check-diff

check-diff is a Mackerel check plugin that detects changes in the output of a command by comparing it with the previous execution.

This is a rewrite of kazeburo/diff-detector with the same functionality.

Installation

Download the latest binary from the releases page, or install it with the Mackerel CLI:

mkr plugin install monitoring-forge/check-diff

Usage

% ./check-diff -h
Usage:
  check-diff [OPTIONS] -- command args1 args2

Application Options:
  -w, --warn        Set the error level to warning
      --identifier= Identify the file used to store the command result with the given string
      --workdir=    Set the working directory
  -v, --version     Show version

Help Options:
  -h, --help        Show this help message

Basic usage

Place -- between options and the command to run. The plugin executes the command, compares its output with the previous run, and reports the result.

./check-diff -- cat /path/to/file

First run

On the first execution, check-diff stores the command output and returns OK because there is nothing to compare with.

% ./check-diff -- cat date.txt
check-diff OK: first time execution command: 'cat date.txt'

Detecting differences

On subsequent runs, check-diff compares the current output with the stored output. If the output is unchanged, it returns OK; otherwise, it returns CRITICAL (or WARNING if -w is used).

% echo "Wed Aug 12 00:39:23 JST 2020" > date.txt
% ./check-diff -- cat date.txt
check-diff OK: no difference: ```Wed Aug 12 00:39:23 JST 2020```

% echo "Wed Aug 12 00:39:40 JST 2020" > date.txt
% ./check-diff -- cat date.txt
check-diff CRITICAL: found difference: ```@@ -1 +1 @@
-Wed Aug 12 00:39:23 JST 2020
+Wed Aug 12 00:39:40 JST 2020```

Options

-w, --warn

Report differences as WARNING instead of CRITICAL.

./check-diff -w -- cat /path/to/file

--identifier

Use a custom identifier for the file that stores the command result. This is useful when you want to run the same command multiple times with different identities.

./check-diff --identifier=production -- cat /etc/passwd
./check-diff --identifier=staging -- cat /etc/passwd

--workdir

Change the directory where the previous output is stored. By default, the plugin uses Mackerel's plugin working directory.

./check-diff --workdir=/var/lib/check-diff -- cat /etc/passwd

Mackerel configuration example

Add entries to mackerel-agent.conf to monitor file or command output changes:

[plugin.checks.uname-changed]
command = "/usr/local/bin/check-diff -- uname -a"

[plugin.checks.passwd-changed]
command = "/usr/local/bin/check-diff -- cat /etc/passwd"

License

See LICENSE.