feat: add overWrite option for commands (#503)

* feat: add overWrite option for commands

* fix: throws an error if a command is overwritten

Author: nfischer

Diff for: src/common.js

@@ -372,13 +372,19 @@ var DEFAULT_WRAP_OPTIONS = {
   pipeOnly: false,
   unix: true,
   wrapOutput: true,
+  overWrite: false,
 };
 
 // Register a new ShellJS command
 function _register(name, implementation, wrapOptions) {
   wrapOptions = wrapOptions || {};
   // If an option isn't specified, use the default
   wrapOptions = objectAssign({}, DEFAULT_WRAP_OPTIONS, wrapOptions);
+
+  if (shell[name] && !wrapOptions.overWrite) {
+    throw new Error('unable to overwrite `' + name + '` command');
+  }
+
   if (wrapOptions.pipeOnly) {
     wrapOptions.canReceivePipe = true;
     shellMethods[name] = wrap(name, implementation, wrapOptions);

Diff for: test/plugin.js

@@ -96,4 +96,11 @@ assert.equal(ret.stdout, '');
 assert.equal(ret.stderr, 'foo: Exited with code 5');
 assert.equal(shell.error(), 'foo: Exited with code 5');
 
+// Cannot overwrite an existing command by default
+var oldCat = shell.cat;
+assert.throws(function () {
+  plugin.register('cat', fooImplementation);
+}, 'Error: unable to overwrite `cat` command');
+assert.equal(shell.cat, oldCat);
+
 shell.exit(123);