Add overwriting to rename

rename now deletes the context (soft-delete, as in does not delete user or
cluster of the context) if the specified name exists.

Fixes #25.

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

kubectx

@@ -103,28 +103,39 @@ swap_context() {
 }
 
 user_of_context() {
+  # TODO(ahmetb) no longer used, consider deleting
   kubectl config view \
     -o=jsonpath="{.contexts[?(@.name==\"${1}\")].context.user}"
 }
 
 cluster_of_context() {
+  # TODO(ahmetb) no longer used, consider deleting
   kubectl config view \
     -o=jsonpath="{.contexts[?(@.name==\"${1}\")].context.cluster}"
 }
 
+context_exists() {
+  grep -q ^"${1}"\$ <(kubectl config get-contexts -o=name)
+}
+
 rename_context() {
   local old_name="${1}"
   local new_name="${2}"
 
+  # TODO(ahmetb) old_user and old_cluster are no longer used, clean up
   local old_user old_cluster
   old_user="$(user_of_context "${old_name}")"
   old_cluster="$(cluster_of_context "${old_name}")"
-
   if [[ -z "$old_user" || -z "$old_cluster" ]]; then
     echo "error: Cannot retrieve context ${old_name}."  >&2
     exit 1
   fi
 
+  if context_exists "${new_name}"; then
+    echo "Context \"${new_name}\" exists, deleting." >&2
+    kubectl config delete-context "${new_name}" 1>/dev/null 2>&1
+  fi
+
   kubectl config rename-context "${old_name}" "${new_name}"
 }