doc/articles/go_command.html

<!--{
	"title": "About the go command"
}-->

<p>The Go distribution includes a command, named
"<code><a href="/cmd/go/">go</a></code>", that
automates the downloading, building, installation, and testing of Go packages
and commands.  This document talks about why we wrote a new command, what it
is, what it's not, and how to use it.</p>

<h2>Motivation</h2>

<p>You might have seen early Go talks in which Rob Pike jokes that the idea
for Go arose while waiting for a large Google server to compile.  That
really was the motivation for Go: to build a language that worked well
for building the large software that Google writes and runs. It was
clear from the start that such a language must provide a way to
express dependencies between code libraries clearly, hence the package
grouping and the explicit import blocks.  It was also clear from the
start that you might want arbitrary syntax for describing the code
being imported; this is why import paths are string literals.</p>

<p>An explicit goal for Go from the beginning was to be able to build Go
code using only the information found in the source itself, not
needing to write a makefile or one of the many modern replacements for
makefiles.  If Go needed a configuration file to explain how to build
your program, then Go would have failed.</p>

<p>At first, there was no Go compiler, and the initial development
focused on building one and then building libraries for it. For
expedience, we postponed the automation of building Go code by using
make and writing makefiles.  When compiling a single package involved
multiple invocations of the Go compiler, we even used a program to
write the makefiles for us.  You can find it if you dig through the
repository history.</p>

<p>The purpose of the new go command is our return to this ideal, that Go
programs should compile without configuration or additional effort on
the part of the developer beyond writing the necessary import
statements.</p>

<h2>Configuration versus convention</h2>

<p>The way to achieve the simplicity of a configuration-free system is to
establish conventions. The system works only to the extent that those conventions
are followed. When we first launched Go, many people published packages that
had to be installed in certain places, under certain names, using certain build
tools, in order to be used. That's understandable: that's the way it works in
most other languages. Over the last few years we consistently reminded people
about the <code>goinstall</code> command
(now replaced by <a href="/cmd/go/#hdr-Download_and_install_packages_and_dependencies"><code>go get</code></a>)
and its conventions: first, that the import path is derived in a known way from
the URL of the source code; second, that the place to store the sources in
the local file system is derived in a known way from the import path; third,
that each directory in a source tree corresponds to a single package; and
fourth, that the package is built using only information in the source code.
Today, the vast majority of packages follow these conventions.
The Go ecosystem is simpler and more powerful as a result.</p>

<p>We received many requests to allow a makefile in a package directory to
provide just a little extra configuration beyond what's in the source code.
But that would have introduced new rules. Because we did not accede to such
requests, we were able to write the go command and eliminate our use of make
or any other build system.</p>

<p>It is important to understand that the go command is not a general
build tool. It cannot be configured and it does not attempt to build
anything but Go packages.  These are important simplifying
assumptions: they simplify not only the implementation but also, more
important, the use of the tool itself.</p>

<h2>Go's conventions</h2>

<p>The <code>go</code> command requires that code adheres to a few key,
well-established conventions.</p>

<p>First, the import path is derived in an known way from the URL of the
source code.  For Bitbucket, GitHub, Google Code, and Launchpad, the
root directory of the repository is identified by the repository's
main URL, without the <code>http://</code> prefix.  Subdirectories are named by
adding to that path.
For example, the Go example programs are obtained by running</p>

<pre>
git clone https://github.com/golang/example
</pre>

<p>and thus the import path for the root directory of that repository is
"<code>github.com/golang/example</code>".
The <a href="https://godoc.org/github.com/golang/example/stringutil">stringutil</a>
package is stored in a subdirectory, so its import path is
"<code>github.com/golang/example/stringutil</code>".</p>

<p>These paths are on the long side, but in exchange we get an
automatically managed name space for import paths and the ability for
a tool like the go command to look at an unfamiliar import path and
deduce where to obtain the source code.</p>

<p>Second, the place to store sources in the local file system is derived
in a known way from the import path, specifically
<code>$GOPATH/src/&lt;import-path&gt;</code>.
If unset, <code>$GOPATH</code> defaults to a subdirectory
named <code>go</code> in the user's home directory.
If <code>$GOPATH</code> is set to a list of paths, the go command tries
<code>&lt;dir&gt;/src/&lt;import-path&gt;</code> for each of the directories in
that list.
</p>

<p>Each of those trees contains, by convention, a top-level directory named
"<code>bin</code>", for holding compiled executables, and a top-level directory
named "<code>pkg</code>", for holding compiled packages that can be imported,
and the "<code>src</code>" directory, for holding package source files.
Imposing this structure lets us keep each of these directory trees
self-contained: the compiled form and the sources are always near each
other.</p>

<p>These naming conventions also let us work in the reverse direction,
from a directory name to its import path. This mapping is important
for many of the go command's subcommands, as we'll see below.</p>

