Merge pull request #35144 from pipejakob/generate-token

Automatic merge from submit-queue

New command: "kubeadm token generate"

As part of #33930, this PR adds a new top-level command to kubeadm to just generate a token for use with the init/join commands. Otherwise, users are left to either figure out how to generate a token on their own, or let `kubeadm init` generate a token, capture and parse the output, and then use that token for `kubeadm join`.

At this point, I was hoping for feedback on the CLI experience, and then I can add tests. I spoke with @mikedanese and he didn't like the original propose of `kubeadm util generate-token`, so here are the runners up:

```
$ kubeadm generate-token          # <--- current implementation
$ kubeadm generate token          # in case kubeadm might generate other things in the future?
$ kubeadm init --generate-token   # possibly as a subcommand of an existing one
```

Currently, the output is simply the token on one line without any padding/formatting:

```
$ kubeadm generate-token
1087fd.722b60cdd39b1a5f
```

CC: @kubernetes/sig-cluster-lifecycle 

**Release note**:

<!--  Steps to write your release note:
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2. Enter your extended release note in the below block; leaving it blank means using the PR title as the release note. If no release note is required, just write `NONE`. 
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``` release-note
New kubeadm command: generate-token
```

cluster/kubeadm.sh

@@ -0,0 +1,106 @@
+#!/bin/bash
+
+# Copyright 2016 The Kubernetes Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+KUBE_ROOT=${KUBE_ROOT:-$(dirname "${BASH_SOURCE}")/..}
+source "${KUBE_ROOT}/cluster/kube-util.sh"
+
+# Get the absolute path of the directory component of a file, i.e. the
+# absolute path of the dirname of $1.
+get_absolute_dirname() {
+  echo "$(cd "$(dirname "$1")" && pwd)"
+}
+
+# Detect the OS name/arch so that we can find our binary
+case "$(uname -s)" in
+  Darwin)
+    host_os=darwin
+    ;;
+  Linux)
+    host_os=linux
+    ;;
+  *)
+    echo "Unsupported host OS.  Must be Linux or Mac OS X." >&2
+    exit 1
+    ;;
+esac
+
+case "$(uname -m)" in
+  x86_64*)
+    host_arch=amd64
+    ;;
+  i?86_64*)
+    host_arch=amd64
+    ;;
+  amd64*)
+    host_arch=amd64
+    ;;
+  arm*)
+    host_arch=arm
+    ;;
+  i?86*)
+    host_arch=386
+    ;;
+  s390x*)
+    host_arch=s390x
+    ;;
+  ppc64le*)
+    host_arch=ppc64le
+    ;;
+  *)
+    echo "Unsupported host arch. Must be x86_64, 386, arm, s390x or ppc64le." >&2
+    exit 1
+    ;;
+esac
+
+# If KUBEADM_PATH isn't set, gather up the list of likely places and use ls
+# to find the latest one.
+if [[ -z "${KUBEADM_PATH:-}" ]]; then
+  locations=(
+    "${KUBE_ROOT}/_output/bin/kubeadm"
+    "${KUBE_ROOT}/_output/dockerized/bin/${host_os}/${host_arch}/kubeadm"
+    "${KUBE_ROOT}/_output/local/bin/${host_os}/${host_arch}/kubeadm"
+    "${KUBE_ROOT}/platforms/${host_os}/${host_arch}/kubeadm"
+  )
+  kubeadm=$( (ls -t "${locations[@]}" 2>/dev/null || true) | head -1 )
+
+  if [[ ! -x "$kubeadm" ]]; then
+    {
+      echo "It looks as if you don't have a compiled kubeadm binary"
+      echo
+      echo "If you are running from a clone of the git repo, please run"
+      echo "'./build/run.sh make cross'. Note that this requires having"
+      echo "Docker installed."
+      echo
+      echo "If you are running from a binary release tarball, something is wrong. "
+      echo "Look at http://kubernetes.io/ for information on how to contact the "
+      echo "development team for help."
+    } >&2
+    exit 1
+  fi
+elif [[ ! -x "${KUBEADM_PATH}" ]]; then
+  {
+    echo "KUBEADM_PATH environment variable set to '${KUBEADM_PATH}', but "
+    echo "this doesn't seem to be a valid executable."
+  } >&2
+  exit 1
+fi
+kubeadm="${KUBEADM_PATH:-${kubeadm}}"
+
+"${kubeadm}" "${@+$@}"