Shell Script To Broadcast A Message

Broadcasting a message in a shell script allows system-wide communication with all logged-in users. This technique is useful for administrators to send notifications, warnings, or maintenance alerts. Shell scripts automate this process, making it easier to ensure that messages reach the intended users quickly.

• Messages can be sent to all logged-in users simultaneously
• Commonly used for system alerts and administrative notifications
• Requires appropriate user permissions to broadcast messages
• Uses standard Linux utilities available on most distributions
• Can be automated using shell scripts for repeated use

Example: Broadcasting a Message Using wall

Demonstrates how to send a message to all logged-in users using a shell script.

Script:

#!/bin/bash
# Broadcasting a maintenance notification to all users
wall "System maintenance will begin in 10 minutes. Please save your work."

• The wall command sends the specified message to all logged-in users’ terminals.
• The message will appear on every active terminal session of logged-in users

Output:

Syntax

wall "Your message here"

• "Your message here": The text to broadcast to all users.

Step-by-Step Guide to Broadcast a Message

Script (broadcast_message.sh):

#!/bin/bash
# Send a message to all users
wall "System maintenance will begin in 10 minutes. Please save your work."

Steps to Run

1. Create the script file:

nano broadcast_message.sh

2. Paste the script into the file and save it.

• In nano, press CTRL+O to save and CTRL+X to exit.

3. Make the script executable:

chmod +x broadcast_message.sh

4. Run the script:

./broadcast_message.sh

Message to a Specific User

The write command allows sending a message to a specific user’s terminal. This is useful for direct communication without notifying all users.

Syntax:

write username [tty]

• username: Name of the target user.
• tty: Terminal session of the user (optional).

Example: Sending a Message to a Specific User

Script:

#!/bin/bash
# Send a message to the user "captain-levi" on their terminal
echo "Hello, Captain Levi ! System maintenance starts in 10 minutes." | write captain-levi

• echo: Generates the message.
• write: Sends the message directly to the specified user (captain-levi).

Output:

Broadcasting a Message to All Users

The wall command broadcasts a message to all logged-in users. This method is useful for system-wide alerts and notifications.

Example: Broadcasting a System Alert

Script:

#!/bin/bash
# Send a system update message to all users
wall "Scheduled system reboot at midnight. Please save your work."

• wall: sends the specified message to every active terminal session.

Output:

Broadcasting a Message to a User Group

Messages can also be sent to a specific group of users using the -g option of the wall command. This is useful for team-specific notifications.

Syntax:

wall -g groupname "Your message here"

• groupname: Name of the target user group.
• "Your message here": Message to broadcast.

Example: Broadcasting a Message to a Specific User Group

Script:

#!/bin/bash
# Send a message to all users in the "developers" group
wall -g developers "Reminder: Daily build process starts in 5 minutes."

• wall -g developers: sends the message to all members of the developers group.
• The message appears on the active terminals of all group members.

Output:

Important Note (Distribution Limitation):

• On some Linux distributions (including certain Ubuntu/Debian systems), the -g option may not correctly restrict the message to the specified group.
• In such cases, the message may be broadcast to all logged-in users, even those not in the target group.
• If this happens, use the reliable method below.

Manually Target Group Members

This method ensures the message is sent only to logged-in users who belong to the specified group.

Script:

#!/bin/bash
# Send a message to all users in the developers group

for user in $(getent group developers | awk -F: '{print $4}' | tr ',' ' '); do
    # send message to all terminals of that user
    for tty in $(who | awk -v u="$user" '$1==u {print $2}'); do
        echo "Reminder: Daily build process starts in 5 minutes." | write "$user" "$tty"
    done
done

• getent group retrieves members of the group.
• who checks which users are currently logged in.
• write sends the message only to active terminals of those users.

Output: