go - The Go Programming Language


Command go

Go is a tool for managing Go source code.


go command [arguments]

The commands are:

build       compile packages and dependencies
clean       remove object files
doc         run godoc on package sources
env         print Go environment information
fix         run go tool fix on packages
fmt         run gofmt on package sources
get         download and install packages and dependencies
install     compile and install packages and dependencies
list        list packages
run         compile and run Go program
test        test packages
tool        run specified go tool
version     print Go version
vet         run go tool vet on packages

Use "go help [command]" for more information about a command.

Additional help topics:

gopath      GOPATH environment variable
packages    description of package lists
remote      remote import path syntax
testflag    description of testing flags
testfunc    description of testing functions

Use "go help [topic]" for more information about that topic.

Compile packages and dependencies


go build [-o output] [build flags] [packages]

Build compiles the packages named by the import paths, along with their dependencies, but it does not install the results.

If the arguments are a list of .go files, build treats them as a list of source files specifying a single package.

When the command line specifies a single main package, build writes the resulting executable to output. Otherwise build compiles the packages but discards the results, serving only as a check that the packages can be built.

The -o flag specifies the output file name. If not specified, the name is packagename.a (for a non-main package) or the base name of the first source file (for a main package).

The build flags are shared by the build, install, run, and test commands:

	force rebuilding of packages that are already up-to-date.
	print the commands but do not run them.
-p n
	the number of builds that can be run in parallel.
	The default is the number of CPUs available.
	print the names of packages as they are compiled.
	print the name of the temporary work directory and
	do not delete it when exiting.
	print the commands.

-compiler name
	name of compiler to use, as in runtime.Compiler (gccgo or gc)
-gccgoflags 'arg list'
	arguments to pass on each gccgo compiler/linker invocation
-gcflags 'arg list'
	arguments to pass on each 5g, 6g, or 8g compiler invocation
-ldflags 'flag list'
	arguments to pass on each 5l, 6l, or 8l linker invocation
-tags 'tag list'
	a list of build tags to consider satisfied during the build.
	See the documentation for the go/build package for
	more information about build tags.

For more about specifying packages, see 'go help packages'. For more about where packages and binaries are installed, see 'go help gopath'.

See also: go install, go get, go clean.

Remove object files


go clean [-i] [-r] [-n] [-x] [packages]

Clean removes object files from package source directories. The go command builds most objects in a temporary directory, so go clean is mainly concerned with object files left by other tools or by manual invocations of go build.

Specifically, clean removes the following files from each of the source directories corresponding to the import paths:

_obj/            old object directory, left from Makefiles
_test/           old test directory, left from Makefiles
_testmain.go     old gotest file, left from Makefiles
test.out         old test log, left from Makefiles
build.out        old test log, left from Makefiles
*.[568ao]        object files, left from Makefiles

DIR(.exe)        from go build
DIR.test(.exe)   from go test -c
MAINFILE(.exe)   from go build MAINFILE.go

In the list, DIR represents the final path element of the directory, and MAINFILE is the base name of any Go source file in the directory that is not included when building the package.

The -i flag causes clean to remove the corresponding installed archive or binary (what 'go install' would create).

The -n flag causes clean to print the remove commands it would execute, but not run them.

The -r flag causes clean to be applied recursively to all the dependencies of the packages named by the import paths.

The -x flag causes clean to print remove commands as it executes them.

For more about specifying packages, see 'go help packages'.

Run godoc on package sources


go doc [packages]

Doc runs the godoc command on the packages named by the import paths.

For more about godoc, see 'godoc godoc'. For more about specifying packages, see 'go help packages'.

To run godoc with specific options, run godoc itself.

See also: go fix, go fmt, go vet.


go env [var ...]

Env prints Go environment information.

By default env prints information as a shell script (on Windows, a batch file). If one or more variable names is given as arguments, env prints the value of each named variable on its own line.

Run go tool fix on packages


go fix [packages]

Fix runs the Go fix command on the packages named by the import paths.

For more about fix, see 'godoc fix'. For more about specifying packages, see 'go help packages'.

To run fix with specific options, run 'go tool fix'.

See also: go fmt, go vet.

Run gofmt on package sources


go fmt [packages]

Fmt runs the command 'gofmt -l -w' on the packages named by the import paths. It prints the names of the files that are modified.

For more about gofmt, see 'godoc gofmt'. For more about specifying packages, see 'go help packages'.

To run gofmt with specific options, run gofmt itself.

See also: go doc, go fix, go vet.

Download and install packages and dependencies


go get [-a] [-d] [-fix] [-n] [-p n] [-u] [-v] [-x] [packages]

Get downloads and installs the packages named by the import paths, along with their dependencies.

