Add nvm_validate_implicit_alias

nvm.sh

@@ -368,6 +368,13 @@ nvm_print_versions() {
   done
 }
 
+nvm_validate_implicit_alias() {
+  if [ "_$1" != "_stable" ] && [ "_$1" != "_unstable" ]; then
+    echo "Only implicit aliases 'stable' and 'unstable' are supported." >&2
+    return 1
+  fi
+}
+
 nvm() {
   if [ $# -lt 1 ]; then
     nvm help

test/fast/Unit tests/nvm_validate_implicit_alias

@@ -0,0 +1,19 @@
+#!/bin/sh
+
+die () { echo $@ ; exit 1; }
+
+. ../../../nvm.sh
+
+EXPECTED_MSG="Only implicit aliases 'stable' and 'unstable' are supported."
+[ "_$(nvm_validate_implicit_alias 2>&1)" = "_$EXPECTED_MSG" ] \
+  || die "nvm_validate_implicit_alias did not require stable|unstable"
+[ "_$(nvm_validate_implicit_alias foo 2>&1)" = "_$EXPECTED_MSG" ] \
+  || die "nvm_validate_implicit_alias did not require stable|unstable"
+
+EXIT_CODE="$(nvm_validate_implicit_alias >/dev/null 2>&1 ; echo $?)"
+[ "_$EXIT_CODE" = "_1" ] \
+  || die "nvm_validate_implicit_alias without stable|unstable had wrong exit code: expected 1, got $EXIT_CODE"
+
+nvm_validate_implicit_alias stable || die "nvm_validate_implicit_alias stable did not exit 0"
+nvm_validate_implicit_alias unstable || die "nvm_validate_implicit_alias unstable did not exit 0"
+