Docs: Add recipe for running shell commands with child_process or gulp-exec

Author: phated

docs/recipes/README.md

@@ -27,3 +27,4 @@
 * [Exports as tasks](exports-as-tasks.md)
 * [Rollup with rollup-stream](rollup-with-rollup-stream.md)
 * [Run gulp task via cron job](cron-task.md)
+* [Running shell commands](running-shell-commands.md)

docs/recipes/running-shell-commands.md

@@ -0,0 +1,31 @@
+# Running Shell Commands
+
+Sometimes it is helpful to be able to call existing command line tools from gulp.
+
+There are 2 ways to handle this: node's [`child_process`](https://nodejs.org/api/child_process.html)
+built-in module or [`gulp-exec`](https://github.com/robrich/gulp-exec) if you need to integrate the
+command with an existing pipeline.
+
+```js
+'use strict';
+
+var cp = require('child_process');
+var gulp = require('gulp');
+
+gulp.task('reset', function() {
+  // In gulp 4, you can return a child process to signal task completion
+  return cp.execFile('git checkout -- .');
+});
+```
+
+```js
+'use strict';
+
+var gulp = require('gulp');
+var exec = require('gulp-exec');
+
+gulp.task('reset', function() {
+  return gulp.src('./**/**')
+    .pipe(exec('git checkout -- <%= file.path %>'));
+});
+```