Namespace: go.std.embed

v1.0

Contents

Summary

Provides a low-level interface to the embed package.

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.
Patterns must not match files whose names include the special punctuation characters " * < > ? ` ' | / \ and :.
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.

Index

Legend

Constants

Constants are variables with :const true in their metadata. Joker currently does not recognize them as special; as such, it allows redefining them or their values.

Variables

Functions, Macros, and Special Forms

Types