Shared memory error while connecting to Oracle

I get this error message each time I try to connect to my Oracle XE local database under Ubuntu 15.10 :

user@user-X220:/u01/app/oracle/product/11.2.0/xe/network/admin$ sqlplus

SQL*Plus: Release 11.2.0.2.0 Production on Thu May 5 20:58:13 2016

Copyright (c) 1982, 2011, Oracle.  All rights reserved.

Enter user-name: SYS
Enter password: 
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux-x86_64 Error: 2: No such file or directory
Process ID: 0
Session ID: 0 Serial number: 0

I have read on some forums that i have to delete my /dev/shm repository :

 sudo rm -rf /dev/shm
 sudo mkdir /dev/shm
 sudo mount -t tmpfs shmfs -o size=2048m /dev/shm

I tried actually to do that but it seems that /dev/shm is used by several processors including my firefox navigator.

Any help will more than welcome.