}
            {
                condition: {|before, _| $before == /some/path/to/directory }
                code: 'hide foo'
            }
        ]
    }
})
```

When defining a hook as a string, the `$before` and `$after` variables are set to the previous and current environment variable value, respectively, similarly to the previous examples:

```nu
$env.config = ($env.config | upsert hooks {
    env_change: {
        PWD: {
            code: 'print $"changing directory from ($before) to ($after)"'
        }
    }
}
```

## Examples

### Adding a Single Hook to Existing Config

An example for PWD env change hook:

```nu
$env.config = ($env.config | upsert hooks.env_change.PWD {|config|
    let val = ($config | get -i hooks.env_change.PWD)

    if $val == null {
        $val | append {|before, after| print $"changing directory from ($before) to ($after)" }
    } else {
        [
            {|before, after| print $"changing directory from ($before) to ($after)" }
        ]
    }
})
```

### Automatically Activating an Environment when Entering a Directory

This one looks for `test-env.nu` in a directory

```nu
$env.config = ($env.config | upsert hooks.env_change.PWD {
    [
        {
            condition: {|_, after|
                ($after == '/path/to/target/dir'
                    and ($after | path join test-env.nu | path exists))
            }
            code: "overlay use test-env.nu"
        }
        {
            condition: {|before, after|
                ('/path/to/target/dir' not-in $after
                    and '/path/to/target/dir' in ($before | default "")
                    and 'test-env' in (overlay list))
            }
            code: "overlay hide test-env --keep-env [ PWD ]"
        }
    ]
})
```

### Filtering or Diverting Command Output

You can use the `display_output` hook to redirect the output of commands.
You should define a block that works on all value types.
The output of external commands is not filtered through `display_output`.

This hook can display the output in a separate window,
perhaps as rich HTML text. Here is the basic idea of how to do that:

```nu
$env.config = ($env.config | upsert hooks {
    display_output: { to html --partial --no-color | save --raw /tmp/nu-output.html }
})
```

You can view the result by opening `file:///tmp/nu-output.html` in
a web browser.
Of course this isn't very convenient unless you use
a browser that automatically reloads when the file changes.
Instead of the [`save`](/commands/docs/save.md) command, you would normally customize this