mac下如何使用命令行打开App呢?

可以使用open命令。

例如:

open -a /Applications/mou.app

可以查看下open都支持哪些参数

ksnowlvdeMacBook-Pro:lvweigithubblog ksnowlv$ open
Usage: open [-e] [-t] [-f] [-W] [-R] [-n] [-g] [-h] [-b <bundle identifier>] [-a <application>] [filenames] [--args arguments]
Help: Open opens files from a shell.
  By default, opens each file using the default application for that file.  
  If the file is in the form of a URL, the file will be opened as a URL.
Options: 
  -a                Opens with the specified application.
  -b                Opens with the specified application bundle identifier.
  -e                Opens with TextEdit.
  -t                Opens with default text editor.
  -f                Reads input from standard input and opens with TextEdit.
  -F  --fresh       Launches the app fresh, that is, without restoring windows. Saved persistent state is lost, excluding Untitled documents.
  -R, --reveal      Selects in the Finder instead of opening.
  -W, --wait-apps   Blocks until the used applications are closed (even if they were already running).
      --args        All remaining arguments are passed in argv to the application's main() function instead of opened.
  -n, --new         Open a new instance of the application even if one is already running.
  -j, --hide        Launches the app hidden.
  -g, --background  Does not bring the application to the foreground.
  -h, --header      Searches header file locations for headers matching the given filenames, and opens them.

如何使用mou打开markdown文件呢?

open -a /Applications/mou.app /Users/ksnowlv/Movies/lvweigithubblog/source/_posts/2014-07-26-macxia-ming-ling-xing-da-kai-app.markdown

正常打开,你可以定义一个脚本来做这件事。