Open tab when running next dev (vercel#8)

* Open tab when running next dev

* Added NEXT_OPEN_BROWSER env var to prevent open a browser tab on next dev

* using regexp for testing false on env var

* adding ! to if

bin/next-dev

@@ -1,4 +1,5 @@
 #!/usr/bin/env node
+import { exec } from 'child_process'
 import { resolve, join } from 'path'
 import parseArgs from 'minimist'
 import Server from '../server'
@@ -16,6 +17,12 @@ const argv = parseArgs(process.argv.slice(2), {
   }
 })
 
+const open = url => {
+  const openers = { darwin: 'open', win32: 'start' }
+  const cmdName = openers[process.platform] || 'xdg-open'
+  exec(`${cmdName} ${url}`)
+}
+
 const dir = resolve(argv._[0] || '.')
 
 build(dir)
@@ -25,13 +32,17 @@ build(dir)
   console.log('> Ready on http://localhost:%d', argv.port)
 
   // Check if pages dir exists and warn if not
-  if (!await exists(join(dir, 'pages'))) {
+  if (!(await exists(join(dir, 'pages')))) {
     if (await exists(join(dir, '..', 'pages'))) {
       console.warn('> No `pages` directory found. Did you mean to run `next` in the parent (`../`) directory?')
     } else {
       console.warn('> Couldn\'t find a `pages` directory. Please create one under the project root')
     }
   }
+
+  if (!/^(false|0)$/i.test(process.env.NEXT_OPEN_BROWSER)) {
+    open(`http://localhost:${argv.port}`)
+  }
 })
 .catch((err) => {
   console.error(err)