Newby here (it has been decades since I have used UNIX.) Linux (ubuntu) on a chromebook. From the opening of the command shell I issue ls and I get a bunch of directories. I want to create a sub-directory under "/opt" and issue the command mkdir opt/newdirectory. Ubuntu responds "Cannot create directory 'opt/newdirectory': Read-only file system." How do I change the permissions so I can create a directory? I have tried all sorts of chmod commands but nothing seems to work. It really can't be that hard.