embed

Overview ¶

Package embed provides access to files embedded in the running Go program.

Go source files that import "embed" can use the //go:embed directive to initialize a variable of type string, []byte, or FS with the contents of files read from the package directory or subdirectories at compile time.

For example, here are three ways to embed a file named hello.txt and then print its contents at run time.

Embedding one file into a string:

import _ "embed"

//go:embed hello.txt
var s string
print(s)

Embedding one file into a slice of bytes:

import _ "embed"

//go:embed hello.txt
var b []byte
print(string(b))

Embedded one or more files into a file system:

import "embed"

//go:embed hello.txt
var f embed.FS
data, _ := f.ReadFile("hello.txt")
print(string(data))

Directives ¶

A //go:embed directive above a variable declaration specifies which files to embed, using one or more path.Match patterns.

The directive must immediately precede a line containing the declaration of a single variable. Only blank lines and ‘//’ line comments are permitted between the directive and the declaration.

The type of the variable must be a string type, or a slice of a byte type, or FS (or an alias of FS).

For example:

package server

import "embed"

// content holds our static web server content.
//go:embed image/* template/*
//go:embed html/index.html
var content embed.FS

The Go build system will recognize the directives and arrange for the declared variable (in the example above, content) to be populated with the matching files from the file system.

The //go:embed directive accepts multiple space-separated patterns for brevity, but it can also be repeated, to avoid very long lines when there are many patterns. The patterns are interpreted relative to the package directory containing the source file. The path separator is a forward slash, even on Windows systems. Patterns may not contain ‘.’ or ‘..’ or empty path elements, nor may they begin or end with a slash. To match everything in the current directory, use ‘*’ instead of ‘.’. To allow for naming files with spaces in their names, patterns can be written as Go double-quoted or back-quoted string literals.

If a pattern names a directory, all files in the subtree rooted at that directory are embedded (recursively), except that files with names beginning with ‘.’ or ‘_’ are excluded. So the variable in the above example is almost equivalent to:

// content is our static web server content.
//go:embed image template html/index.html
var content embed.FS

The difference is that ‘image/*’ embeds ‘image/.tempfile’ while ‘image’ does not. Neither embeds ‘image/dir/.tempfile’.

If a pattern begins with the prefix ‘all:’, then the rule for walking directories is changed to include those files beginning with ‘.’ or ‘_’. For example, ‘all:image’ embeds both ‘image/.tempfile’ and ‘image/dir/.tempfile’.

The //go:embed directive can be used with both exported and unexported variables, depending on whether the package wants to make the data available to other packages. It can only be used with variables at package scope, not with local variables.

Patterns must not match files outside the package's module, such as ‘.git/*’ or symbolic links. Matches for empty directories are ignored. After that, each pattern in a //go:embed line must match at least one file or non-empty directory.

If any patterns are invalid or have invalid matches, the build will fail.

Strings and Bytes ¶

The //go:embed line for a variable of type string or []byte can have only a single pattern, and that pattern can match only a single file. The string or []byte is initialized with the contents of that file.

The //go:embed directive requires importing "embed", even when using a string or []byte. In source files that don't refer to embed.FS, use a blank import (import _ "embed").

File Systems ¶

For embedding a single file, a variable of type string or []byte is often best. The FS type enables embedding a tree of files, such as a directory of static web server content, as in the example above.

FS implements the io/fs package's FS interface, so it can be used with any package that understands file systems, including net/http, text/template, and html/template.

For example, given the content variable in the example above, we can write:

http.Handle("/static/", http.StripPrefix("/static/", http.FileServer(http.FS(content))))

template.ParseFS(content, "*.tmpl")

Tools ¶

To support tools that analyze Go packages, the patterns found in //go:embed lines are available in “go list” output. See the EmbedPatterns, TestEmbedPatterns, and XTestEmbedPatterns fields in the “go help list” output.

package main

import (
	"embed"
	"log"
	"net/http"
)

//go:embed internal/embedtest/testdata/*.txt
var content embed.FS

func main() {
	mutex := http.NewServeMux()
	mutex.Handle("/", http.FileServer(http.FS(content)))
	err := http.ListenAndServe(":8080", mutex)
	if err != nil {
		log.Fatal(err)
	}
}

Output: