Linux, Ubuntu: how to tell if 32 or 64 bit installed

5 Sep 2012

A question I get regularly from other sysadmins - how to tell if you’re running a 32 or 64 bit install (vs CPU) of Linux?

Here’s one way - use the file command on /sbin/init:

# on a 32 bit install
% file /sbin/init
/sbin/init: ELF 32-bit LSB executable....

# on a 64 bit install
% file /sbin/init
/sbin/init: ELF 64-bit LSB shared object, x86-64...

Update

I got a few comments on this (thank you) saying that I was wrong or “haven’t you heard of uname?”…

1. CPU vs Kernel

One comment said I was wrong and should use grep against the cpu info:

$ grep ^flags /proc/cpuinfo | grep lm

Unfortunately the commenter hadn’t read my post correctly - I’m interested in whether the operating system is ³²⁄₆₄ bit, not the cpu.

2. Use uname

Other comments said I should use uname with particular flags (as if I’d never heard of uname before….). Unfortunately the manpage for uname is a good example of manpage considered harmful. Let’s have a look at what it says for the various options:

% man uname
...
-m, --machine
    print the machine hardware name

-p, --processor
    print the processor type or "unknown"

-i, --hardware-platform
    print the hardware platform or "unknown"
...

What is the difference between the “machine hardware name”, the “processor type”, and the “hardware platform”? Googling doesn’t turn up a good explanation. I could look at the source code for uname, or run uname on a machine I which already has known hardware and interpret the results, then work out the flags to use on the target machine. Or I could just rote-memorise uname -m and not know what it means.

Bzzt, fail. I know what the file command does, I use it regularly for cross-compiling stuff.

file /sbin/init is what I use.