Linux / Ubuntu system: 32-bit or 64-bit ?

arch
uname -m
getconf LONG_BIT
uname -a
lscpu | grep bit

• 32 bits: i686
• 64 bits: amd64 or x86_64

Note: with lscpu, you know if your computer accepts a 32-bit and/or a 64-bit OS

ljouhet@myboard:~$ arch
x86_64
ljouhet@myboard:~$ uname -m
x86_64
ljouhet@myboard:~$ getconf LONG_BIT
64
ljouhet@myboard:~$ uname -a
Linux myboard 4.4.0-47-generic #68-Ubuntu SMP Wed Oct 26 19:39:52 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
ljouhet@myboard:~$ lscpu | grep bit
Mode(s) opératoire(s) des processeurs :32-bit, 64-bit