QEMU is a powerful and handy tool to modify a disk image, which gem5 cannot do. To boot from a given disk image without opening GUI, run this command.

qemu-kvm -nographic -enable-kvm -m 1024 -hda <disk-img> -net nic,model=virtio -net user,hostfwd=tcp::2222-:22

In this case, the command below should be supplied for access.

ssh gem5@localhost -p 2222

The above method is handy in that the emulator makes it simple to use common utilities such as git. However, if there is need of moving a file from the host to the disk image, the method below may be more straightforward.

fdisk -l <disk-img> # To get the proper offset
dzdo mount -o loop,offset=1048576 <disk-img> fsmount
# Do whatever to modify the disk
dzdo umount fsmount