1 <!--{ 2 "title": "About the go command" 3 }--> 4 5 <p>The Go distribution includes a command, named 6 "<code><a href="/cmd/go/">go</a></code>", that 7 automates the downloading, building, installation, and testing of Go packages 8 and commands. This document talks about why we wrote a new command, what it 9 is, what it's not, and how to use it.</p> 10 11 <h2>Motivation</h2> 12 13 <p>You might have seen early Go talks in which Rob Pike jokes that the idea 14 for Go arose while waiting for a large Google server to compile. That 15 really was the motivation for Go: to build a language that worked well 16 for building the large software that Google writes and runs. It was 17 clear from the start that such a language must provide a way to 18 express dependencies between code libraries clearly, hence the package 19 grouping and the explicit import blocks. It was also clear from the 20 start that you might want arbitrary syntax for describing the code 21 being imported; this is why import paths are string literals.</p> 22 23 <p>An explicit goal for Go from the beginning was to be able to build Go 24 code using only the information found in the source itself, not 25 needing to write a makefile or one of the many modern replacements for 26 makefiles. If Go needed a configuration file to explain how to build 27 your program, then Go would have failed.</p> 28 29 <p>At first, there was no Go compiler, and the initial development 30 focused on building one and then building libraries for it. For 31 expedience, we postponed the automation of building Go code by using 32 make and writing makefiles. When compiling a single package involved 33 multiple invocations of the Go compiler, we even used a program to 34 write the makefiles for us. You can find it if you dig through the 35 repository history.</p> 36 37 <p>The purpose of the new go command is our return to this ideal, that Go 38 programs should compile without configuration or additional effort on 39 the part of the developer beyond writing the necessary import 40 statements.</p> 41 42 <h2>Configuration versus convention</h2> 43 44 <p>The way to achieve the simplicity of a configuration-free system is to 45 establish conventions. The system works only to the extent that those conventions 46 are followed. When we first launched Go, many people published packages that 47 had to be installed in certain places, under certain names, using certain build 48 tools, in order to be used. That's understandable: that's the way it works in 49 most other languages. Over the last few years we consistently reminded people 50 about the <code>goinstall</code> command 51 (now replaced by <a href="/cmd/go/#hdr-Download_and_install_packages_and_dependencies"><code>go get</code></a>) 52 and its conventions: first, that the import path is derived in a known way from 53 the URL of the source code; second, that the place to store the sources in 54 the local file system is derived in a known way from the import path; third, 55 that each directory in a source tree corresponds to a single package; and 56 fourth, that the package is built using only information in the source code. 57 Today, the vast majority of packages follow these conventions. 58 The Go ecosystem is simpler and more powerful as a result.</p> 59 60 <p>We received many requests to allow a makefile in a package directory to 61 provide just a little extra configuration beyond what's in the source code. 62 But that would have introduced new rules. Because we did not accede to such 63 requests, we were able to write the go command and eliminate our use of make 64 or any other build system.</p> 65 66 <p>It is important to understand that the go command is not a general 67 build tool. It cannot be configured and it does not attempt to build 68 anything but Go packages. These are important simplifying 69 assumptions: they simplify not only the implementation but also, more 70 important, the use of the tool itself.</p> 71 72 <h2>Go's conventions</h2> 73 74 <p>The <code>go</code> command requires that code adheres to a few key, 75 well-established conventions.</p> 76 77 <p>First, the import path is derived in an known way from the URL of the 78 source code. For Bitbucket, GitHub, Google Code, and Launchpad, the 79 root directory of the repository is identified by the repository's 80 main URL, without the <code>http://</code> prefix. Subdirectories are named by 81 adding to that path. 82 For example, the Go example programs are obtained by running</p> 83 84 <pre> 85 git clone https://github.com/golang/example 86 </pre> 87 88 <p>and thus the import path for the root directory of that repository is 89 "<code>github.com/golang/example</code>". 90 The <a href="https://godoc.org/github.com/golang/example/stringutil">stringutil</a> 91 package is stored in a subdirectory, so its import path is 92 "<code>github.com/golang/example/stringutil</code>".</p> 93 94 <p>These paths are on the long side, but in exchange we get an 95 automatically managed name space for import paths and the ability for 96 a tool like the go command to look at an unfamiliar import path and 97 deduce where to obtain the source code.</p> 98 99 <p>Second, the place to store sources in the local file system is derived 100 in a known way from the import path, specifically 101 <code>$GOPATH/src/<import-path></code>. 102 If unset, <code>$GOPATH</code> defaults to a subdirectory 103 named <code>go</code> in the user's home directory. 104 If <code>$GOPATH</code> is set to a list of paths, the go command tries 105 <code><dir>/src/<import-path></code> for each of the directories in 106 that list. 107 </p> 108 109 <p>Each of those trees contains, by convention, a top-level directory named 110 "<code>bin</code>", for holding compiled executables, and a top-level directory 111 named "<code>pkg</code>", for holding compiled packages that can be imported, 112 and the "<code>src</code>" directory, for holding package source files. 113 Imposing this structure lets us keep each of these directory trees 114 self-contained: the compiled form and the sources are always near each 115 other.