使用 `systemd-nspawn` 这个命令我们可以很方便的创建一个 Linux 容器，需要的只是一个使用 systemd 作为 init 的 Linux 发行版的根文件系统。通过创建容器，我们可以获得一个可以随便折腾而不用担心损坏的 Linux 环境。这里用 Ubuntu 16.04 和 CentOS 7 为例，整个过程可以说是非常简单（虽然比起 Docker 还是麻烦了点）
对于 Ubuntu，可以直接从源里下载到它的根文件系统。下载一份，并解压到 `/var/lib/machines/ubuntu1604`：
sudo mkdir -p /var/lib/machines/ubuntu1604 wget http://mirrors.ustc.edu.cn/ubuntu-cdimage/ubuntu-base/releases/16.04.3/release/ubuntu-base-16.04.1-base-amd64.tar.gz -O /tmp/rootfs.tgz sudo tar xpzf /tmp/rootfs.tgz -C /var/lib/machines/ubuntu1604
OK，到此为止我们就得到了一个可以被 `systemd-nspawn` 启动的 rootfs，不过我们还需要一些配置，例如修改 root 密码等等：
chroot /var/lib/machines/ubuntu1604 /usr/bin/passwd root echo ubuntu > /var/lib/machines/ubuntu1604/etc/hostname
下面只需要用 `systemd-nspawn` 来“启动”这个容器：
systemd-nspawn -b -D /var/lib/machines/ubuntu1604 --bind=/lib/firmware
这样就完成了！相当简单吧～输出内容大概是这样：
Spawning container ubuntu1604 on /var/lib/machines/ubuntu1604. Press ^] three times within 1s to kill container. systemd 229 running in system mode. (+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ -LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN) Detected virtualization systemd-nspawn. Detected architecture x86-64. Welcome to Ubuntu 16.04.1 LTS! Set hostname to. Failed to install release agent, ignoring: No such file or directory [ OK ] Listening on Journal Socket (/dev/log). [ OK ] Started Dispatch Password Requests to Console Directory Watch. [ OK ] Started Forward Password Requests to Wall Directory Watch. [ OK ] Reached target Paths. [ OK ] Reached target Remote File Systems (Pre). [ OK ] Reached target Remote File Systems. [ OK ] Listening on /dev/initctl Compatibility Named Pipe. [ OK ] Created slice System Slice. [ OK ] Reached target Slices. [ OK ] Created slice system-getty.slice. [ OK ] Reached target Swap. [ OK ] Reached target Encrypted Volumes. [ OK ] Listening on Journal Socket. Mounting Huge Pages File System... [ OK ] Reached target Sockets. Starting Remount Root and Kernel File Systems... Mounting POSIX Message Queue File System... Starting Journal Service... Mounting FUSE Control File System... [ OK ] Mounted POSIX Message Queue File System. [ OK ] Mounted Huge Pages File System. [ OK ] Mounted FUSE Control File System. [ OK ] Started Remount Root and Kernel File Systems. [ OK ] Reached target Local File Systems (Pre). [ OK ] Reached target Local File Systems. Starting Load/Save Random Seed... [ OK ] Started Load/Save Random Seed. [ OK ] Started Journal Service. Starting Flush Journal to Persistent Storage... [ OK ] Started Flush Journal to Persistent Storage. Starting Create Volatile Files and Directories... [ OK ] Started Create Volatile Files and Directories. Starting Update UTMP about System Boot/Shutdown... [ OK ] Reached target System Time Synchronized. [ OK ] Started Update UTMP about System Boot/Shutdown. [ OK ] Reached target System Initialization. [ OK ] Started Daily Cleanup of Temporary Directories. [ OK ] Reached target Basic System. Starting Permit User Sessions... Starting LSB: Set the CPU Frequency Scaling