The -a, -n, -v, -x, and -p flags have the same meaning as in 'go build' and 'go install'. See 'go help build'.

The -d flag instructs get to stop after downloading the packages; that is, it instructs get not to install the packages.

The -fix flag instructs get to run the fix tool on the downloaded packages before resolving dependencies or building the code.

The -u flag instructs get to use the network to update the named packages and their dependencies. By default, get uses the network to check out missing packages but does not use it to look for updates to existing packages.

When checking out or updating a package, get looks for a branch or tag that matches the locally installed version of Go. The most important rule is that if the local installation is running version "go1", get searches for a branch or tag named "go1". If no such version exists it retrieves the most recent version of the package.

For more about specifying packages, see 'go help packages'.

For more about how 'go get' finds source code to download, see 'go help remote'.

See also: go build, go install, go clean.

Compile and install packages and dependencies


go install [build flags] [packages]

Install compiles and installs the packages named by the import paths, along with their dependencies.

For more about the build flags, see 'go help build'. For more about specifying packages, see 'go help packages'.

See also: go build, go get, go clean.

List packages


go list [-e] [-f format] [-json] [packages]

List lists the packages named by the import paths, one per line.

The default output shows the package import path:


The -f flag specifies an alternate format for the list, using the syntax of package template. The default output is equivalent to -f '{{.ImportPath}}'. The struct being passed to the template is:

type Package struct {
    Dir        string // directory containing package sources
    ImportPath string // import path of package in dir
    Name       string // package name
    Doc        string // package documentation string
    Target     string // install path
    Goroot     bool   // is this package in the Go root?
    Standard   bool   // is this package part of the standard Go library?
    Stale      bool   // would 'go install' do anything for this package?
    Root       string // Go root or Go path dir containing this package

    // Source files
    GoFiles  []string  // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
    CgoFiles []string  // .go sources files that import "C"
    CFiles   []string  // .c source files
    HFiles   []string  // .h source files
    SFiles   []string  // .s source files
    SysoFiles []string // .syso object files to add to archive

    // Cgo directives
    CgoCFLAGS    []string // cgo: flags for C compiler
    CgoLDFLAGS   []string // cgo: flags for linker
    CgoPkgConfig []string // cgo: pkg-config names

    // Dependency information
    Imports []string // import paths used by this package
    Deps    []string // all (recursively) imported dependencies

    // Error information
    Incomplete bool            // this package or a dependency has an error
    Error      *PackageError   // error loading package
    DepsErrors []*PackageError // errors loading dependencies

    TestGoFiles  []string // _test.go files in package
    TestImports  []string // imports from TestGoFiles
    XTestGoFiles []string // _test.go files outside package
    XTestImports []string // imports from XTestGoFiles

The -json flag causes the package data to be printed in JSON format instead of using the template format.

The -e flag changes the handling of erroneous packages, those that cannot be found or are malformed. By default, the list command prints an error to standard error for each erroneous package and omits the packages from consideration during the usual printing. With the -e flag, the list command never prints errors to standard error and instead processes the erroneous packages with the usual printing. Erroneous packages will have a non-empty ImportPath and a non-nil Error field; other information may or may not be missing (zeroed).

For more about specifying packages, see 'go help packages'.

Compile and run Go program


go run [build flags] gofiles... [arguments...]

Run compiles and runs the main package comprising the named Go source files.

For more about build flags, see 'go help build'.

See also: go build.

Test packages


go test [-c] [-i] [build flags] [packages] [flags for test binary]

'Go test' automates testing the packages named by the import paths. It prints a summary of the test results in the format:

ok   archive/tar   0.011s
FAIL archive/zip   0.022s
ok   compress/gzip 0.033s

followed by detailed output for each failed package.

'Go test' recompiles each package along with any files with names matching the file pattern "*_test.go". These additional files can contain test functions, benchmark functions, and example functions. See 'go help testfunc' for more.

By default, go test needs no arguments. It compiles and tests the package with source in the current directory, including tests, and runs the tests.

The package is built in a temporary directory so it does not interfere with the non-test installation.

In addition to the build flags, the flags handled by 'go test' itself are:

-c  Compile the test binary to pkg.test but do not run it.

    Install packages that are dependencies of the test.
    Do not run the test.

The test binary also accepts flags that control execution of the test; these flags are also accessible by 'go test'. See 'go help testflag' for details.

For more about build flags, see 'go help build'. For more about specifying packages, see 'go help packages'.

See also: go build, go vet.

Run specified go tool


go tool [-n] command [args...]

Tool runs the go tool command identified by the arguments. With no arguments it prints the list of known tools.

The -n flag causes tool to print the command that would be executed but not execute it.

