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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)
```
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