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.
To get an idea of the what and why of packr, please enjoy this short video: https://vimeo.com/219863271.
To install Packr utility
$ go get -u github.com/gobuffalo/packr/packr
To get the dependency
$ go get -u github.com/gobuffalo/packr
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 (relative) path to a folder on disk:
box := packr.NewBox("./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.FindBytes("index.html")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 packr 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 (
"fmt"
"github.com/gobuffalo/packr"
)
func main() {
box := packr.NewBox("./templates")
s, err := box.FindString("admin/index.html")
if err != nil {
log.Fatal(err)
}
fmt.Println(s)
}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:
- Look for the file in-memory (inside a Go binary)
- 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.
A box implements the http.FileSystem interface, meaning it can be used to serve static files.
package main
import (
"net/http"
"github.com/gobuffalo/packr"
)
func main() {
box := packr.NewBox("./templates")
http.Handle("/", http.FileServer(box))
http.ListenAndServe(":3000", nil)
}When it comes time to build, or install, your Go binary, simply use packr build or packr install just as you would go build or go install. All flags for the go tool are supported and everything works the way you expect, the only difference is your static assets are now bundled in the generated binary. If you want more control over how this happens, looking at the following section on building binaries (the hard way).
Before you build your Go binary, run the packr 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.
$ packr
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 packr clean after running the packr tool.
When you're done it is recommended that you run the packr clean command. This will remove all of the generated files that Packr created for you.
$ packr 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.
When it comes to building multiple releases you typically want that release to be built in a specific directory.
For example: ./releases
However, because passing a .go file requires absolute paths, we must compile the release in the appropriate absolute path.
GOOS=linux GOARCH=amd64 packr buildNow your project_name binary will be built at the root of your project dir. Great!
All that is left to do is to move that binary to your release dir:
Linux/macOS/Windows (bash)
mv ./project_name ./releasesWindows (cmd):
move ./project_name ./releasesPowershell:
Move-Item -Path .\project_name -Destination .\releases\If you target for Windows when building don't forget that it's project_name.exe
Now you can make multiple releases and all of your needed static files will be available!
Example Script for building to 3 common targets:
GOOS=darwin GOARCH=amd64 packr build && mv ./project_name ./releases/darwin-project_name \
&& GOOS=linux GOARCH=amd64 packr build && mv ./project_name ./releases/linux-project_name \
&& GOOS=windows GOARCH=386 packr build && mv ./project_name.exe ./releases/project_name.exe \
&& packr cleanThe packr 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.