最近遇到个两年前遇到的问题，使用virt-manager提示(virt-manager:873): Gtk-WARNING **: 14:53:28.147: cannot open display: :1，当时专门运维的同事帮忙临时调了下DISPLAY变量，好像是将:1改成了SSH用户本地IP:10.0，当时的确好了，用完就关了再没用到，也没深究原因，那个运维同事也不大理解(网上查到的解决办法)。然而最近在做资产盘点，领导让我把我挂名管理的服务器作置换申请，需要知道虚拟机的信息，赶上盘到两年前有问题的机器上，又出现同样的问题，经过查找了资料找到了个几乎万无一失的理解，记一记。
首先，它是Linux X11 server（显示服务）用到的一个环境变量，用来指示你的显示(也可以包含键盘和鼠标)指向的显示服务地址，通常桌面PC该值会被设为:0.0 。
其次，它的格式有三部分： [主机名]:显示服务端口号-6000.显示器编号
[主机名] ：一般是可以省略的，可以不写，也可以写成$HOSTNAME变量表示的主机名 或 localhost显示服务端口号-6000：意思是sshd服务的X11Forwarding占用端口减去6000的值显示器编号：一般都是0，表示第一个显示器分两种情况：
:0.0检查步骤如下：
[root@hz ~]# netstat -anpt |grep sshd |grep LISTEN |grep 60
tcp 0 0 127.0.0.1:6010 0.0.0.0:* LISTEN 30346/sshd: root@pt
tcp6 0 0 ::1:6010 :::* LISTEN 30346/sshd: root@pt
找到60开头的sshd端口，这时是6010，减去6000是10，SSH只写第一显示器编号
则我的DISPLAY变量可设为 :10.0 或者 hz:10.0
如果上边的命令查不出来6000左右的端口号，请检查 /etc/ssh/sshd_config，确认
X11Forwarding yes参数已配置并systemctl restart sshd，使用exit退出当前ssh，重新连接再尝试。
The magic word in the X window system is DISPLAY. A display consists (simplified) of:
A display is managed by a server program, known as an X server. The server serves displaying capabilities to other programs that connect to it.
The remote server knows where it has to redirect the X network traffic via the definition of the DISPLAY environment variable which generally points to an X Display server located on your local computer.
The value of the display environment variable is:
hostname:D.S
where:
hostname is the name of the computer where the X server runs. An omitted hostname means the localhost.
D is a sequence number (usually 0). It can be varied if there are multiple displays connected to one computer.
S is the screen number. A display can actually have multiple screens. Usually, there's only one screen though where 0 is the default.
Example of values
localhost:4
google.com:0
:0.0
hostname:D.S means screen S on display D of host hostname; the X server for this display is listening at TCP port 6000+D.
host/unix:D.S means screen S on display D of host host; the X server for this display is listening at UNIX domain socket /tmp/.X11-unix/XD (so it's only reachable from host).
:D.S is equivalent to host/unix:D.S, where host is the local hostname.
:0.0 means that we are talking about the first screen attached to your first display in your local host
Read more here: support.objectplanet.com and here: superuser.com and here: docstore.mik.ua.
From a X(7) man page:
From the user's perspective, every X server has a display name of the form:
hostname:displaynumber.screennumber
This information is used by the application to determine how it should connect to the server and which screen it should use by default (on displays with multiple monitors):
hostname The hostname specifies the name of the machine to which the display is physically connected. If the hostname is not given, the most efficient way of communicating to a server on the same machine will be used. displaynumber The phrase "display" is usually used to refer to a collection of monitors that share a common keyboard and pointer (mouse, tablet, etc.). Most workstations tend to only have one keyboard, and therefore, only one display. Larger, multi-user systems, however, frequently have several displays so that more than one person can be doing graphics work at once. To avoid confusion, each display on a machine is assigned a display number (beginning at 0) when the X server for that display is started. The display number must always be given in a display name. screennumber Some displays share a single keyboard and pointer among two or more monitors. Since each monitor has its own set of windows, each screen is assigned a screen number (beginning at 0) when the X server for that display is started. If the screen number is not given, screen 0 will be used.