When you SSH into an Ubuntu server, the welcome message (Message of the Day or MotD) is generated by several components working together:

1. /etc/motd - Static message file
2. /etc/update-motd.d/ - Dynamic message scripts
3. PAM modules that control message display

The modern Ubuntu systems use dynamic MOTD generation through scripts in /etc/update-motd.d/. Here's how to list them:

ls -la /etc/update-motd.d/
total 40
drwxr-xr-x   2 root root  4096 Jul 12  2021 .
drwxr-xr-x 142 root root 12288 Mar 15 09:30 ..
-rwxr-xr-x   1 root root   122 Aug 14  2019 00-header
-rwxr-xr-x   1 root root   982 Aug 14  2019 10-help-text
-rwxr-xr-x   1 root root  1315 Aug 14  2019 50-motd-news
-rwxr-xr-x   1 root root   154 Aug 14  2019 80-esm
-rwxr-xr-x   1 root root   704 Aug 14  2019 80-livepatch
-rwxr-xr-x   1 root root  4417 Aug 14  2019 90-updates-available
-rwxr-xr-x   1 root root   982 Aug 14  2019 98-reboot-required
-rwxr-xr-x   1 root root   526 Aug 14  2019 98-fsck-at-reboot

To modify the system load information (like removing memory usage display), edit the header script:

sudo nano /etc/update-motd.d/00-header

Find and comment out the memory-related lines:

# Fetch memory usage
#memory_used=$(free -m | awk 'NR==2{printf "%.2f%%", $3*100/$2 }')
#memory_available=$(free -m | awk 'NR==2{printf "%sMB", $2 }')

Add your own script to the MOTD directory (make sure it's executable):

sudo nano /etc/update-motd.d/99-custom
#!/bin/sh
echo ""
echo "=== Custom Server Message ==="
echo "Maintenance Window: Every Sunday 2-4 AM UTC"
echo "Contact: admin@example.com"
exit 0

sudo chmod +x /etc/update-motd.d/99-custom

If you prefer static messages only:

sudo chmod -x /etc/update-motd.d/*
sudo nano /etc/motd
Welcome to Production Server
Authorized Use Only
All activities are logged

For more sophisticated messages based on system state:

#!/bin/bash
if [ $(cat /proc/loadavg | cut -d' ' -f1 | awk '{printf "%.0f", $1}') -gt 2 ]; then
    echo "WARNING: High system load detected!"
fi

If changes don't appear:

1. Check script permissions: ls -la /etc/update-motd.d/
2. Test script execution: run-parts /etc/update-motd.d/
3. Verify PAM configuration: /etc/pam.d/sshd

---

The login message you see when SSH-ing into Ubuntu servers is controlled by the Message of the Day (MOTD) system. This consists of several components that generate different parts of the message:

/etc/update-motd.d/ - Directory containing executable scripts that generate dynamic content
/etc/motd - Static message file (deprecated in modern Ubuntu)
/run/motd.dynamic - Combined output from all MOTD scripts

The dynamic MOTD system in Ubuntu uses these important files:

/etc/default/motd-news - Controls Ubuntu news updates
/etc/legal - Contains legal notices
/etc/update-motd.d/00-header - System information header
/etc/update-motd.d/10-help-text - Help information
/etc/update-motd.d/50-motd-news - Ubuntu news
/etc/update-motd.d/90-updates-available - Package updates
/etc/update-motd.d/98-reboot-required - Reboot notices

To customize specific parts of the message, edit the corresponding script in /etc/update-motd.d/. For example, to modify the system information section:

sudo nano /etc/update-motd.d/00-header

The script uses simple bash commands to generate output. For instance, the memory usage information comes from:

printf "  Memory usage: %s" "$(free -m | awk 'NR==2 { printf "%.1f%%", $3*100/$2 }')"

You can create new scripts in /etc/update-motd.d/ following these conventions:

#!/bin/sh
# Add your custom content here
echo "Custom Server Message"
echo "Current time: $(date)"

Remember to:

1. Name files with two-digit prefixes (e.g., 99-custom)
2. Make them executable: chmod +x /etc/update-motd.d/99-custom
3. Keep them fast (they run on every login)

To disable any component, either:

sudo chmod -x /etc/update-motd.d/50-motd-news  # Remove execute permission
or
sudo mv /etc/update-motd.d/50-motd-news /etc/update-motd.d/50-motd-news.disabled  # Rename

Here's a complete example of a custom MOTD script that shows disk usage per mount point:

#!/bin/sh
[ -r /etc/lsb-release ] && . /etc/lsb-release

if [ -z "$DISTRIB_DESCRIPTION" ] && [ -x /usr/bin/lsb_release ]; then
    DISTRIB_DESCRIPTION=$(lsb_release -s -d)
fi

printf "\nDisk Usage Information:\n"
df -h | awk '
NR==1 {print}
NR>1 && $1 !~ /tmpfs/ && $1 !~ /udev/ {
    printf "%-25s %-8s %-8s %-8s %s\n", $1, $2, $3, $4, $6
}'

To test changes without logging in/out:

run-parts /etc/update-motd.d/

Or to see the complete MOTD output:

cat /run/motd.dynamic

When customizing MOTD:

• Avoid revealing sensitive system information
• Keep scripts simple to prevent security issues
• Consider performance impact on frequent logins