# Custom commands

Nu's ability to compose long pipelines allow you a lot of control over your data and system, but it comes at the price of a lot of typing. Ideally, you'd be able to save your well-crafted pipelines to use again and again.

This is where custom commands come in.

An example definition of a custom command:

def greet [name] {
  echo "hello" $name

In this definition, we define the greet command, which takes a single parameter name. Following this parameter is the block that represents what will happen with the custom command runs. When called, the custom command will set the value passed for name as the $name variable, which will be available to the block.

To run the above, we can call it like we would call built-in commands:

> greet "world"

As we do, we also get output just as we would with built-in commands:

 0 │ hello 
 1 │ world 

# Command names

In Nushell, a command name a string of characters or a quoted string. Here are some examples of valid command names: greet, get-size, mycommand123, "mycommand", 😊, and 123.

Note: It's common practice in Nushell to separate the words of the command with - for better readability. For example get-size instead of getsize or get_size.

# Sub-commands

You can also define subcommands to commands using a space. For example, if we wanted to add a new subcommand to str, we can create it my naming our subcommand to start with "str ". For example:

def "str mycommand" [] {
  echo hello

Now we can call our custom command as if it were a built-in subcommand of str:

> str mycommand

# Parameter types

When defining custom commands, you can name and optionally set the type for each parameter. For example, you can write the above as:

def greet [name: string] {
  echo "hello" $name

The types of parameters are optional. Nushell supports leaving them off, and treating the parameter as any if so. If you annotated a type on a parameter, Nushell will check this type when you call the function.

For example, let's say you wanted to take in an int instead:

def greet [name: int] {
  echo "hello" $name

greet world

If we try to run the above, Nushell will tell us that the types don't match:

error: Type Error
  ┌─ shell:6:7
5 │ greet world
  │       ^^^^^ Expected int, found world

This can help you guide users of your definitions to call them with only the supported types.

# Flags

In addition to passing positional parameters, you can also pass named parameters by defining flags for your custom commands.

For example:

def greet [
  name: string
  --age: int
] {
  echo $name $age

In the greet definition above, we define the name positional parameter as well as an age flag. This allows the caller of greet to optionally pass the age parameter as well.

You can call the above using:

> greet world --age 10


> greet --age 10 world

Or even leave the flag off altogether:

> greet world

Flags can also be defined to have a shorthand version. This allows you to pass a simpler flag as well as a longhand, easier-to-read flag.

Let's extend the previous example to use a shorthand flag for the age value:

def greet [
  name: string
  --age (-a): int
] {
  echo $name $age

Note: Flags are named by their longhand name, so the above example would need to use $age and not $a.

Now, we can call this updated definition using the shorthand flag:

> greet -a 10 hello

# Documenting your command

In order to best help users of your custom commands, you can also document them with additional descriptions for the commands and the parameters.

Taking our previous example:

def greet [
  name: string
  --age (-a): int
] {
  echo $name $age

Once defined, we can run help greet to get the help information for the command:

  > greet <name> {flags} 


  -h, --help: Display this help message
  -a, --age <integer>  

You can see the parameter and flag that we defined, as well as the -h help flag that all commands get.

To improve this help, we can add descriptions to our definition that will show up in the help:

# A greeting command that can greet the caller
def greet [
  name: string      # The name of the person to greet
  --age (-a): int   # The age of the person
] {
  echo $name $age

The comments that we put on the definition and its parameters then appear as descriptions inside the help of the command.

Now, if we run help greet, we're given a more helpful help text:

A greeting command that can greet the caller

  > greet <name> {flags} 

  <name> The name of the person to greet

  -h, --help: Display this help message
  -a, --age <integer>: The age of the person

# Output

Custom commands stream their output just like built-in commands. For example, let's say we wanted to refactor this pipeline:

> ls | get name

Let's move ls into a command that we've written:

def my-ls [] { ls }

We can use the output from this command just as we would ls.

> my-ls | get name
 0 │ myscript.nu           
 1 │ myscript2.nu          
 2 │ welcome_to_nushell.md 

This lets us easily build custom commands and process their output. Note, we don't use return statements like other languages. Instead, we build pipelines that output streams of data that can be connected to other pipelines.

# Input

Custom commands can also take input, just like other commands. This input is passed from the pipeline to the block that the custom command uses.

Let's make our own echo command that also outputs a line after each value that it gets from the input:

def my-echo [] {
  each {
    echo $it "--"

Now, if we call the above command later in a pipeline, we can see what it does with the input:

> echo foo bar | my-echo
 0 │ foo 
 1 │ --  
 2 │ bar 
 3 │ --  

# Persisting

For information about how to persist custom commands so that they're visible when you start up Nushell, see the configuration chapter and add your custom commands to the startup section.