I'm also not sure how one would prevent go install from downloading code.

Maybe you want #26361. Or perhaps one of the existing -getmode modes.

I couldn't find any clarification on this.

I think you want go help module-get. From the last paragraph in go help get:

This text describes the behavior of get when using GOPATH
to manage source code and dependencies.
If instead the go command is running in module-aware mode,
the details of get's flags and effects change, as does 'go help get'.
See 'go help modules' and 'go help module-get'.

Milestoned for 1.11 in case there's anything else to do here. cc @rsc @bcmills

I'm also not sure how one would prevent go install from downloading code.

From downloading code for existing dependencies, or from adding new dependencies?
(Adding new dependencies is #26361.)

If I want to install an arbitrary command, like go get used to do, what do I do now? If I want to install an arbitrary command, without downloading anything (what go install used to do), what now?

I find the new behavior confusing as well.

$ mkdir /tmp/gopath
$ GO111MODULE=on GOPATH=/tmp/gopath go1.11beta2 get -v github.com/baltimore-sun-data/boreas
go: cannot find main module root; see 'go help modules'
$ GO111MODULE=on GOPATH=/tmp/gopath go1.11beta2 install -v github.com/baltimore-sun-data/boreas
go: cannot find main module root; see 'go help modules'

Is there a way to install arbitrary CLI tools with the go command any more, or do I have to git clone instead?

@carlmjohnson yes you can use go install, but you will need a go.mod file, i.e. a module, as the starting point for that.

So if you want these tools to be generally available you could do something like:

mkdir -p $HOME/gotools
cd $HOME/gotools
mkdir bin
export GOBIN=$HOME/gotools/bin
go mod init gotools

GOBIN is a way of specifying exactly where programs should be installed.

Then:

go install github.com/baltimore-sun-data/boreas

Notice how you don't care what value is set for GOPATH; GOBIN becomes the relevant directory.

You might then also want to add the export above to your .bashrc or equivalent.

Doesn't work:

$ GO111MODULE=on go1.11beta2 mod init whatever
go: cannot find main module root; see 'go help modules'

Is there a way to install arbitrary CLI tools with the go command any more, or do I have to git clone instead?

That's #24250.

@carlmjohnson, mod -init was changed to mod init after go1.11beta2. Either build from head or use the older spelling of the flag.

$ GO111MODULE=on GOPATH=/tmp/gopath go1.11beta2 mod -init
go: cannot determine module path for source directory /tmp/gopath (outside GOPATH, no import comments)
$ GO111MODULE=on GOPATH=/tmp/gopath go1.11beta2 mod -init whatever
go mod: mod takes no arguments

My Go projects (including Boreas) all include these installation instructions in the README:

First install Go. If you just want to install the binary to your current directory and don't care about the source code, run GOBIN="$(pwd)" GOPATH="$(mktemp -d)" go get github.com/baltimore-sun-data/boreas.

It looks like for Go 1.12, if #24250 doesn't happen, I will need to change the instructions to

git clone $REPO
cd $PKG
GOBIN=$(pwd)/../bin GOPATH=$(mktemp -d) go install -v ./...

Three lines instead of one is not the end of the world, but it would be nice if I could just say to run a single command.

I think (per #26591 (comment)) this is covered by #24250; please shout if that's not the correct conclusion.