msgexec

msgexec KEY TYPE SUBTYPE TEXT [COMMAND [ARGUMENTS...]]

msgexec is a utility for inter-process communication (IPC) within the Linux kernel, specifically utilizing System V message queues. It serves a dual purpose: first, to send a message to a pre-existing message queue, and second, to conditionally execute another command. The command accepts a numerical KEY to identify the target message queue, a TYPE and SUBTYPE for categorizing the message, and TEXT as the message content. A unique aspect of msgexec is its ability to execute an optional COMMAND (with its ARGUMENTS) only if the message is successfully sent to the queue. This makes it a useful tool for signaling between processes or for orchestrating tasks where a process should proceed only after successfully communicating a specific event or data via a message queue.

Permissions: The user executing msgexec must have write permissions to the specified System V message queue.

Queue Existence: The message queue identified by KEY must already exist. If it does not, msgexec will fail.

System V IPC Limits: Operations may be subject to system-wide limits on message queue size, total message bytes, and the number of queues. Failures can occur if these limits are exceeded.

Error Handling: The optional COMMAND is only executed if the message is successfully sent. If an error occurs during message sending (e.g., queue full, invalid key, permission denied), msgexec will exit with a non-zero status, and the COMMAND will not be run.

msgexec exits with status 0 upon successful message sending and, if provided, successful execution of the COMMAND. A non-zero exit status indicates an error during message sending or if the COMMAND itself failed to execute (if it was launched).

The message sent by msgexec consists of four parts: a numerical KEY for the queue identifier, a numerical TYPE and SUBTYPE for categorizing the message, and a TEXT string as the actual message payload. This structured format allows receiving processes to filter and interpret messages based on these fields.

msgexec is part of the util-linux project, a collection of essential Linux utilities. It has been a long-standing component for managing and interacting with System V IPC facilities. Its design reflects a straightforward approach to sending structured messages and conditionally executing subsequent commands, fitting into the traditional Unix philosophy of small, specialized tools.

ipcs(1), ipcrm(1), ipcmk(1), msgsnd(2), msgrcv(2)