Reference

Unix Signal Numbers

Signals are software interrupts that the kernel or other processes send to notify a program of an event. Signal numbers shown are for x86-64 Linux; some values differ on other architectures.

POSIX Standard Signals

#	Name	Default Action	Description
1	SIGHUP	Terminate	Hangup — terminal disconnected or controlling process ended. Daemons often reload config on SIGHUP.
2	SIGINT	Terminate	Interrupt — sent by Ctrl+C. Graceful shutdown request.
3	SIGQUIT	Core dump	Quit — sent by Ctrl+\. Like SIGINT but produces a core dump.
4	SIGILL	Core dump	Illegal instruction — CPU encountered an invalid opcode.
5	SIGTRAP	Core dump	Trace/breakpoint trap — used by debuggers.
6	SIGABRT	Core dump	Abort — sent by abort(). Indicates a fatal internal error.
8	SIGFPE	Core dump	Floating-point exception — division by zero or overflow.
9	SIGKILL	Terminate	Kill — cannot be caught or ignored. Immediate process termination.
10	SIGUSR1	Terminate	User-defined signal 1 — application-specific (e.g. log rotation).
11	SIGSEGV	Core dump	Segmentation fault — invalid memory access.
12	SIGUSR2	Terminate	User-defined signal 2 — application-specific.
13	SIGPIPE	Terminate	Broken pipe — write to a pipe with no reader.
14	SIGALRM	Terminate	Alarm clock — timer set by alarm() expired.
15	SIGTERM	Terminate	Termination — default signal sent by kill command. Graceful shutdown.
17	SIGCHLD	Ignore	Child stopped or terminated — sent to parent process.
18	SIGCONT	Continue	Continue — resume a stopped process (sent by fg or kill -CONT).
19	SIGSTOP	Stop	Stop — cannot be caught or ignored. Suspends the process.
20	SIGTSTP	Stop	Terminal stop — sent by Ctrl+Z. Can be caught unlike SIGSTOP.
21	SIGTTIN	Stop	Background read — background process tried to read from terminal.
22	SIGTTOU	Stop	Background write — background process tried to write to terminal.

Linux-specific Signals

#	Name	Default Action	Description
7	SIGBUS	Core dump	Bus error — unaligned memory access or non-existent physical address.
16	SIGSTKFLT	Terminate	Stack fault on coprocessor (unused on modern Linux).
23	SIGURG	Ignore	Urgent data on socket — out-of-band data arrived.
24	SIGXCPU	Core dump	CPU time limit exceeded (setrlimit).
25	SIGXFSZ	Core dump	File size limit exceeded (setrlimit).
26	SIGVTALRM	Terminate	Virtual timer expired (ITIMER_VIRTUAL).
27	SIGPROF	Terminate	Profiling timer expired (ITIMER_PROF).
28	SIGWINCH	Ignore	Window resize — terminal window size changed.
29	SIGIO	Terminate	I/O now possible — async I/O event (SIGPOLL).
30	SIGPWR	Terminate	Power failure — UPS indicating imminent shutdown.
31	SIGSYS	Core dump	Bad system call — invalid syscall number (seccomp violations).
34-64	SIGRTMIN to SIGRTMAX	Terminate	Real-time signals — queued, priority-ordered, application-defined.

Common Use Cases

Command	Signal	Purpose
kill -15 <pid>	SIGTERM	Graceful shutdown — let the process clean up
kill -9 <pid>	SIGKILL	Force kill — when SIGTERM is ignored
kill -1 <pid>	SIGHUP	Reload config — nginx, Apache, sshd
kill -USR1 <pid>	SIGUSR1	Reopen log files — used by log rotation
kill -STOP <pid>	SIGSTOP	Pause a process
kill -CONT <pid>	SIGCONT	Resume a paused process
Ctrl+C	SIGINT	Interrupt foreground process
Ctrl+Z	SIGTSTP	Suspend foreground process (use fg to resume)
Ctrl+\	SIGQUIT	Quit with core dump for debugging

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