How to fix “Error: Can’t open display: (null)” with ssh and xclip command in headless mode

When you try to use the xclip command over the ssh-based session to copy and paste data, you will get the following message on screen:
Error: Can't open display: (null)
Here is how to fix this error by editing the ssh client and OpenSSH (SSHD server) configuration file.

Error: Can’t open display: (null) when using Xclip over the ssh-based session under Linux or Unix

Tutorial details
Difficulty level	Easy
Root privileges	Yes
Requirements	Linux or Unix terminal
Category	Terminal/ssh
OS compatibility	AIX • AlmaLinux • Alpine • Amazon Linux • Arch • BSD • CentOS • Debian • Fedora • FreeBSD • HP-UX • Linux • macOS • Mint • NetBSD • OpenBSD • openSUSE • Pop!_OS • RHEL • Rocky • Stream • SUSE • Ubuntu • Unix • WSL
Est. reading time	3 minutes

The xclip is a command line interface to X selections. It works with X GUI sessions. You can copy and paste using your Linux or Unix terminal and remote/local ssh server easily. However, you need to configure a few things. Otherwise, you will see an error that read as follows:

1. On your Linux or Unix desktop client, edit the ~/.ssh/config file and add ForwardX11 yes directive. For example:
   {vivek@desktop:~}$ vim ~/.ssh/config
   Set it as follows for all or selected host:

   Host *
       ForwardX11 yes

   Save and close the file.

2. Next log into your ssh server:
   {vivek@desktop:~}$ ssh user@server-ip-here
{vivek@desktop:~}$ ssh vivek@ln.cbz-www-004
3. Edit the /etc/ssh/sshd_config file using a text editor such as Vim or nano. For instance:
   {vivek@server1:~}$ sudo vim /etc/ssh/sshd_config
   Append or set X11Forwarding to yes as follows:

   X11Forwarding yes

   Save and close the file. Restart or reload the sshd service. For example, here is how to restart ssh on Linux with systemd:
   {vivek@server1:~}$ sudo systemctl restart ssh.service

4. Log in using the ssh command and enable X11 forwarding when log into the remote server. On macOS, you may need to pass the -Y option if -X is not working out for you. For example:
   {vivek@desktop:~}$ ssh -X vivek@ln.cbz-www-004
   OR
   {vivek@MacBookPro:~}$ ssh -Y vivek@ln.cbz-www-004
5. Verify that $DISPLAY is set using the echo command or printf command:
   {vivek@server1:~}$ printf "%s\n" "$DISPLAY"
Outputs:
localhost:10.0

6. Test it using the xclip command:
   {vivek@server1:~}$ command | xclip #copy
{vivek@server1:~}$ xclip -o #paste
{vivek@server1:~}$ date | xclip
{vivek@server1:~}$ xclip -o
   
   

   Click to enlarge

Summing up

You must set proper X11 forwarding between your SSH client and server to fix the “Error: Can’t open display: (null)” error under Linux, macOS, and Unix-like systems. For more information, see the following manual pages using the man command:
$ man 5 ssh_config
$ man 5 sshd_config
$ man 1 ssh
See my following tutorial pages for more info about using the xclip command under Linux for CLI copy and pasting:

• How To Copy Command Output To Linux Clipboard Directly
• How do I copy a file to the clipboard in Linux?
• Use oathtool Linux command line for 2 step verification (2FA)

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