Quick question, how to determine if CPU is 64bit or not under Linux?

A. You need to use uname command, which prints system information including kernel version and whether kernel is 32 bit or 64 bit. You can also use less /proc/cpuinfo command determine if it is 64 bit cpu or not.
Example - Find out if Running kernel is 32 or 64 bit</p>

$ uname -a
Output:

Linux ora100 2.6.5-7.252-smp #1 SMP Tue Feb 14 11:11:04 UTC 2006 x86_64 x86_64 x86_64 GNU/Linux

x86_64 GNU/Linux indicates I have 64bit kernel running. If you use see i386/i486/i586/i686 it is a 32 bit kernel.
How do I find out CPU is 32bit or 64bit?

Simply type the following command and if you see lm in output, you have a 64 bit system:
$ grep flags /proc/cpuinfo
Output:

flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm constant_tsc pni monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm

flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm constant_tsc pni monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm constant_tsc pni monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm

flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm constant_tsc pni monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm

    * lm means Long mode - 64 bit CPU
    * Real mode 16 bit CPU
    * Protected Mode is 32-bit CPU</div> </div>

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