+++
title = "Targets"
weight = 10
+++
A target is any exported function that has an optional first argument of context.Context, has either
no return or just an error return, and where the arguments are all of type `string`, `int`, `float64`, `bool`, or
`time.Duration`. Pointer types of these (`*string`, `*int`, `*float64`, `*bool`, `*time.Duration`) are
also accepted and treated as optional arguments (see [Optional Arguments](#optional-arguments) below).

e.g. these are all acceptable targets

```
func Build()
func Install(ctx context.Context) error
func Run(what string) error
func Exec(ctx context.Context, name string, count int, debug bool, timeout time.Duration) error
func Greet(name string, greeting *string)
```

A target is effectively a subcommand of mage while running mage in
this directory.  i.e. you can run a target by running `mage <target>`

## Arguments

Arguments aside from context are taken from the CLI arguments after the target name.
`string` is passed as-is, `int` are converted with `strconv.Atoi`, `bool` are converted with
`strconv.ParseBool`, and `time.Duration` are converted with `time.ParseDuration`.

Thus you could call Exec above by running

`mage exec somename 5 true 100ms`

All arguments are mandatory and must be specified in the order they appear in the function.

You can intersperse multiple targets with arguments as you'd expect:

`mage run foo.exe exec somename 5 true 100ms`

### Optional Arguments (v1.16.0+)

You can define optional arguments by using pointer types for any of the
supported argument types (`*string`, `*int`, `*float64`, `*bool`,
`*time.Duration`). Optional arguments are passed on the command line using
`-name=value` syntax, where `name` is the Go parameter name (case-insensitive).
If an optional argument is not provided, the pointer will be `nil`.

For `*bool` parameters, you can use the shorthand `-name` without a value,
which is equivalent to `-name=true`.

```go
func Greet(name string, greeting *string) {
    if greeting != nil {
        fmt.Printf("%s, %s!\n", *greeting, name)
    } else {
        fmt.Printf("Hello, %s!\n", name)
    }
}
```

You can call this target with or without the optional argument:

```plain
$ mage greet World
Hello, World!

$ mage greet World -greeting=Hi
Hi, World!
```

Optional arguments can be mixed freely with required arguments in the function
signature. Required arguments are always positional, while optional arguments use
the `-name=value` flag syntax and can appear in any order after the required
arguments.

```go
func Deploy(ctx context.Context, env string, version *string, dryRun *bool) error {
    // env is required, version and dryRun are optional
}
```

```plain
$ mage deploy production -version=1.2.3 -dryrun=true
```

The help output shows optional arguments in brackets:

```plain
$ mage -h greet
Greet greets someone with an optional greeting.

Usage:

	mage greet <name> [-greeting=<string>]
```

## Errors

If the function has an error return, errors returned from the function will
print to stdout and cause the magefile to exit with an exit code of 1.  Any
functions that do not fit this pattern are not considered targets by mage.

Comments on the target function will become documentation accessible by running
`mage -l` which will list all the build targets in this directory with the first
sentence from their docs, or `mage -h <target>` which will show the full comment
from the docs on the function, and a list of aliases if specified.

A target may be designated the default target, which is run when the user runs
`mage` with no target specified. To denote the default, create a `var Default =
<targetname>`  If no default target is specified, running `mage` with no target
will print the list of targets, like `mage -l`.

## Multiple Targets

Multiple targets can be specified as args to Mage, for example `mage foo bar
baz`.  Targets will be run serially, from left to right (so in this case, foo,
then once foo is done, bar, then once bar is done, baz).  Dependencies run using
mg.Deps will still only run once per mage execution, so if each of the targets
depend on the same function, that function will only be run once for all
targets.  If any target panics or returns an error, no later targets will be run.

## Contexts and Cancellation

A default context is passed into any target with a context argument.  This
context will have a timeout if mage was run with -t, and thus will cancel the
running targets and dependencies at that time.  To pass this context to
dependencies, use mg.CtxDeps(ctx, ...) to pass the context from the target to
its dependencies (and pass the context to sub-dependencies).  Dependencies run
with mg.Deps will not get the starting context, and thus will not be cancelled
when the timeout set with -t expires.

mg.CtxDeps will pass along whatever context you give it, so if you want to
modify the original context, or pass in your own, that will work like you expect
it to.

## Aliases

Target aliases can be specified using the following notation:

```go
var Aliases = map[string]interface{} {
  "i":     Install,
  "build": Install,
  "ls":    List,
}
```

The key is an alias and the value is a function identifier.
An alias can be used interchangeably with it's target.

## Namespaces

Namespaces are a way to group related commands, much like subcommands in a
normal application.   To define a namespace in your magefile, simply define an
exported named type of type `mg.Namespace`.  Then, every method on that type which
matches the normal target signature becomes a target under that namespace.

```go
import "github.com/magefile/mage/mg"

type Build mg.Namespace

// Builds the site using hugo.
func (Build) Site() error {
  return nil
}

// Builds the pdf docs.
func (Build) Docs() {}
```

To call a namespaced target, type it as `namespace:target`. For example, the
above would be called by typing

```plain
$ mage build:site
```

Similarly, the help for the target will show how it may be called:

```plain
$ mage -l

build:docs    Builds the pdf docs.
build:site    Builds the site using hugo.
```
