print $"And a special welcome to our VIP today, ($vip)!"
}
```

Now run `help vip-greet` again to see the difference:

```text
Greet guests along with a VIP

Use for birthdays, graduation parties,
retirements, and any other event which
celebrates an event # for a particular
person.

Category: default

This command:
- does not create a scope.
- is not a built-in command.
- is not a subcommand.
- is not part of a plugin.
- is a custom command.
- is not a keyword.

Usage:
  > vip-greet <vip>


Flags:


  -h, --help - Display the help message for this command

Signatures:

  <any> | vip-greet[ <string>] -> <any>

Parameters:

  vip: <string> The special guest
  ...rest: <string> The other guests
```

Notice that the comments on the lines immediately before the `def` statement become a description of the command in the help system. Multiple lines of comments can be used. The first line (before the blank-comment line) becomes the Help `description`. This information is also shown when tab-completing commands.

The remaining comment lines become its `extra_description` in the help data.

::: tip
Run:

```nu
scope commands
| where name == 'vip-greet'
| wrap help
```

This will show the Help _record_ that Nushell creates.
:::

The comments following the parameters become their description. Only a single-line comment is valid for parameters.

::: tip Note
A Nushell comment that continues on the same line for argument documentation purposes requires a space before the ` #` pound sign.
:::

## Changing the Environment in a Custom Command

Normally, environment variable definitions and changes are [_scoped_ within a block](./environment.html#scoping). This means that changes to those variables are lost when they go out of scope at the end of the block, including the block of a custom command.

```nu
def foo [] {
    $env.FOO = 'After'
}

$env.FOO = "Before"
foo
$env.FOO
# => Before
```

However, a command defined using [`def --env`](/commands/docs/def.md) or [`export def --env`](/commands/docs/export_def.md) (for a [Module](modules.md)) will preserve the environment on the caller's side:

```nu
def --env foo [] {
    $env.FOO = 'After'
}

$env.FOO = "Before"
foo
$env.FOO
# => After
```

### Changing Directories (cd) in a Custom Command

Likewise, changing the directory using the `cd` command results in a change of the `$env.PWD` environment variable. This means that directory changes (the `$env.PWD` variable) will also be reset when a custom command ends. The solution, as above, is to use `def --env` or `export def --env`.

```nu
def --env go-home [] {
  cd ~
}

cd /
go-home
pwd
# => Your home directory
```

## Persisting

To make custom commands available in future Nushell sessions, you'll want to add them to your startup configuration. You can add command definitions:

- Directly in your `config.nu`
- To a file sourced by your `config.nu`
- To a [module](./modules.html) imported by your `config.nu`

See the [configuration chapter](configuration.md) for more details.