Go Import Path

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Content from command : go help importpath

An import path (see 'go help packages') denotes(表示) a package stored in the local file system. In general,an import path denotes（表示） either a standard package (such
as "unicode/utf8") or a package found in one of the work spaces (For more details see: 'go help gopath').

Relative import paths

An import path beginning with ./ or ../ is called a relative path.
The toolchain supports relative import paths as a shortcut(快捷方式) in two ways.

First,a relative path can be used as a shorthand（速记） on the command line.
If you are working in the directory containing the code imported as
"unicode" and want to run the tests for "unicode/utf8",you can type
"go test ./utf8" instead of needing to specify the full path.
Similarly,in the reverse situation,"go test .." will test "unicode" from
the "unicode/utf8" directory. Relative patterns are also allowed,like
"go test ./..." to test all subdirectories. See 'go help packages' for details
on the pattern syntax.

Second,if you are compiling a Go program not in a work space,
you can use a relative path in an import statement in that program
to refer to nearby code also not in a work space.
This makes it easy to experiment(实验) with small multipackage programs
outside of the usual work spaces,but such programs cannot be
installed with "go install" (there is no work space in which to install them),
so they are rebuilt from scratch(from scratch 白手起家；从头做起) each time they are built.
To avoid ambiguity,Go programs cannot use relative import paths
within a work space.

1.一般情况下,不能在work spackes 中使用相对路径; demo:
--ch2  
    --cte.go  (package ch2  var Y = 4)
    --main
                  -- main.go 
                                 package main  
                                 import (
                                             "fmt"
                                            "../../ch2"  注意这里要锁定到 ch2这个包,../ch2是不行的
                                            )
                                            func main(){
                                                fmt.Printf(ch2.Y)    
                                            }
cd main and go run main.go (succed)

Remote import paths

Certain import paths also
describe how to obtain the source code for the package using
a revision(版本) control system.

A few common code hosting sites have special syntax:

Bitbucket (Git,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"
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"
IBM DevOps Services (Git)
import "hub.jazz.net/git/user/project"
import "hub.jazz.net/git/user/project/sub/directory"

For code hosted on other servers,import paths may either be qualified(受限于)
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 tag in the HTML.

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

repository.vcs/path

specifies the given repository,with or without the .vcs suffix(前缀),
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.org/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 then git+ssh://.

By default,downloads are restricted to known secure protocols
(e.g. https,ssh). To override this setting for Git downloads,the
GIT_ALLOW_PROTOCOL environment variable can be set (For more details see:
'go help environment').

If the import path is not a known code hosting site and also lacks a
version control qualifier(修饰),the go tool attempts to fetch the import
over https/http and looks for a tag in the document's HTML
. ---如果不是像git 或者是Mercurial这样的代码托管的情况---

The meta tag has the form:

import "example.org/pkg/foo"

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

package math // import "path"
package math /* import "path" */