</p> 116 117 <p>These naming conventions also let us work in the reverse direction, 118 from a directory name to its import path. This mapping is important 119 for many of the go command's subcommands, as we'll see below.</p> 120 121 <p>Third, each directory in a source tree corresponds to a single 122 package. By restricting a directory to a single package, we don't have 123 to create hybrid import paths that specify first the directory and 124 then the package within that directory. Also, most file management 125 tools and UIs work on directories as fundamental units. Tying the 126 fundamental Go unit—the package—to file system structure means 127 that file system tools become Go package tools. Copying, moving, or 128 deleting a package corresponds to copying, moving, or deleting a 129 directory.</p> 130 131 <p>Fourth, each package is built using only the information present in 132 the source files. This makes it much more likely that the tool will 133 be able to adapt to changing build environments and conditions. For 134 example, if we allowed extra configuration such as compiler flags or 135 command line recipes, then that configuration would need to be updated 136 each time the build tools changed; it would also be inherently tied 137 to the use of a specific tool chain.</p> 138 139 <h2>Getting started with the go command</h2> 140 141 <p>Finally, a quick tour of how to use the go command. 142 As mentioned above, the default <code>$GOPATH</code> on Unix is <code>$HOME/go</code>. 143 We'll store our programs there. 144 To use a different location, you can set <code>$GOPATH</code>; 145 see <a href="/doc/code.html">How to Write Go Code</a> for details. 146 147 <p>We first add some source code. Suppose we want to use 148 the indexing library from the codesearch project along with a left-leaning 149 red-black tree. We can install both with the "<code>go get</code>" 150 subcommand:</p> 151 152 <pre> 153 $ go get github.com/google/codesearch/index 154 $ go get github.com/petar/GoLLRB/llrb 155 $ 156 </pre> 157 158 <p>Both of these projects are now downloaded and installed into <code>$HOME/go</code>, 159 which contains the two directories 160 <code>src/github.com/google/codesearch/index/</code> and 161 <code>src/github.com/petar/GoLLRB/llrb/</code>, along with the compiled 162 packages (in <code>pkg/</code>) for those libraries and their dependencies.</p> 163 164 <p>Because we used version control systems (Mercurial and Git) to check 165 out the sources, the source tree also contains the other files in the 166 corresponding repositories, such as related packages. The "<code>go list</code>" 167 subcommand lists the import paths corresponding to its arguments, and 168 the pattern "<code>./...</code>" means start in the current directory 169 ("<code>./</code>") and find all packages below that directory 170 ("<code>...</code>"):</p> 171 172 <pre> 173 $ cd $HOME/go/src 174 $ go list ./... 175 github.com/google/codesearch/cmd/cgrep 176 github.com/google/codesearch/cmd/cindex 177 github.com/google/codesearch/cmd/csearch 178 github.com/google/codesearch/index 179 github.com/google/codesearch/regexp 180 github.com/google/codesearch/sparse 181 github.com/petar/GoLLRB/example 182 github.com/petar/GoLLRB/llrb 183 $ 184 </pre> 185 186 <p>We can also test those packages:</p> 187 188 <pre> 189 $ go test ./... 190 ? github.com/google/codesearch/cmd/cgrep [no test files] 191 ? github.com/google/codesearch/cmd/cindex [no test files] 192 ? github.com/google/codesearch/cmd/csearch [no test files] 193 ok github.com/google/codesearch/index 0.203s 194 ok github.com/google/codesearch/regexp 0.017s 195 ? github.com/google/codesearch/sparse [no test files] 196 ? github.com/petar/GoLLRB/example [no test files] 197 ok github.com/petar/GoLLRB/llrb 0.231s 198 $ 199 </pre> 200 201 <p>If a go subcommand is invoked with no paths listed, it operates on the 202 current directory:</p> 203 204 <pre> 205 $ cd github.com/google/codesearch/regexp 206 $ go list 207 github.com/google/codesearch/regexp 208 $ go test -v 209 === RUN TestNstateEnc 210 --- PASS: TestNstateEnc (0.00s) 211 === RUN TestMatch 212 --- PASS: TestMatch (0.00s) 213 === RUN TestGrep 214 --- PASS: TestGrep (0.00s) 215 PASS 216 ok github.com/google/codesearch/regexp 0.018s 217 $ go install 218 $ 219 </pre> 220 221 <p>That "<code>go install</code>" subcommand installs the latest copy of the 222 package into the pkg directory. Because the go command can analyze the 223 dependency graph, "<code>go install</code>" also installs any packages that 224 this package imports but that are out of date, recursively.</p> 225 226 <p>Notice that "<code>go install</code>" was able to determine the name of the 227 import path for the package in the current directory, because of the convention 228 for directory naming. It would be a little more convenient if we could pick 229 the name of the directory where we kept source code, and we probably wouldn't 230 pick such a long name, but that ability would require additional configuration 231 and complexity in the tool. Typing an extra directory name or two is a small 232 price to pay for the increased simplicity and power.</p> 233 234 <h2>Limitations</h2> 235 236 <p>As mentioned above, the go command is not a general-purpose build 237 tool. 238 In particular, it does not have any facility for generating Go 239 source files <em>during</em> a build, although it does provide 240 <a href="/cmd/go/#hdr-Generate_Go_files_by_processing_source"><code>go</code> 241 <code>generate</code></a>, 242 which can automate the creation of Go files <em>before</em> the build. 243 For more advanced build setups, you may need to write a 244 makefile (or a configuration file for the build tool of your choice) 245 to run whatever tool creates the Go files and then check those generated source files 246 into your repository. This is more work for you, the package author, 247 but it is significantly less work for your users, who can use 248 "<code>go get</code>" without needing to obtain and build 249 any additional tools.</p> 250 251 <h2>More information</h2> 252 253 <p>For more information, read <a href="/doc/code.html">How to Write Go Code</a> 254 and see the <a href="/cmd/go/">go command documentation</a>.</p> 255
