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+(event-handler)=
+
+# Event Handler
+
+## Event Handler Technology Overview
+
+Event handler allows you to execute scripts when a string that matches
+a regex or a regex with a service name appears in journald logs. You
+can pass variables, arguments, and a full matching string to the script.
+
+## How to configure Event Handler
+
+> [1. Create an event handler](#create-an-event-handler)
+>
+> [2. Add regex to the script](#add-regex-to-the-script)
+>
+> [3. Add a full path to the script](#add-a-full-path-to-the-script)
+>
+> [4. Add optional parameters](#add-optional-parameters)
+
+## Event Handler Configuration Steps
+
+### 1. Create an event handler
+
+```{cfgcmd} set service event-handler event \<event-handler name\>
+
+This is an optional command because the event handler will be
+automatically created after any of the next commands.
+```
+
+
+### 2. Add regex to the script
+
+```{cfgcmd} set service event-handler event \<event-handler name\> filter pattern \<regex\>
+
+This is a mandatory command. Sets regular expression to match
+against log string message.
+
+:::{note}
+The regular expression matches if and only if the entire
+string matches the pattern.
+:::
+```
+
+
+### 3. Add a full path to the script
+
+```{cfgcmd} set service event-handler event \<event-handler name\> script path \<path to script\>
+
+This is a mandatory command. Sets the full path to the script.
+The script file must be executable.
+```
+
+
+### 4. Add optional parameters
+
+```{cfgcmd} set service event-handler event \<event-handler name\> filter syslog-identifier \<syslogid name\>
+
+This is an optional command. Filters log messages by
+syslog-identifier.
+```
+
+```{cfgcmd} set service event-handler event \<event-handler name\> script environment \<env name\> value \<env value\>
+
+This is an optional command. Adds environment and its value to
+the script. Use separate commands for each environment.
+
+One implicit environment exists.
+
+* ``message``: Full message that has triggered the script.
+```
+
+```{cfgcmd} set service event-handler event \<event-handler name\> script arguments \<arguments\>
+
+This is an optional command. Adds arguments to the script.
+Arguments must be separated by spaces.
+
+:::{note}
+We don't recommend using arguments. Using environment variables is preferable.
+:::
+```
+
+
+## Example
+
+Event handler that monitors the state of interface eth0.
+
+```none
+set service event-handler event INTERFACE_STATE_DOWN filter pattern '.*eth0.*,RUNNING,.*->.*'
+set service event-handler event INTERFACE_STATE_DOWN filter syslog-identifier 'netplugd'
+set service event-handler event INTERFACE_STATE_DOWN script environment interface_action value 'down'
+set service event-handler event INTERFACE_STATE_DOWN script environment interface_name value 'eth0'
+set service event-handler event INTERFACE_STATE_DOWN script path '/config/scripts/eventhandler.py'
+```
+
+Event handler script
+
+```none
+#!/usr/bin/env python3
+#
+# VyOS event-handler script example
+from os import environ
+import subprocess
+from sys import exit
+
+# Perform actions according to requirements
+def process_event() -> None:
+    # Get variables
+    message_text = environ.get('message')
+    interface_name = environ.get('interface_name')
+    interface_action = environ.get('interface_action')
+    # Print the message that triggered this script
+    print(f'Logged message: {message_text}')
+    # Prepare a command to run
+    command = f'sudo ip link set {interface_name} {interface_action}'.split()
+    # Execute a command
+    subprocess.run(command)
+
+if __name__ == '__main__':
+    try:
+        # Run script actions and exit
+        process_event()
+        exit(0)
+    except Exception as err:
+        # Exit properly in case if something in the script goes wrong
+        print(f'Error running script: {err}')
+        exit(1)
+```
+