# Custom Filters

Bashly supports custom filter functions for your commands. These filters allow
you to define custom conditions that can prevent your command from running
unless they are met.

This is how it works:

1. In your Bashly configuration file, commands can include a `filters` option,
   which specifies an array of one or more function names.
2. Whenever you run your script, it will search for functions with those names,
   prefixed by `filter_`, and execute them before running the command code.
3. If any of the functions return (echo) a string, it will be treated as an
   error. The returned string will be displayed on the screen as the error message.

+++ Configuration

```yaml bashly.yml
name: viewer
filters:
- docker_running
```

+++ Custom Function

```shell src/lib/filter_docker_running.sh
filter_docker_running() {
  docker info > /dev/null 2>&1 || echo "Docker must be running"
}
```

+++

[!button variant="primary" icon="code-review" text="Filters Example"](https://github.com/bashly-framework/bashly/tree/master/examples/filters#readme)

!!!success Tips
- To verify a program is installed, use [`dependencies`](/configuration/command/#dependencies) instead.
- To verify an environment variable is defined, use [`environment_variables`](/configuration/command/#environment_variables) instead.
!!!