Virtual Machines from the command line with QEMU

I recently went through the Arch Linux Wiki page about QEMU and realized it is quite easy to work with virtual machines from the command line. Not only that: for the simple scenarios I usually need, the command line is even simpler than using a GUI application.

Here are the steps!

Create a disk image

Create a disk image in QCOW2 format (qcow2 is slower than the raw format, but uses less disk space).

qemu-img create -f qcow2 ubuntu.cow 20G

The Arch Wiki explains how to add an overlay image, if you want to take snapshots.

Install the OS

qemu-system-x86_64 -m 4G -enable-kvm \
                   -cdrom ~/Downloads/ubuntu-21.10-desktop-amd64.iso \
                   -drive file=ubuntu.cow,format=qcow2 \
                   -boot order=d

where:

-enable-kvm increases performances dramatically but it requires the host to load some modules. See the Arch page for more details.
-m 4G allocates 4G of RAM, in place of the 128M otherwise available

You can skip this passage and download prebuilt images, such as, for instance:

Arch Linux boxes, from: https://gitlab.archlinux.org/archlinux/arch-boxes/

Guix boxes, from: https://guix.gnu.org/en/download/

I did not look into it, but the option -bios /usr/share/qemu/bios.bin -boot menu=on might be necessary for booting certain OSs, such as Arch Linux.

Run the machine

qemu-system-x86_64 -m 4G -smp 2 -enable-kvm \
                   -nic user,hostfwd=tcp::60022-:22 \
                   ubuntu.cow

qemu-system-x86_64 -m 4G -smp 2 -enable-kvm \
                   -display none \
                   -nic user,hostfwd=tcp::60022-:22 \
                   ubuntu.cow

where:

-display none hides the graphical display
-smp 2 tells qemu to use 2 CPUs
-nic user,... makes port 60022 on the host point to port 22 (ssh) on the guest. This allows to ssh on the machine with ssh localhost -p 60022, is sshd is running on the guest.

Share Files

Mount a guest directory on the host (assuming sshd is running on the guest):

sshfs remote_user@localhost:/home/remote_user ./tmp/ -C -p 60022

Serve Web pages

Similar to the previous example, we also forward port 80 to 8080 on the host:

qemu-system-x86_64 -m 4G -smp 2 -enable-kvm  ubuntu.cow \
                   -display none \
                   -nic user,hostfwd=tcp::60022-:22,hostfwd=tcp::8080-:80

We assume, of course, a web server runs on the guest.