For more about each tool command, see 'go tool command -h'.


go version

Version prints the Go version, as reported by runtime.Version.

Run go tool vet on packages


go vet [packages]

Vet runs the Go vet command on the packages named by the import paths.

For more about vet, see 'godoc vet'. For more about specifying packages, see 'go help packages'.

To run the vet tool with specific options, run 'go tool vet'.

See also: go fmt, go fix.

GOPATH environment variable

The Go path is used to resolve import statements. It is implemented by and documented in the go/build package.

The GOPATH environment variable lists places to look for Go code. On Unix, the value is a colon-separated string. On Windows, the value is a semicolon-separated string. On Plan 9, the value is a list.

GOPATH must be set to build and install packages outside the standard Go tree.

Each directory listed in GOPATH must have a prescribed structure:

The src/ directory holds source code. The path below 'src' determines the import path or executable name.

The pkg/ directory holds installed package objects. As in the Go tree, each target operating system and architecture pair has its own subdirectory of pkg (pkg/GOOS_GOARCH).

If DIR is a directory listed in the GOPATH, a package with source in DIR/src/foo/bar can be imported as "foo/bar" and has its compiled form installed to "DIR/pkg/GOOS_GOARCH/foo/bar.a".

The bin/ directory holds compiled commands. Each command is named for its source directory, but only the final element, not the entire path. That is, the command with source in DIR/src/foo/quux is installed into DIR/bin/quux, not DIR/bin/foo/quux. The foo/ is stripped so that you can add DIR/bin to your PATH to get at the installed commands. If the GOBIN environment variable is set, commands are installed to the directory it names instead of DIR/bin.

Here's an example directory layout:


            bar/               (go code in package bar)
            quux/              (go code in package main)
        quux                   (installed command)
                bar.a          (installed package object)

Go searches each directory listed in GOPATH to find source code, but new packages are always downloaded into the first directory in the list.

Description of package lists

Many commands apply to a set of packages:

go action [packages]

Usually, [packages] is a list of import paths.

An import path that is a rooted path or that begins with a . or .. element is interpreted as a file system path and denotes the package in that directory.

Otherwise, the import path P denotes the package found in the directory DIR/src/P for some DIR listed in the GOPATH environment variable (see 'go help gopath').

If no import paths are given, the action applies to the package in the current directory.

The special import path "all" expands to all package directories found in all the GOPATH trees. For example, 'go list all' lists all the packages on the local system.

The special import path "std" is like all but expands to just the packages in the standard Go library.

An import path is a pattern if it includes one or more "..." wildcards, each of which can match any string, including the empty string and strings containing slashes. Such a pattern expands to all package directories found in the GOPATH trees with names matching the patterns. As a special case, x/... matches x as well as x's subdirectories. For example, net/... expands to net and packages in its subdirectories.

An import path can also name a package to be downloaded from a remote repository. Run 'go help remote' for details.

Every package in a program must have a unique import path. By convention, this is arranged by starting each path with a unique prefix that belongs to you. For example, paths used internally at Google all begin with 'google', and paths denoting remote repositories begin with the path to the code, such as 'code.google.com/p/project'.

As a special case, if the package list is a list of .go files from a single directory, the command is applied to a single synthesized package made up of exactly those files, ignoring any build constraints in those files and ignoring any other files in the directory.

Remote import path syntax

An import path (see 'go help importpath') denotes a package stored in the local file system. Certain import paths also describe how to obtain the source code for the package using a revision control system.

A few common code hosting sites have special syntax:

BitBucket (Mercurial)

	import "bitbucket.org/user/project"
	import "bitbucket.org/user/project/sub/directory"

GitHub (Git)

	import "github.com/user/project"
	import "github.com/user/project/sub/directory"

Google Code Project Hosting (Git, Mercurial, Subversion)

	import "code.google.com/p/project"
	import "code.google.com/p/project/sub/directory"

	import "code.google.com/p/project.subrepository"
	import "code.google.com/p/project.subrepository/sub/directory"

Launchpad (Bazaar)

	import "launchpad.net/project"
	import "launchpad.net/project/series"
	import "launchpad.net/project/series/sub/directory"

	import "launchpad.net/~user/project/branch"
	import "launchpad.net/~user/project/branch/sub/directory"

For code hosted on other servers, import paths may either be qualified with the version control type, or the go tool can dynamically fetch the import path over https/http and discover where the code resides from a <meta> tag in the HTML.

To declare the code location, an import path of the form


specifies the given repository, with or without the .vcs suffix, using the named version control system, and then the path inside that repository. The supported version control systems are:

Bazaar      .bzr
Git         .git
Mercurial   .hg
Subversion  .svn

For example,

import "example.org/user/foo.hg"

denotes the root directory of the Mercurial repository at example.org/user/foo or foo.hg, and

import "example.org/repo.git/foo/bar"

denotes the foo/bar directory of the Git repository at example.com/repo or repo.git.

