Add --runtime-id flag to dotnet-install.sh

This lets us specify the runtime id of the desired .NET Core SDK on the
command line. This makes it easier to get the SDK for the desired
runtime without having to modify the install script for new runtimes
unsupported by the current version of this script.

scripts/obtain/dotnet-install.sh

@@ -56,6 +56,59 @@ say_verbose() {
     fi
 }
 
+get_os_download_name_from_platform() {
+    eval $invocation
+
+    platform="$1"
+    case "$platform" in
+        "centos.7")
+            echo "centos"
+            return 0
+            ;;
+        "debian.8")
+            echo "debian"
+            return 0
+            ;;
+        "fedora.23")
+            echo "fedora.23"
+            return 0
+            ;;
+        "fedora.24")
+            echo "fedora.24"
+            return 0
+            ;;
+        "opensuse.13.2")
+            echo "opensuse.13.2"
+            return 0
+            ;;
+        "opensuse.42.1")
+            echo "opensuse.42.1"
+            return 0
+            ;;
+        "rhel.7"*)
+            echo "rhel"
+            return 0
+            ;;
+        "ubuntu.14.04")
+            echo "ubuntu"
+            return 0
+            ;;
+        "ubuntu.16.04")
+            echo "ubuntu.16.04"
+            return 0
+            ;;
+        "ubuntu.16.10")
+            echo "ubuntu.16.10"
+            return 0
+            ;;
+        "alpine.3.4.3")
+            echo "alpine"
+            return 0
+            ;;
+    esac
+    return 1
+}
+
 get_current_os_name() {
     eval $invocation
 
@@ -66,56 +119,17 @@ get_current_os_name() {
     elif [ "$uname" = "Darwin" ]; then
         echo "osx"
         return 0
+    elif [ -n "$runtime_id" ]; then
+        echo $(get_os_download_name_from_platform "${runtime_id%-*}" || echo "${runtime_id%-*}")
+        return 0
     else
         if [ -e /etc/os-release ]; then
             . /etc/os-release
-
+            os=$(get_os_download_name_from_platform "$ID.$VERSION_ID" || echo "")
-            case "$ID.$VERSION_ID" in
+            if [ -n "$os" ]; then
-                "centos.7")
+                echo "$os"
-                    echo "centos"
+                return 0
-                    return 0
+            fi
-                    ;;
-                "debian.8")
-                    echo "debian"
-                    return 0
-                    ;;
-                "fedora.23")
-                    echo "fedora.23"
-                    return 0
-                    ;;
-                "fedora.24")
-                    echo "fedora.24"
-                    return 0
-                    ;;
-                "opensuse.13.2")
-                    echo "opensuse.13.2"
-                    return 0
-                    ;;
-                "opensuse.42.1")
-                    echo "opensuse.42.1"
-                    return 0
-                    ;;
-                "rhel.7"*)
-                    echo "rhel"
-                    return 0
-                    ;;
-                "ubuntu.14.04")
-                    echo "ubuntu"
-                    return 0
-                    ;;
-                "ubuntu.16.04")
-                    echo "ubuntu.16.04"
-                    return 0
-                    ;;
-                "ubuntu.16.10")
-                    echo "ubuntu.16.10"
-                    return 0
-                    ;;
-                "alpine.3.4.3")
-                    echo "alpine"
-                    return 0
-                    ;;
-            esac
         fi
     fi
     
@@ -588,6 +602,7 @@ uncached_feed="https://dotnetcli.blob.core.windows.net/dotnet"
 verbose=false
 shared_runtime=false
 linux_portable=false
+runtime_id=""
 
 while [ $# -ne 0 ]
 do
@@ -631,6 +646,10 @@ do
         --linux-portable|-[Ll]inux[Pp]ortable)
             linux_portable=true
             ;;
+        --runtime-id|-[Rr]untime[Ii]d)
+            shift
+            runtime_id="$1"
+            ;;
         -?|--?|-h|--help|-[Hh]elp)
             script_name="$(basename $0)"
             echo ".NET Tools Installer"
@@ -657,6 +676,8 @@ do
             echo "  --azure-feed,-AzureFeed        Azure feed location. Defaults to $azure_feed"
             echo "  --linux-portable               Installs the Linux portable .NET Tools instead of a distro-specific version."
             echo "      -LinuxPortable"
+            echo "  --runtime-id                   Installs the .NET Tools for the given platform (such as linux-x64)."
+            echo "      -RuntimeId"
             echo "  -?,--?,-h,--help,-Help         Shows this help message"
             echo ""
             echo "Install Location:"