improve readme

README.md

@@ -1,8 +1,16 @@
 # check-diff
 
-Check the difference between the results of the command. check-diff is written to work as a mackerel check plugin.
+`check-diff` is a [Mackerel](https://mackerel.io/) 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.
+This is a rewrite of [kazeburo/diff-detector](https://github.com/kazeburo/diff-detector) with the same functionality.
+
+## Installation
+
+Download the latest binary from the [releases page](https://github.com/monitoring-forge/check-diff/releases), or install it with the [Mackerel CLI](https://github.com/mackerelio/mkr):
+
+```sh
+mkr plugin install monitoring-forge/check-diff
+```
 
 ## Usage
 
@@ -12,39 +20,87 @@ Usage:
   check-diff [OPTIONS] -- command args1 args2
 
 Application Options:
-      --identifier= indetify a file store the command result with given string
+  -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
-
 ```
 
-## Example
+### 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.
 
+```sh
+./check-diff -- cat /path/to/file
 ```
-% echo $(date) > date.txt
-% ./check-diff -- cat date.txt 
+
+### First run
+
+On the first execution, `check-diff` stores the command output and returns `OK` because there is nothing to compare with.
+
+```sh
+% ./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).
+
+```sh
+% 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 $(date) > date.txt     
+% 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```
 ```
 
-## mackerel.conf example
+## Options
 
+### `-w`, `--warn`
+
+Report differences as `WARNING` instead of `CRITICAL`.
+
+```sh
+./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.
+
+```sh
+./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.
+
+```sh
+./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:
+
+```conf
 [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"
 ```
 
-## Install
+## License
 
-Please download release page or `mkr plugin install monitoring-forge/check-diff`.
+See [LICENSE](LICENSE).