Home » Is there a command to list all open displays on a machine?

Is there a command to list all open displays on a machine?


If you want the X connection forwarded over SSH, you need to enable it on both the server side and the client side. (Depending on the distribution, it may be enabled or disabled by default.) On the server side, make sure that you have X11Forwarding yes in /etc/sshd_config (or /etc/ssh/sshd_config or wherever the configuration file is). On the client side, pass the -X option to the ssh command, or put ForwardX11 in your ~/.ssh/config.

If you run ssh -X localhost, you should see that $DISPLAY is (probably) localhost:10.0. Contrast with :0.0, which is the value when you’re not connected over SSH. (The .0 part may be omitted; it’s a screen number, but multiple screens are rarely used.) There are two forms of X displays that you’re likely to ever encounter:

  • Local displays, with nothing before the :.
  • TCP displays, with a hostname before the :.

With ssh -X localhost, you can access the X server through both displays, but the applications will use a different method: :NUMBER accesses the server via local sockets and shared memory, whereas HOSTNAME:NUMBER accesses the server over TCP, which is slower and disables some extensions.

Note that you need a form of authorization to access an X server, called a cookie and normally stored behind the scenes in the file ~/.Xauthority. If you’re using ssh to access a different user account, or if your distribution puts the cookies in a different file, you may find that DISPLAY=:0 doesn’t work within the SSH session (but ssh -X will, if it’s enabled in the server; you never need to mess with XAUTHORITY when doing ssh -X). If that’s a problem, you need to set the XAUTHORITY environment variable or obtain the other user’s cookies.

To answer your actual question:

  • Local displays correspond to a socket in /tmp/.X11-unix.

    (cd /tmp/.X11-unix && for x in X*; do echo ":${x#X}"; done)
  • Remote displays correspond to open TCP ports above 6000; accessing display number N on machine M is done by connecting to TCP port 6000+N on machine M. From machine M itself:

    netstat -lnt | awk '
      sub(/.*:/,"",$4) && $4 >= 6000 && $4 < 6100 {
        print ($1 == "tcp6" ? "ip6-localhost:" : "localhost:") ($4 - 6000)

    (The rest of this bullet point is of academic interest only.)

    From another machine, you can use nmap -p 6000-6099 host_name to probe open TCP ports in the usual range. It’s rare nowadays to have X servers listening on a TCP socket, especially outside the loopback interface.

    Strictly speaking, another application could be using a port in the range usually used by X servers. You can tell whether an X server is listening by checking which program has the port open.

    lsof -i -n | awk '$9 ~ /:60[0-9][0-9]$/ {print}'

    If that shows something ambiguous like sshd, there’s no way to know for sure whether it’s an X server or a coincidence.

The display is the first argument to Xorg. You can ps then grep Xorg out.

[braga@coleman teste_geom]$ ps aux | grep Xorg
root      1584  5.3  1.0 156628 41708 tty1     Rs+  Jul22  22:56 /usr/bin/Xorg :0 -background none -verbose -auth /var/run/gdm/auth-for-gdm-a3kSKB/database -nolisten tcp vt1
braga     9110  0.0  0.0 109104   804 pts/1    S+   00:26   0:00 grep --color=auto Xorg

You can then awk this into wherever format you need to.

# Show all active login shells, with displays
$ w -oush

trunc-us tty1                      23:02  -bash
trunc-us tty7     :0                4days /sbin/upstart --user
trunc-us pts/4    :0                      w -oush

# Capture the Display part
$ w -oush | grep -Eo ' :[0-9]+'


# only unique lines
$ w -oush | grep -Eo ' :[0-9]+' | uniq


# trim off the leading space
$ w -oush | grep -Eo ' :[0-9]+' | uniq | cut -d   -f 2

[Edit: I ran an Xnest instance to see if this would catch it – it doesn’t; it only captures login shells (‘w’ is short for ‘who’). Back to the drawing board for me.]
[Edit: Found it:

$ ls /tmp/.X11-unix

X0 X2

$ ls /tmp/.X11-unix | tr 'X' ':'


Related Solutions

Joining bash arguments into single string with spaces

[*] I believe that this does what you want. It will put all the arguments in one string, separated by spaces, with single quotes around all: str="'$*'" $* produces all the scripts arguments separated by the first character of $IFS which, by default, is a space....

