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Packr (v2)


Packr is a simple solution for bundling static assets inside of Go binaries. Most importantly it does it in a way that is friendly to developers while they are developing.

Intro Video

To get an idea of the what and why of Packr, please enjoy this short video:

Library Installation

$ go get -u

Binary Installation

$ go get -u

New File Format FAQs

In version v2.0.0 the file format changed and is not backward compatible with the packr-v1.x library.

Can packr-v1.x read the new format?

No, it can not. Because of the way the new file format works porting it to packr-v1.x would be difficult. PRs are welcome though. :)

Can packr-v2.x read packr-v1.x files?

Yes it can, but that ability will eventually be phased out. Because of that we recommend moving to the new format.

Can packr-v2.x generate packr-v1.x files?

Yes it can, but that ability will eventually be phased out. Because of that we recommend moving to the new format.

The --legacy command is available on all commands that generate -packr.go files.

$ packr2 --legacy


In Code

The first step in using Packr is to create a new box. A box represents a folder on disk. Once you have a box you can get string or []byte representations of the file.

// set up a new box by giving it a name and an optional (relative) path to a folder on disk:
box := packr.New("My Box", "./templates")

// Get the string representation of a file, or an error if it doesn't exist:
html, err := box.FindString("index.html")

// Get the []byte representation of a file, or an error if it doesn't exist:
html, err := box.Find("index.html")
What is a Box?

A box represents a folder, and any sub-folders, on disk that you want to have access to in your binary. When compiling a binary using the packr2 CLI the contents of the folder will be converted into Go files that can be compiled inside of a "standard" go binary. Inside of the compiled binary the files will be read from memory. When working locally the files will be read directly off of disk. This is a seamless switch that doesn't require any special attention on your part.


Assume the follow directory structure:

├── main.go
└── templates
    ├── admin
    │   └── index.html
    └── index.html

The following program will read the ./templates/admin/index.html file and print it out.

package main

import (


func main() {
  box := packr.New("myBox", "./templates")

  s, err := box.FindString("admin/index.html")
  if err != nil {
Development Made Easy

In order to get static files into a Go binary, those files must first be converted to Go code. To do that, Packr, ships with a few tools to help build binaries. See below.

During development, however, it is painful to have to keep running a tool to compile those files.

Packr uses the following resolution rules when looking for a file:

  1. Look for the file in-memory (inside a Go binary)
  2. Look for the file on disk (during development)

Because Packr knows how to fall through to the file system, developers don't need to worry about constantly compiling their static files into a binary. They can work unimpeded.

Packr takes file resolution a step further. When declaring a new box you use a relative path, ./templates. When Packr receives this call it calculates out the absolute path to that directory. By doing this it means you can be guaranteed that Packr can find your files correctly, even if you're not running in the directory that the box was created in. This helps with the problem of testing, where Go changes the pwd for each package, making relative paths difficult to work with. This is not a problem when using Packr.

Usage with HTTP

A box implements the http.FileSystem interface, meaning it can be used to serve static files.

package main

import (


func main() {
	box := packr.New("someBoxName", "./templates")

	http.Handle("/", http.FileServer(box))
	http.ListenAndServe(":3000", nil)

Building a Binary

Before you build your Go binary, run the packr2 command first. It will look for all the boxes in your code and then generate .go files that pack the static files into bytes that can be bundled into the Go binary.

$ packr2

Then run your go build command like normal.

NOTE: It is not recommended to check-in these generated -packr.go files. They can be large, and can easily become out of date if not careful. It is recommended that you always run packr2 clean after running the packr2 tool.

Cleaning Up

When you're done it is recommended that you run the packr2 clean command. This will remove all of the generated files that Packr created for you.

$ packr2 clean

Why do you want to do this? Packr first looks to the information stored in these generated files, if the information isn't there it looks to disk. This makes it easy to work with in development.


The packr2 command passes all arguments down to the underlying go command, this includes the -v flag to print out go build information. Packr looks for the -v flag, and will turn on its own verbose logging. This is very useful for trying to understand what the packr command is doing when it is run.


Compilation Errors with Go Templates

Q: I have a program with Go template files, those files are named foo.go and look like the following:

// Copyright {{.Year}} {{.Author}}. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package {{.Project}}

When I run packr2 I get errors like:

expected 'IDENT', found '{'

A: Packr works by searching your .go files for or calls. Because those files aren't "proper" Go files, Packr can't parse them to find the box declarations. To fix this you need to tell Packr to ignore those files when searching for boxes. A couple solutions to this problem are:

  • Name the files something else. The .tmpl extension is the idiomatic way of naming these types of files.
  • Rename the folder containing these files to start with an _, for example _templates. Packr, like Go, will ignore folders starting with the _ character when searching for boxes.
Dynamic Box Paths

Q: I need to set the path of a box using a variable, but packr.New("foo", myVar) doesn't work correctly.

A: Packr attempts to "automagically" set it's resolution directory when using, however, for dynamic paths you need to set it manually:

box := packr.New("foo", "|")
box.ResolutionDir = myVar
I don't want to pack files, but still use the Packr interface.

Q: I want to write code that using the Packr tools, but doesn't actually pack the files into my binary. How can I do that?

A: Using gives you back a *packr.Box that can be used as normal, but is excluded by the Packr tool when compiling.