governor to "ondemand"... Starting /etc/rc.local Compatibility... [ OK ] Started Daily apt activities. [ OK ] Reached target Timers. [ OK ] Started Permit User Sessions. [ OK ] Started /etc/rc.local Compatibility. [ OK ] Started Console Getty. [ OK ] Reached target Login Prompts. [ OK ] Started LSB: Set the CPU Frequency Scaling governor to "ondemand". [ OK ] Reached target Multi-User System. [ OK ] Reached target Graphical Interface. Starting Update UTMP about System Runlevel Changes... [ OK ] Started Update UTMP about System Runlevel Changes. Ubuntu 16.04.1 LTS ubuntu console ubuntu login:
值得注意的是，这个容器和虽然看起来很像那么一回事儿，但是它的内核和网络仍然是使用的宿主机的，如果宿主机已经运行了 sshd，容器里尝试运行则会提示端口被占用，不过 `systemd-nspawn` 提供了桥接网络之类的功能，具体方法可以 Google。关闭容器很简单，在里面执行 `systemctl poweroff` 即可。
CentOS 稍微复杂一点，因为他没有直接提供最小的 rootfs，我们要自己从 ISO 中解压安装，整个过程如下：
# 切换到超级用户
sudo -s
# 建立一些文件夹
mkdir -p /var/lib/machines/centos7
mkdir -p /tmp/iso
mkdir -p /tmp/squashfs
mkdir -p /tmp/rootfs
# 挂载 CentOS 7 的系统盘
mount /mnt/Disk2/OS/Linux/CentOS-7-x86_64-Minimal-1708.iso /tmp/iso
mount /tmp/iso/LiveOS/squashfs.img /tmp/squashfs
mount /tmp/squashfs/LiveOS/rootfs.img /tmp/rootfs
# 复制系统文件，速度看你的硬盘，可能会比较慢_(:з」∠)_
cp -pr /tmp/rootfs/* /var/lib/machines/centos7
# 卸载一些不会再用到的镜像
umount /tmp/{rootfs,squashfs}
# 安装一下 yum
mkdir -p /var/lib/machines/centos7/mnt/iso
mount --bind /tmp/iso /var/lib/machines/centos7/mnt/iso
chroot /var/lib/machines/centos7 /usr/bin/rpm -ivh --nodeps /mnt/iso/Packages/rpm-4.11.3-25.el7.x86_64.rpm
chroot /var/lib/machines/centos7 /usr/bin/rpm -ivh --nodeps /mnt/iso/Packages/yum-3.4.3-154.el7.centos.noarch.rpm
# 配置一下基本系统，执行最小安装
echo "[cdrom]
name=Install CD-ROM
baseurl=file:///mnt/iso
enabled=0
gpgcheck=1
gpgkey=file:///mnt/iso/RPM-GPG-KEY-CentOS-7" > /var/lib/machines/centos7/etc/yum.repos.d/cdrom.repo
chroot /var/lib/machines/centos7 /usr/bin/yum --disablerepo=\* --enablerepo=cdrom -y reinstall yum
chroot /var/lib/machines/centos7 /usr/bin/yum --disablerepo=\* --enablerepo=cdrom -y groupinstall "Minimal Install"
# 删掉 ISO 源
rm /var/lib/machines/centos7/etc/yum.repos.d/cdrom.repo
# 卸载 ISO
umount /var/lib/machines/centos7/mnt/iso /tmp/iso
# 设置一下 root 密码之类的
chroot /var/lib/machines/centos7 /usr/bin/passwd root
# 进入虚拟环境，执行这段脚本
systemd-nspawn -D /var/lib/machines/centos7 --bind=/lib/firmware << _END_POSTINSTALL_
# 换源，先备份
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
curl https://lug.ustc.edu.cn/wiki/_export/code/mirrors/help/centos\?codeblock=3 > /etc/yum.repos.d/CentOS-Base.repo
# 更新
yum makecache
# 安装一个缺失的依赖，为什么会缺我也不知道…
yum install lvm2-libs -y
# 这些服务是作为一个容器不需要的，把他们关掉
systemctl disable auditd.service
systemctl disable kdump.service
systemctl disable multipathd.service
systemctl disable network.service
systemctl disable smartd.service
systemctl disable lvm2-monitor.service
systemctl disable sshd.service
# 设置 locale，如果需要中文就用 zh_CN.UTF-8
echo LANG=en_US.UTF-8 > /etc/locale.conf
# 设置主机名
echo CentOS-7 > /etc/hostname
_END_POSTINSTALL_
# 然后就可以开机了（（（
systemd-nspawn -b -D /var/lib/machines/centos7 --bind=/lib/firmware
参考资料：