Raspberry pi 3 B with Ubuntu 18.04 server arm64: what pi version

How can I find what chip I have (what version of raspberry pi) with Ubuntu 18.04 server for arm64? What file can I check or what command can I run? /proc/cpuinfo does not have useful information, just some generic details without mention of the pi.

At least on Ubuntu 18.04 server for arm64 and with a Raspberry Pi 3 B, the following command gives the board, including revision:

lshw

A less verbose output that's easier to read is given by:

lshw -short

That gave me my board as a "Raspberry Pi 3 B Rev 1.2", which is exactly what was needed in this case.

The command lscpu is what you are looking for. Here's an example output of the command (taken on my Raspberry Pi 3B+):

lscpu

which produces the following output:

Architecture:          armv7l
Byte Order:            Little Endian
CPU(s):                4
On-line CPU(s) list:   0-3
Thread(s) per core:    1
Core(s) per socket:    4
Socket(s):             1
Model:                 4
Model name:            ARMv7 Processor rev 4 (v7l)
CPU max MHz:           1200,0000
CPU min MHz:           600,0000
BogoMIPS:              38.40
Flags:                 half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm crc32

Another option is the inxi command (you might have to install it if not present with sudo apt install inxi). Use the flag -C as follows:

inxi -C

which produces the following output:

CPU:       Quad core ARMv7 rev 4 (v7l) (-MCP-) (ARM) 
           clock speeds: max: 1200 MHz 1: 1200 MHz 2: 1200 MHz 3: 1200 MHz 4: 1200 MHz

As for identifying which chip your board has, you can check the chip on the board as well the manufacturers website for info and as well as the different selling outlets for getting technical details.