背景
最近遇到個兩年前遇到的問題，使用virt-manager提示(virt-manager:873): Gtk-WARNING **: 14:53:28.147: cannot open display: :1，當時專門運維的同事幫忙臨時調了下DISPLAY變數，好像是將:1改成了SSH使用者本地IP:10.0，當時的確好了，用完就關了再沒用到，也沒深究原因，那個運維同事也不大理解(網上查到的解決辦法)。然而最近在做資產盤點，領導讓我把我掛名管理的伺服器作置換申請，需要知道虛擬機器的資訊，趕上盤到兩年前有問題的機器上，又出現同樣的問題，經過查詢了資料找到了個幾乎萬無一失的理解，記一記。
DISPLAY變數是啥
首先，它是Linux X11 server（顯示服務）用到的一個環境變數，用來指示你的顯示(也可以包含鍵盤和滑鼠)指向的顯示服務地址，通常桌面PC該值會被設為:0.0 。
其次，它的格式有三部分： [主機名]:顯示服務埠號-6000.顯示器編號
[主機名]：一般是可以省略的，可以不寫，也可以寫成$HOSTNAME變數表示的主機名 或 localhost顯示服務埠號-6000：意思是sshd服務的X11Forwarding佔用埠減去6000的值顯示器編號：一般都是0，表示第一個顯示器
如何正確設定DISPLAY變數
分兩種情況：
- Linux桌面系統：直接設定
:0.0 - SSH連線的Linux伺服器：需要按照格式進行檢查。
檢查步驟如下：
[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開頭的sshd埠，這時是6010，減去6000是10，SSH只寫第一顯示器編號
則我的DISPLAY變數可設為 :10.0 或者 hz:10.0
如果上邊的命令查不出來6000左右的埠號，請檢查 /etc/ssh/sshd_config，確認
X11Forwarding yes引數已配置並systemctl restart sshd，使用exit退出當前ssh，重新連線再嘗試。
附：參考
The magic word in the X window system is DISPLAY. A display consists (simplified) of:
- a keyboard,
- a mouse
- and a screen.
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.