shell command for ps

What's the difference between the following two co开发者_如何转开发mmands. I know the output is different but what is that = sign before rss?

$ ps -orss -p 62277
   RSS
111636

$ ps -o rss= -p 62277
111636

The -o specifies that you will be listing the columns that you want displayed. Adding an = after the column name (rss in this case), gives you the opportunity to override the default value for the column header, you could put -o rss=ResidentSet for example, but with an empty =, you are saying 'no column header'.