<p>Third, each directory in a source tree corresponds to a single
package. By restricting a directory to a single package, we don't have
to create hybrid import paths that specify first the directory and
then the package within that directory.  Also, most file management
tools and UIs work on  directories as fundamental units.  Tying the
fundamental Go unit&mdash;the package&mdash;to file system structure means
that file system tools become Go package tools.  Copying, moving, or
deleting a package corresponds to copying, moving, or deleting a
directory.</p>

<p>Fourth, each package is built using only the information present in
the source files.  This makes it much more likely that the tool will
be able to adapt to changing build environments and conditions. For
example, if we allowed extra configuration such as compiler flags or
command line recipes, then that configuration would need to be updated
each time the build tools changed; it would also be inherently tied
to the use of a specific toolchain.</p>

<h2>Getting started with the go command</h2>

<p>Finally, a quick tour of how to use the go command.
As mentioned above, the default <code>$GOPATH</code> on Unix is <code>$HOME/go</code>.
We'll store our programs there.
To use a different location, you can set <code>$GOPATH</code>;
see <a href="/doc/code.html">How to Write Go Code</a> for details.

<p>We first add some source code.  Suppose we want to use
the indexing library from the codesearch project along with a left-leaning
red-black tree.  We can install both with the "<code>go get</code>"
subcommand:</p>

<pre>
$ go get github.com/google/codesearch/index
$ go get github.com/petar/GoLLRB/llrb
$
</pre>

<p>Both of these projects are now downloaded and installed into <code>$HOME/go</code>,
which contains the two directories
<code>src/github.com/google/codesearch/index/</code> and
<code>src/github.com/petar/GoLLRB/llrb/</code>, along with the compiled
packages (in <code>pkg/</code>) for those libraries and their dependencies.</p>

<p>Because we used version control systems (Mercurial and Git) to check
out the sources, the source tree also contains the other files in the
corresponding repositories, such as related packages. The "<code>go list</code>"
subcommand lists the import paths corresponding to its arguments, and
the pattern "<code>./...</code>" means start in the current directory
("<code>./</code>") and find all packages below that directory
("<code>...</code>"):</p>

<pre>
$ cd $HOME/go/src
$ go list ./...
github.com/google/codesearch/cmd/cgrep
github.com/google/codesearch/cmd/cindex
github.com/google/codesearch/cmd/csearch
github.com/google/codesearch/index
github.com/google/codesearch/regexp
github.com/google/codesearch/sparse
github.com/petar/GoLLRB/example
github.com/petar/GoLLRB/llrb
$
</pre>

<p>We can also test those packages:</p>

<pre>
$ go test ./...
?   	github.com/google/codesearch/cmd/cgrep	[no test files]
?   	github.com/google/codesearch/cmd/cindex	[no test files]
?   	github.com/google/codesearch/cmd/csearch	[no test files]
ok  	github.com/google/codesearch/index	0.203s
ok  	github.com/google/codesearch/regexp	0.017s
?   	github.com/google/codesearch/sparse	[no test files]
?       github.com/petar/GoLLRB/example          [no test files]
ok      github.com/petar/GoLLRB/llrb             0.231s
$
</pre>

<p>If a go subcommand is invoked with no paths listed, it operates on the
current directory:</p>

<pre>
$ cd github.com/google/codesearch/regexp
$ go list
github.com/google/codesearch/regexp
$ go test -v
=== RUN   TestNstateEnc
--- PASS: TestNstateEnc (0.00s)
=== RUN   TestMatch
--- PASS: TestMatch (0.00s)
=== RUN   TestGrep
--- PASS: TestGrep (0.00s)
PASS
ok  	github.com/google/codesearch/regexp	0.018s
$ go install
$
</pre>

<p>That "<code>go install</code>" subcommand installs the latest copy of the
package into the pkg directory. Because the go command can analyze the
dependency graph, "<code>go install</code>" also installs any packages that
this package imports but that are out of date, recursively.</p>

<p>Notice that "<code>go install</code>" was able to determine the name of the
import path for the package in the current directory, because of the convention
for directory naming.  It would be a little more convenient if we could pick
the name of the directory where we kept source code, and we probably wouldn't
pick such a long name, but that ability would require additional configuration
and complexity in the tool. Typing an extra directory name or two is a small
price to pay for the increased simplicity and power.</p>

<h2>Limitations</h2>

<p>As mentioned above, the go command is not a general-purpose build
tool.
In particular, it does not have any facility for generating Go
source files <em>during</em> a build, although it does provide
<a href="/cmd/go/#hdr-Generate_Go_files_by_processing_source"><code>go</code>
<code>generate</code></a>,
which can automate the creation of Go files <em>before</em> the build.
For more advanced build setups, you may need to write a
makefile (or a configuration file for the build tool of your choice)
to run whatever tool creates the Go files and then check those generated source files
into your repository. This is more work for you, the package author,
but it is significantly less work for your users, who can use
"<code>go get</code>" without needing to obtain and build
any additional tools.</p>

<h2>More information</h2>

<p>For more information, read <a href="/doc/code.html">How to Write Go Code</a>
and see the <a href="/cmd/go/">go command documentation</a>.</p>