Message of the Day

Different ways to implement the 'Message of the Day' in Unix machines

#unix

A “Message of the Day” is a message that shows up any time a user logs in to a Unix machine.

This can be useful if you SSH into a variety of machines throughout the day, and want to leave yourself a note with special information about the machine.

There are actually a ton of different ways to implement this pattern in Unix machines. This blog post will cover some of the ones I know about, as well as the pros and cons of each approach.

/etc/motd

Unix has a built-in handler for outputting the contents of the /etc/motd file. You can confirm this utility is installed by running man motd.

Since this is located in /etc, it is only accessible by root. This is a good place to put a message that you want to display to all users:

Terminal window
sudo echo "Welcome to my machine!" > /etc/motd

Support for this file is included in most Unix/Linux distros without any additional config, which is a plus. But this approach is not by-user specific: the same message will be displayed to all users. The next approach will be more useful if you want to display per-user messages.

Shell profile

While not technically a “Message of the Day”, you can also put a message in your shell’s profile. This is a file that is executed when you start a new shell.

Terminal window
$ echo "Welcome to my machine!" > ~/.profile

You could even replicate the functionality of /etc/motd further by creating ~/.motd and then adding the following to your ~/.profile:

~/.profile
if [ -f ~/.motd ]; then
  cat ~/.motd
fi

This approach is by-user, so it’s useful if you are logging into different users on the same machine. If you switch shells or overwrite your profile file, you will lose this, so you need to keep track of it. Although it’s manual, it’s not super heavy-handed.

SSH banner

An SSH banner is a file that is displayed when you SSH into a machine. You can configure this globally, or per-user.

We’ll focus on per-user. To set this up, you need to create a file called ~/.ssh/banner and add the following to it:

~/.ssh/banner
Welcome to my machine!

In your global SSH config, you can then match per user and set a banner:

/etc/ssh/sshd_config
Match User ubuntu
  Banner /home/ubuntu/.ssh/banner

Finally, restart your SSH server to apply the changes:

Terminal window
sudo systemctl restart sshd

This will display the contents of the ~/.ssh/banner file when you SSH into the machine.

Terminal window
$ ssh ubuntu@my-machine
Welcome to my machine!