Signals¶

Synopsis¶

kill [-s SIGNAL | -SIGNAL] PID…
kill -l [NUMBER | EXIT_STATUS]
pkill [-SIGNAL] [OPTIONS] PATTERN
killall [-s SIGNAL] NAME…
systemctl kill [--signal=SIGNAL] [--kill-whom=main|control|all] UNIT…

Description¶

A signal is an asynchronous notification sent to a process or thread, identified by a number and a name such as SIGTERM. Each signal has a default action that applies unless the process has changed its disposition: ignore the signal, or catch it with a handler. SIGKILL and SIGSTOP can't be caught, blocked or ignored.

Signal numbers are architecture-specific. This page lists the values on x86-64 (and ARM and most other architectures); Alpha, SPARC, MIPS and PA-RISC number several signals differently. Commands and scripts that name signals (kill -TERM, kill -s HUP) rather than numbering them work on every architecture.

The signal list was checked with Bash's kill -l and kill -L from procps-ng (Debian 13, Ubuntu 26.04, Gentoo) and util-linux (AlmaLinux 10, Rocky Linux 10, Arch Linux) on x86-64, and the default actions against signal(7) from man-pages 6.9.1 on Debian 13 (November 2026). The background is explained in Processes and signals.

Default actions¶

Action	Effect
Term	Terminate the process
Core	Terminate the process and write a core dump, if core dumps are enabled (`ulimit -c`, `kernel.core_pattern`)
Ign	Ignore the signal
Stop	Stop (suspend) the process
Cont	Continue the process if it is stopped

Standard signals¶

Number	Name	Default action	Typical source and use
1	`SIGHUP`	Term	Terminal hang-up, or end of the controlling process. Many daemons reload their configuration or reopen log files on `SIGHUP`
2	`SIGINT`	Term	`Ctrl+C` on a terminal
3	`SIGQUIT`	Core	`Ctrl+\` on a terminal; quit with a core dump
4	`SIGILL`	Core	Illegal CPU instruction
5	`SIGTRAP`	Core	Breakpoint or trace trap, used by debuggers
6	`SIGABRT`	Core	`abort(3)`, typically after a failed assertion; `SIGIOT` is a synonym
7	`SIGBUS`	Core	Bus error: invalid memory access, such as touching a mapped file beyond its end
8	`SIGFPE`	Core	Arithmetic error, such as integer division by zero
9	`SIGKILL`	Term	Unconditional termination; can't be caught, blocked or ignored
10	`SIGUSR1`	Term	No predefined meaning; defined by each program
11	`SIGSEGV`	Core	Invalid memory reference (segmentation fault)
12	`SIGUSR2`	Term	No predefined meaning; defined by each program
13	`SIGPIPE`	Term	Write to a pipe or socket with no reader
14	`SIGALRM`	Term	Timer set with `alarm(2)` or `setitimer(2)` expired
15	`SIGTERM`	Term	Request to terminate; the default signal of `kill`, `pkill` and `killall`, and the first signal service managers send to stop a service
16	`SIGSTKFLT`	Term	Coprocessor stack fault; unused on x86-64
17	`SIGCHLD`	Ign	A child process stopped, continued or terminated; `SIGCLD` is a synonym
18	`SIGCONT`	Cont	Continue a stopped process; sent by `fg` and `bg`
19	`SIGSTOP`	Stop	Unconditional stop; can't be caught, blocked or ignored
20	`SIGTSTP`	Stop	`Ctrl+Z` on a terminal
21	`SIGTTIN`	Stop	A background process read from its terminal
22	`SIGTTOU`	Stop	A background process wrote to its terminal while `stty tostop` is set, or changed the terminal's settings
23	`SIGURG`	Ign	Urgent (out-of-band) data on a socket
24	`SIGXCPU`	Core	CPU time limit (`ulimit -t`, `RLIMIT_CPU`) exceeded
25	`SIGXFSZ`	Core	File size limit (`ulimit -f`, `RLIMIT_FSIZE`) exceeded
26	`SIGVTALRM`	Term	Virtual timer (`setitimer(2)`, `ITIMER_VIRTUAL`) expired
27	`SIGPROF`	Term	Profiling timer expired
28	`SIGWINCH`	Ign	Terminal window size changed
29	`SIGIO`	Term	I/O possible on a file descriptor set up for it; `SIGPOLL` is a synonym
30	`SIGPWR`	Term	Power failure; sent by UPS software, handled by some init systems
31	`SIGSYS`	Core	Bad system call, or a system call blocked by a `seccomp` filter

Numbers 32 and 33 are real-time signals that the GNU C library reserves for its threads implementation, which is why kill -l doesn't list them.

Real-time signals¶

Numbers	Names	Default action
34 to 64	`SIGRTMIN` to `SIGRTMAX`, also written `SIGRTMIN+n` and `SIGRTMAX-n`	Term

Real-time signals have no predefined meaning. Unlike standard signals, multiple instances of the same real-time signal are queued, are delivered in order, and can carry a value (sigqueue(3)). systemd uses several of them as commands to PID 1, listed in systemd(1).

Exit status after a signal¶

A shell reports a command killed by a signal as an exit status of 128 plus the signal number.

Signal	Exit status
`SIGHUP` (1)	129
`SIGINT` (2)	130
`SIGQUIT` (3)	131
`SIGABRT` (6)	134
`SIGKILL` (9)	137
`SIGSEGV` (11)	139
`SIGPIPE` (13)	141
`SIGTERM` (15)	143

Bash's kill -l converts an exit status back into a signal name:

$ kill -l 137
KILL

Commands¶

Command	Package	Selects processes by	Notes
`kill` (Bash built-in)	bash	PID, or job (`%1`)	Used when you type `kill` in Bash; `kill -l` and `kill -L` list signals
`/usr/bin/kill`	procps-ng (Debian, Ubuntu, Gentoo), util-linux (RHEL, AlmaLinux, Rocky Linux, Arch Linux)	PID; the util-linux version also accepts process names	`-L` prints a table of numbers and names
`pkill`, `pgrep`	procps-ng	Name pattern, user (`-u`), exact name (`-x`), full command line (`-f`), parent (`-P`)	`pgrep` lists the PIDs `pkill` would signal
`killall`	psmisc	Exact process name	psmisc is installed in all six lab cloud images; a minimal installation may lack it
`systemctl kill`	systemd	Unit	Signals the unit's main process or every process in its cgroup (`--kill-whom=`)
`rc-service <SERVICE> stop`	OpenRC	Service	Sends the service's stop signal following its retry schedule (default `SIGTERM/5`: `SIGTERM`, then wait up to 5 seconds)

A PID of -1 sends the signal to every process the sender may signal, and a negative PID (kill -TERM -- -<PGID>) sends it to every process in that process group. Signal 0 checks whether a process exists and can be signalled, without sending anything.

Examples¶

kill -TERM 1234                 # ask process 1234 to terminate
kill -HUP "$(pgrep -o -x nginx)"  # reload a daemon that handles SIGHUP
pkill -u alice -x sleep         # terminate alice's processes named exactly "sleep"
kill -0 1234 && echo running    # test whether process 1234 exists

Files¶

Path	Purpose
`/proc/<PID>/status`	`SigPnd`, `ShdPnd` (pending), `SigBlk` (blocked), `SigIgn` (ignored) and `SigCgt` (caught) signal masks, as hexadecimal bit masks where bit n-1 is signal n
`/proc/sys/kernel/core_pattern`	Where and how core dumps for Core signals are written