On multi-user Unix/Linux systems, which command broadcasts a message to all users currently logged in (for example, for maintenance notices)?

Introduction / Context:
System administrators often need to alert every active user about impending maintenance, shutdowns, or policy changes. Unix-like systems provide terminal-messaging utilities that can target one user, a group, or everyone with an interactive session.

Given Data / Assumptions:

• Multiple users are logged in to terminals or pseudo-terminals.
• You need immediate, real-time notification.
• Permissions allow writing to user terminals.

Concept / Approach:
The wall (write all) command sends a message to all logged-in users by writing to their terminals. By contrast, write targets a single user (for example, write alice), mail delivers e-mail and is not instantaneous, and mesg controls whether a user's terminal can receive messages (mesg y/n). For urgent broadcast communication, wall is the canonical choice.

Step-by-Step Solution:

Verification / Alternative check:
Run who or w to list logged-in users and their TTYs. After sending with wall, observe that all TTYs display the same message, confirming system-wide broadcast.

Why Other Options Are Wrong:

• write: One-to-one messaging; not a broadcast.
• mail: Asynchronous e-mail; not immediate terminal broadcast.
• mesg: Toggles permission to receive writes; not a messaging command itself.
• None of the above: Incorrect because wall accomplishes the broadcast.

Common Pitfalls:
Assuming wall overrides a user's mesg n setting or forgetting sudo/root may be required to reach all terminals on some systems.

Final Answer:
wall