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GOPATH
The GOPATH environment variable is used to specify directories outside of $GOROOT that contain the source for Go projects and their binaries.
See the go command and go/build package documentation for more details.
Since the $GOPATH variable can be a list, the rest of this document will use $GOPATH to mean the first element unless otherwise specified.
On OS X or Linux (bash), adding the following expression to PATH will add all $GOPATH/bin directories.
${GOPATH//://bin:}/bin
The source for a package with the import path "X/Y/Z" is in the directory
$GOPATH/src/X/Y/Z
The binary for a package with the import path "X/Y/Z" is in
$GOPATH/pkg/$GOOS_$GOARCH/X/Y/Z.a
The binary for a command whose source is in $GOPATH/src/A/B is
$GOPATH/bin/B
When fetching a package the go tool looks at the package's import path to discover a URL. For instance, if you attempt to
go get code.google.com/p/gomatrix/matrix
the go tool will get the source from the project hosted at http://code.google.com/p/gomatrix/. It will clone the repository to
$GOPATH/src/code.google.com/p/gomatrix
As a result, if (from your repository project) you import a package that is in the same repository, you need to use its "full" import path - the place "go get" puts it. In this example, if something else wants to import the "matrix" package, it should import "code.google.com/p/gomatrix/matrix" rather than "matrix".
It is useful to have two GOPATH entries. One for a location for 3rd party goinstalled packages, and the second for your own projects. List the 3rd party GOPATH first, so that goinstall will use it as a default destination. Then you can work in the second GOPATH directory and have all your packages be importable by using the "go" command, goinstall, or a GOPATH-aware 3rd party build tool like gb.
When the go command is looking for packages, it always looks in $GOROOT first. This includes directories, so if it finds (as in the case above) a cmd/ directory in $GOROOT it won't proceed to look in any of the GOPATH directories. This prevents you from defining your own math/matrix package as well as your own cmd/mycmd commands.