exec

The exec input plugin, allows to execute external program and collects event logs.

WARNING: Because this plugin invokes commands via a shell, its inputs are subject to shell metacharacter substitution. Careless use of untrusted input in command arguments could lead to malicious command execution.

Container support

This plugin will not function in the distroless production images (AMD64 currently) as it needs a functional /bin/sh which is not present. The debug images use the same binaries so even though they have a shell, there is no support for this plugin as it is compiled out.

Configuration Parameters

The plugin supports the following configuration parameters:

Key Description
Command The command to execute, passed to popen(...) without any additional escaping or processing. May include pipelines, redirection, command-substitution, etc.
Parser Specify the name of a parser to interpret the entry as a structured message.
Interval_Sec Polling interval (seconds).
Interval_NSec Polling interval (nanosecond).
Buf_Size Size of the buffer (check unit sizes for allowed values)
Oneshot Only run once at startup. This allows collection of data precedent to fluent-bit's startup (bool, default: false)
Exit_After_Oneshot Exit as soon as the one-shot command exits. This allows the exec plugin to be used as a wrapper for another command, sending the target command's output to any fluent-bit sink(s) then exiting. (bool, default: false)
Propagate_Exit_Code When exiting due to Exit_After_Oneshot, cause fluent-bit to exit with the exit code of the command exited by this plugin. Follows shell conventions for exit code propagation. (bool, default: false)

Getting Started

You can run the plugin from the command line or through the configuration file:

Command Line

The following example will read events from the output of ls.

$ fluent-bit -i exec -p 'command=ls /var/log' -o stdout
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

[2018/03/21 17:46:49] [ info] [engine] started
[0] exec.0: [1521622010.013470159, {"exec"=>"ConsoleKit"}]
[1] exec.0: [1521622010.013490313, {"exec"=>"Xorg.0.log"}]
[2] exec.0: [1521622010.013492079, {"exec"=>"Xorg.0.log.old"}]
[3] exec.0: [1521622010.013493443, {"exec"=>"anaconda.ifcfg.log"}]
[4] exec.0: [1521622010.013494707, {"exec"=>"anaconda.log"}]
[5] exec.0: [1521622010.013496016, {"exec"=>"anaconda.program.log"}]
[6] exec.0: [1521622010.013497225, {"exec"=>"anaconda.storage.log"}]

Configuration File

In your main configuration file append the following Input & Output sections:

[INPUT]
    Name          exec
    Tag           exec_ls
    Command       ls /var/log
    Interval_Sec  1
    Interval_NSec 0
    Buf_Size      8mb
    Oneshot       false

[OUTPUT]
    Name   stdout
    Match  *

Use as a command wrapper

To use fluent-bit with the exec plugin to wrap another command, use the Exit_After_Oneshot and Propagate_Exit_Code options, e.g.:

[INPUT]
    Name                exec
    Tag                 exec_oneshot_demo
    Command             for s in $(seq 1 10); do echo "count: $s"; sleep 1; done; exit 1
    Oneshot             true
    Exit_After_Oneshot  true
    Propagate_Exit_Code true

[OUTPUT]
    Name   stdout
    Match  *

fluent-bit will output

[0] exec_oneshot_demo: [[1681702172.950574027, {}], {"exec"=>"count: 1"}]
[1] exec_oneshot_demo: [[1681702173.951663666, {}], {"exec"=>"count: 2"}]
[2] exec_oneshot_demo: [[1681702174.953873724, {}], {"exec"=>"count: 3"}]
[3] exec_oneshot_demo: [[1681702175.955760865, {}], {"exec"=>"count: 4"}]
[4] exec_oneshot_demo: [[1681702176.956840282, {}], {"exec"=>"count: 5"}]
[5] exec_oneshot_demo: [[1681702177.958292246, {}], {"exec"=>"count: 6"}]
[6] exec_oneshot_demo: [[1681702178.959508200, {}], {"exec"=>"count: 7"}]
[7] exec_oneshot_demo: [[1681702179.961715745, {}], {"exec"=>"count: 8"}]
[8] exec_oneshot_demo: [[1681702180.963924140, {}], {"exec"=>"count: 9"}]
[9] exec_oneshot_demo: [[1681702181.965852990, {}], {"exec"=>"count: 10"}]

then exit with exit code 1.

Translation of command exit code(s) to fluent-bit exit code follows the usual shell rules for exit code handling. Like with a shell, there is no way to differentiate between the command exiting on a signal and the shell exiting on a signal, and no way to differentiate between normal exits with codes greater than 125 and abnormal or signal exits reported by fluent-bit or the shell. Wrapped commands should use exit codes between 0 and 125 inclusive to allow reliable identification of normal exit. If the command is a pipeline, the exit code will be the exit code of the last command in the pipeline unless overridden by shell options.

Parsing command output

By default the exec plugin emits one message per command output line, with a single field exec containing the full message. Use the Parser directive to specify the name of a parser configuration to use to process the command input.

Security concerns

Take great care with shell quoting and escaping when wrapping commands. A script like

#!/bin/bash
# This is a DANGEROUS example of what NOT to do, NEVER DO THIS
exec fluent-bit \
  -o stdout \
  -i exec \
  -p exit_after_oneshot=true \
  -p propagate_exit_code=true \
  -p command='myscript $*'

can ruin your day if someone passes it the argument $(rm -rf /my/important/files; echo "deleted your stuff!")'

The above script would be safer if written with:

  -p command='echo '"$(printf '%q' "$@")" \

... but it's generally best to avoid dynamically generating the command or handling untrusted arguments to it at all.