embed

Documentation

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:

import "embed"

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

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

//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 variable must be of type string, []byte, or FS exactly. Named types or type aliases derived from those types are not allowed.

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. 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.

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. Similarly, it can be used with both global and function-local variables, depending on what is more convenient in context.

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.

Patterns must not contain ‘.’ or ‘..’ path elements nor begin with a leading slash. To match everything in the current directory, use ‘*’ instead of ‘.’.

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.

Index

• type FS
  • func (f FS) Open(name string) (fs.File, error)
  • func (f FS) ReadDir(name string) ([]fs.DirEntry, error)
  • func (f FS) ReadFile(name string) ([]byte, error)

Types

type FS

type FS struct {
	// contains filtered or unexported fields
}

An FS is a read-only collection of files, usually initialized with a //go:embed directive. When declared without a //go:embed directive, an FS is an empty file system.

An FS is a read-only value, so it is safe to use from multiple goroutines simultaneously and also safe to assign values of type FS to each other.

FS implements fs.FS, so it can be used with any package that understands file system interfaces, including net/http, text/template, and html/template.

See the package documentation for more details about initializing an FS.

func (FS) Open

func (f FS) Open(name string) (fs.File, error)

Open opens the named file for reading and returns it as an fs.File.

func (FS) ReadDir

func (f FS) ReadDir(name string) ([]fs.DirEntry, error)

ReadDir reads and returns the entire named directory.

func (FS) ReadFile

func (f FS) ReadFile(name string) ([]byte, error)

ReadFile reads and returns the content of the named file.