# Standard Library (Preview)

Nushell ships with a standard library of useful commands written in native Nu. By default, the standard library is loaded into memory (but not automatically imported) when Nushell starts.

[[toc]]

## Overview

The standard library currently includes:

- Assertions
- An alternative `help` system with support for completions.
- Additional JSON variant formats
- XML Access
- Logging
- And more

To see a complete list of the commands available in the standard library, run the following:

```nu
nu -c "
  use std
  scope commands
  | where name =~ '^std '
  | select name description extra_description
  | wrap 'Standard Library Commands'
  | table -e
"
```

::: note
The `use std` command above loads the entire standard library so that you can see all of the commands at once. This is typically not how it will be used (more info below). It is also run in a separate Nu subshell simply so that it is not loaded into scope in the shell you are using.
:::

## Importing the Standard Library

The Standard Library modules and submodules are imported with the [`use`](/commands/docs/use.md) command, just as any other module. See [Using Modules](./modules/using_modules.md) for more information.

While working at the commandline, it can be convenient to load the entire standard library using:

```nu
use std *
```

However, this form should be avoided in custom commands and scripts since it has the longest load time.

::: important Optimal Startup when Using the Standard Library
See the [notes below](#optimal-startup) on how to ensure that your configuration isn't loading the entire Standard Library.
:::

### Importing Submodules

Each submodule of the standard library can be loaded separately. Again, _for best performance, load only the submodule(s) that you need in your code._

See [Importing Modules](./modules/using_modules.md#importing-modules) for general information on using modules. The recommended import for each of the Standard Library submodules is listed below:

#### 1. Submodules with `<command> <subcommand>` form

These submodules are normally imported with `use std/<submodule>` (without a glob/`*`):

- `use std/assert`: `assert` and its subcommands
- `use std/bench`: The benchmarking command `bench`
- `use std/dirs`: The directory stack command `dirs` and its subcommands
- `use std/input`: The `input display` command
- `use std/help`: An alternative version of the `help` command and its subcommands which supports completion and other features
- `use std/iters`: Additional `iters`-prefixed iteration commands.
- `use std/log`: The `log <subcommands>` such as `log warning <msg>`
- `use std/math`: Mathematical constants such as `$math.E`. These can also be imported as definitions as in Form #2 below.

#### 2. Import the _definitions_ (contents) of the module directly

Some submodules are easier to use when their definitions (commands, aliases, constants, etc.) are loaded into the current scope. For instance:

```nu
use std/formats *
ls | to jsonl
```

Submodules that are normally imported with `use std/<submodule> *` (**with** a glob/`*`):

- `use std/dt *`: Additional commands for working with `date` values
- `use std/formats *`: Additional `to` and `from` format conversions
- `use std/math *`: The math constants without a prefix, such as `$E`. Note that the prefixed form #1 above is likely more understandable when reading and maintaining code.
- `use std/xml *`: Additional commands for working with XML data

#### 3. `use std <submodule>`

It is _possible_ to import Standard Library submodules using a space-separated form:

```nu
use std log
use std formats *
```

::: important
As mentioned in [Using Modules](./modules/using_modules.md#module-definitions), this form (like `use std *`) first loads the _entire_ Standard Library into scope and _then_ imports the submodules. In contrast, the slash-separated versions in #1 and #2 above _only_ import the submodule and will be much faster as a result.
:::

## The Standard Library Candidate Module

`std-rfc`, found in the [nushell Repository](https://github.com/nushell/nushell/tree/main/crates/nu-std/std-rfc), serves as a staging ground for possible Standard Library additions.

If you are interested in adding to the Standard Library, please submit your code via PR to the `std-rfc` module in that repository. We also encourage you to install this module and provide feedback on upcoming candidate commands.

::: details More details

Candidate commands for the Standard Library should, in general:

- Have broad appeal - Be useful to a large number of users or use cases
- Be well-written and clearly commented for future maintainers
- Implement help comments with example usage
- Have a description that explains why you feel the command should be a part of the standard library. Think of this as an "advertisement" of sorts to convince people to try the command and provide feedback so that it can be promoted in the future.

In order for a command to be graduated from RFC to the Standard Library, it must have:

- Positive feedback
- Few (or no) outstanding issues and, of course, no significant issues
- A PR author for the `std` submission. This does not necessarily have to be the original author of the command.
- Test cases as part of the `std` submission PR

Ultimately a member of the core team will decide when and if to merge the command into `std` based on these criteria.

Of course, if a candidate command in `std-rfc` no longer works or has too many issues, it may be removed from or disabled in `std-rfc`.

:::

## Disabling the Standard Library

To disable the standard library, you can start Nushell using:

```nu
nu --no-std-lib
```

This can be especially useful to minimize overhead when running a command in a subshell using `nu -c`. For example:

```nu
nu --no-std-lib -n -c "$nu.startup-time"
# => 1ms 125µs 10ns

nu -n -c "$nu.startup-time"
# => 4ms 889µs 576ns
```

You will not be able to import the library, any of its submodules, nor use any of its commands, when it is disabled in this way.

## Using `std/log` in Modules

::: warning Important!
`std/log` exports environment variables. To use the `std/log` module in your own module, please see [this caveat](./modules/creating_modules.md#export-env-runs-only-when-the-use-call-is-evaluated) in the "Creating Modules" Chapter.

:::

## Optimal Startup

If Nushell's startup time is important to your workflow, review your [startup configuration]([./configuration.md]) in `config.nu`, `env.nu`, and potentially others for inefficient use of the standard library. The following command should identify any problem areas:

```nu
view files
| enumerate | flatten
| where filename !~ '^std'
| where filename !~ '^entry'
| where {|file|
    (view span $file.start $file.end) =~ 'use\W+std[^\/]'
  }
```

Edit those files to use the recommended syntax in the [Importing Submodules](#importing-submodules) section above.

::: note
If a Nushell library (e.g., from [the `nu_scripts` repository](https://github.com/nushell/nu_scripts)), example, or doc is using this syntax, please report it via an issue or PR. These will be updated over time after Nushell 0.99.0 is released.

If a third-party module is using this syntax, please report it to the author/maintainers to update.
:::