Signals

TERM targets group G: A exits using its default disposition while B ignores TERM and survives. C is outside the group and is not targeted. These are the article fixture outcomes.

Process groups select signal recipients, not a family tree

A process group is a kernel-maintained set of processes identified by a process group ID, or PGID. It provides a target for operations such as delivering a signal to a job. Parent-child relationships and group membership are separate: related processes can occupy different groups, and a group’s leader does not need to remain alive for other members to continue belonging to it. ...

An explicit normal exit and INT or TERM handlers that explicitly exit converge on a shared EXIT cleanup step. A red KILL route ends at a stop bar before cleanup.

Shell traps turn a signal into a cleanup step

A trap lets a shell arrange an action for a signal or a shell event. It is useful for removing temporary files and coordinating shutdown, but it does not make interruption atomic or guarantee cleanup under every termination. In Bash, trap is a builtin, and EXIT is a shell event rather than an operating system signal. A reliable design names the exits it handles and preserves evidence of why execution stopped. ...

With signals blocked first, the fixture’s five SIGUSR1 sends coalesce into one pending instance, while three SIGRTMIN sends remain three queued instances. Real-time queue resources remain finite.

A signal has a disposition, a mask, and delivery rules

Signals notify processes or threads of events using kernel-defined delivery rules. A signal’s disposition selects a default action, an installed handler, or ignoring it where permitted. A thread’s mask can block delivery of eligible signals while they remain pending. Separating disposition, masking, and pending state is essential when a program appears to miss repeated notifications or fails to stop as expected. ...