AddTransient, AddScoped and AddSingleton Services Differences

TL;DR Transient objects are always different; a new instance is provided to every controller and every service. Scoped objects are the same within a request, but different across different requests. Singleton objects are the same for every object and every...

How to download package not install it with apt-get command?

Use --download-only: sudo apt-get install --download-only pppoe This will download pppoe and any dependencies you need, and place them in /var/cache/apt/archives. That way a subsequent apt-get install pppoe will be able to complete without any extra downloads....

What defines the maximum size for a command single argument?

Answers Definitely not a bug. The parameter which defines the maximum size for one argument is MAX_ARG_STRLEN. There is no documentation for this parameter other than the comments in binfmts.h: /* * These are the maximum length and maximum number of strings...

Bulk rename, change prefix

I'd say the simplest it to just use the rename command which is common on many Linux distributions. There are two common versions of this command so check its man page to find which one you have: ## rename from Perl (common in Debian systems -- Ubuntu, Mint,...

Output from ls has newlines but displays on a single line. Why?

When you pipe the output, ls acts differently. This fact is hidden away in the info documentation: If standard output is a terminal, the output is in columns (sorted vertically) and control characters are output as question marks; otherwise, the output is...

mv: Move file only if destination does not exist

mv -vn file1 file2. This command will do what you want. You can skip -v if you want. -v makes it verbose - mv will tell you that it moved file if it moves it(useful, since there is possibility that file will not be moved) -n moves only if file2 does not exist....

Is it possible to store and query JSON in SQLite?

SQLite 3.9 introduced a new extension (JSON1) that allows you to easily work with JSON data . Also, it introduced support for indexes on expressions, which (in my understanding) should allow you to define indexes on your JSON data as well. PostgreSQL has some...

Combining tail && journalctl

You could use: journalctl -u service-name -f -f, --follow Show only the most recent journal entries, and continuously print new entries as they are appended to the journal. Here I've added "service-name" to distinguish this answer from others; you substitute...

how can shellshock be exploited over SSH?

One example where this can be exploited is on servers with an authorized_keys forced command. When adding an entry to ~/.ssh/authorized_keys, you can prefix the line with command="foo" to force foo to be run any time that ssh public key is used. With this...

Why doesn’t the tilde (~) expand inside double quotes?

The reason, because inside double quotes, tilde ~ has no special meaning, it's treated as literal. POSIX defines Double-Quotes as: Enclosing characters in double-quotes ( "" ) shall preserve the literal value of all characters within the double-quotes, with the...

What is GNU Info for?

GNU Info was designed to offer documentation that was comprehensive, hyperlinked, and possible to output to multiple formats. Man pages were available, and they were great at providing printed output. However, they were designed such that each man page had a...

Set systemd service to execute after fstab mount

a CIFS network location is mounted via /etc/fstab to /mnt/ on boot-up. No, it is not. Get this right, and the rest falls into place naturally. The mount is handled by a (generated) systemd mount unit that will be named something like mnt-wibble.mount. You can...

Merge two video clips into one, placing them next to each other

To be honest, using the accepted answer resulted in a lot of dropped frames for me. However, using the hstack filter_complex produced perfectly fluid output: ffmpeg -i left.mp4 -i right.mp4 -filter_complex hstack output.mp4 ffmpeg -i input1.mp4 -i input2.mp4...

How portable are /dev/stdin, /dev/stdout and /dev/stderr?

It's been available on Linux back into its prehistory. It is not POSIX, although many actual shells (including AT&T ksh and bash) will simulate it if it's not present in the OS; note that this simulation only works at the shell level (i.e. redirection or...

How can I increase the number of inodes in an ext4 filesystem?

It seems that you have a lot more files than normal expectation. I don't know whether there is a solution to change the inode table size dynamically. I'm afraid that you need to back-up your data, and create new filesystem, and restore your data. To create new...

Why doesn’t cp have a progress bar like wget?

The tradition in unix tools is to display messages only if something goes wrong. I think this is both for design and practical reasons. The design is intended to make it obvious when something goes wrong: you get an error message, and it's not drowned in...