Packr Finds No Boxes

Q: I run packr2 -v but it doesn't find my boxes:

DEBU[2019-03-18T18:48:52+01:00] *parser.Parser#NewFromRoots found prospects=0
DEBU[2019-03-18T18:48:52+01:00] found 0 boxes

A: Packr works by parsing .go files to find and declarations. If there aren't any .go in the folder that packr2 is run in it can not find those declarations. To fix this problem run the packr2 command in the directory containing your .go files.

Box Interfaces

Q: I want to be able to easily test my applications by passing in mock boxes. How do I do that?

A: Packr boxes and files conform to the interfaces found at Change your application to use those interfaces instead of the concrete Packr types.

// using concrete type
func myFunc(box *packr.Box) {}

// using interfaces
func myFunc(box packd.Box) {}
Expand ▾ Collapse ▴




View Source
const Version = "v2.8.1"

    Version of Packr


    View Source
    var (
    	// ErrResOutsideBox gets returned in case of the requested resources being outside the box
    	// Deprecated
    	ErrResOutsideBox = fmt.Errorf("can't find a resource outside the box")


    func PackBytes

    func PackBytes(box string, name string, bb []byte)

      PackBytes packs bytes for a file into a box. Deprecated

      func PackBytesGzip

      func PackBytesGzip(box string, name string, bb []byte) error

        PackBytesGzip packets the gzipped compressed bytes into a box. Deprecated

        func PackJSONBytes

        func PackJSONBytes(box string, name string, jbb string) error

          PackJSONBytes packs JSON encoded bytes for a file into a box. Deprecated


          type Box

          type Box struct {
          	Path            string            `json:"path"`
          	Name            string            `json:"name"`
          	ResolutionDir   string            `json:"resolution_dir"`
          	DefaultResolver resolver.Resolver `json:"default_resolver"`
          	// contains filtered or unexported fields

            Box represent a folder on a disk you want to have access to in the built Go binary.

            func Folder

            func Folder(path string) *Box

              Folder returns a Box that will NOT be packed. This is useful for writing tests or tools that need to work with a folder at runtime.

              func New

              func New(name string, path string) *Box

                New returns a new Box with the name of the box and the path of the box.

                func NewBox

                func NewBox(path string) *Box

                  NewBox returns a Box that can be used to retrieve files from either disk or the embedded binary. Deprecated: Use New instead.

                  func (*Box) AddBytes

                  func (b *Box) AddBytes(path string, t []byte) error

                    AddBytes sets t in by the given path

                    func (*Box) AddString

                    func (b *Box) AddString(path string, t string) error

                      AddString converts t to a byteslice and delegates to AddBytes to add to

                      func (*Box) Bytes

                      func (b *Box) Bytes(name string) []byte

                        Bytes is deprecated. Use Find instead

                        func (*Box) Find

                        func (b *Box) Find(name string) ([]byte, error)

                          Find returns either the byte slice of the requested file or an error if it can not be found.

                          func (*Box) FindString

                          func (b *Box) FindString(name string) (string, error)

                            FindString returns either the string of the requested file or an error if it can not be found.

                            func (*Box) Has

                            func (b *Box) Has(name string) bool

                              Has returns true if the resource exists in the box

                              func (*Box) HasDir

                              func (b *Box) HasDir(name string) bool

                                HasDir returns true if the directory exists in the box

                                func (*Box) List

                                func (b *Box) List() []string

                                  List shows "What's in the box?"

                                  func (*Box) MustBytes

                                  func (b *Box) MustBytes(name string) ([]byte, error)

                                    MustBytes is deprecated. Use Find instead.

                                    func (*Box) MustString

                                    func (b *Box) MustString(name string) (string, error)

                                      MustString is deprecated. Use FindString instead

                                      func (*Box) Open

                                      func (b *Box) Open(name string) (http.File, error)

                                        Open returns a File using the http.File interface

                                        func (*Box) Resolve

                                        func (b *Box) Resolve(key string) (file.File, error)

                                          Resolve will attempt to find the file in the box, returning an error if the find can not be found.

                                          func (*Box) SetResolver

                                          func (b *Box) SetResolver(file string, res resolver.Resolver)

                                            SetResolver allows for the use of a custom resolver for the specified file

                                            func (*Box) String

                                            func (b *Box) String(name string) string

                                              String is deprecated. Use FindString instead

                                              func (*Box) Walk

                                              func (b *Box) Walk(wf WalkFunc) error

                                                Walk will traverse the box and call the WalkFunc for each file in the box/folder.

                                                func (*Box) WalkPrefix

                                                func (b *Box) WalkPrefix(prefix string, wf WalkFunc) error

                                                  WalkPrefix will call box.Walk and call the WalkFunc when it finds paths that have a matching prefix

                                                  type File

                                                  type File = file.File

                                                    File has been deprecated and file.File should be used instead

                                                    type Pointer

                                                    type Pointer struct {
                                                    	ForwardBox  string
                                                    	ForwardPath string

                                                      Pointer is a resolvr which resolves a file from a different box.

                                                      func (Pointer) Resolve

                                                      func (p Pointer) Resolve(box string, path string) (file.File, error)

                                                        Resolve attempts to find the file in the specific box with the specified key

                                                        type WalkFunc

                                                        type WalkFunc = packd.WalkFunc

                                                          WalkFunc is used to walk a box