When a version control system supports multiple protocols, each is tried in turn when downloading. For example, a Git download tries git://, then https://, then http://.

If the import path is not a known code hosting site and also lacks a version control qualifier, the go tool attempts to fetch the import over https/http and looks for a <meta> tag in the document's HTML <head>.

The meta tag has the form:

<meta name="go-import" content="import-prefix vcs repo-root">

The import-prefix is the import path correponding to the repository root. It must be a prefix or an exact match of the package being fetched with "go get". If it's not an exact match, another http request is made at the prefix to verify the <meta> tags match.

The vcs is one of "git", "hg", "svn", etc,

The repo-root is the root of the version control system containing a scheme and not containing a .vcs qualifier.

For example,

import "example.org/pkg/foo"

will result in the following request(s):

https://example.org/pkg/foo?go-get=1 (preferred)
http://example.org/pkg/foo?go-get=1  (fallback)

If that page contains the meta tag

<meta name="go-import" content="example.org git https://code.org/r/p/exproj">

the go tool will verify that https://example.org/?go-get=1 contains the same meta tag and then git clone https://code.org/r/p/exproj into GOPATH/src/example.org.

New downloaded packages are written to the first directory listed in the GOPATH environment variable (see 'go help gopath').

The go command attempts to download the version of the package appropriate for the Go release being used. Run 'go help install' for more.

Description of testing flags

The 'go test' command takes both flags that apply to 'go test' itself and flags that apply to the resulting test binary.

The test binary, called pkg.test, where pkg is the name of the directory containing the package sources, has its own flags:

    Verbose output: log all tests as they are run.

-test.run pattern
    Run only those tests and examples matching the regular

-test.bench pattern
    Run benchmarks matching the regular expression.
    By default, no benchmarks run.

-test.cpuprofile cpu.out
    Write a CPU profile to the specified file before exiting.

-test.memprofile mem.out
    Write a memory profile to the specified file when all tests
    are complete.

-test.memprofilerate n
    Enable more precise (and expensive) memory profiles by setting
    runtime.MemProfileRate.  See 'godoc runtime MemProfileRate'.
    To profile all memory allocations, use -test.memprofilerate=1
    and set the environment variable GOGC=off to disable the
    garbage collector, provided the test can run in the available
    memory without garbage collection.

-test.parallel n
    Allow parallel execution of test functions that call t.Parallel.
    The value of this flag is the maximum number of tests to run
    simultaneously; by default, it is set to the value of GOMAXPROCS.

    Tell long-running tests to shorten their run time.
    It is off by default but set during all.bash so that installing
    the Go tree can run a sanity check but not spend time running
    exhaustive tests.

-test.timeout t
	If a test runs longer than t, panic.

-test.benchtime n
	Run enough iterations of each benchmark to take n seconds.
	The default is 1 second.

-test.cpu 1,2,4
    Specify a list of GOMAXPROCS values for which the tests or
    benchmarks should be executed.  The default is the current value

For convenience, each of these -test.X flags of the test binary is also available as the flag -X in 'go test' itself. Flags not listed here are passed through unaltered. For instance, the command

go test -x -v -cpuprofile=prof.out -dir=testdata -update

will compile the test binary and then run it as

pkg.test -test.v -test.cpuprofile=prof.out -dir=testdata -update

Description of testing functions

The 'go test' command expects to find test, benchmark, and example functions in the "*_test.go" files corresponding to the package under test.

A test function is one named TestXXX (where XXX is any alphanumeric string not starting with a lower case letter) and should have the signature,

func TestXXX(t *testing.T) { ... }

A benchmark function is one named BenchmarkXXX and should have the signature,

func BenchmarkXXX(b *testing.B) { ... }

An example function is similar to a test function but, instead of using *testing.T to report success or failure, prints output to os.Stdout and os.Stderr. That output is compared against the function's "Output:" comment, which must be the last comment in the function body (see example below). An example with no such comment, or with no text after "Output:" is compiled but not executed.

Godoc displays the body of ExampleXXX to demonstrate the use of the function, constant, or variable XXX. An example of a method M with receiver type T or *T is named ExampleT_M. There may be multiple examples for a given function, constant, or variable, distinguished by a trailing _xxx, where xxx is a suffix not beginning with an upper case letter.

Here is an example of an example:

func ExamplePrintln() {
	Println("The output of\nthis example.")
	// Output: The output of
	// this example.

The entire test file is presented as the example when it contains a single example function, at least one other function, type, variable, or constant declaration, and no test or benchmark functions.

See the documentation of the testing package for more information.