The whole point of a virtual machine is to make it as difficult as possible to distinguish between it and a real physical machine.
You may be able to see some differences in the DMESG boot report, since often the hardware virtualization isn't complete. Some things aren't actually necessary for each individual VM and can be handle by the VM host. Also, depending on the virtualization method used, there may be vm-assist packages installed, although that doesn't necessarily indicate that the machine is currently running virtualized. Other cues include doing an "ifconfig" and looking for virtual NICs.
Finally, a VM often shows less RAM and disk than a full physical machine, since you're usually using it for a specialized purpose and therefore don't need worst-case hardware. Instead you share the resources among VMs.
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Joined: Oct 06, 2008
Thanks for all replies. They are really helpful to me.
subject: How to know the linux is running on a virtual server?