Process Signals

View list of process signals

# MacOS
kill -l
# HUP INT QUIT ILL TRAP ABRT EMT FPE KILL BUS SEGV SYS PIPE ALRM TERM URG STOP TSTP CONT CHLD TTIN TTOU IO XCPU XFSZ VTALRM PROF WINCH INFO USR1 USR2
 
# Ubuntu
kill -l
 1) SIGHUP	 2) SIGINT	 3) SIGQUIT	 4) SIGILL	 5) SIGTRAP
 6) SIGABRT	 7) SIGBUS	 8) SIGFPE	 9) SIGKILL	10) SIGUSR1
11) SIGSEGV	12) SIGUSR2	13) SIGPIPE	14) SIGALRM	15) SIGTERM
16) SIGSTKFLT	17) SIGCHLD	18) SIGCONT	19) SIGSTOP	20) SIGTSTP
21) SIGTTIN	22) SIGTTOU	23) SIGURG	24) SIGXCPU	25) SIGXFSZ
26) SIGVTALRM	27) SIGPROF	28) SIGWINCH	29) SIGIO	30) SIGPWR
31) SIGSYS	34) SIGRTMIN	35) SIGRTMIN+1	36) SIGRTMIN+2	37) SIGRTMIN+3
38) SIGRTMIN+4	39) SIGRTMIN+5	40) SIGRTMIN+6	41) SIGRTMIN+7	42) SIGRTMIN+8
43) SIGRTMIN+9	44) SIGRTMIN+10	45) SIGRTMIN+11	46) SIGRTMIN+12	47) SIGRTMIN+13
48) SIGRTMIN+14	49) SIGRTMIN+15	50) SIGRTMAX-14	51) SIGRTMAX-13	52) SIGRTMAX-12
53) SIGRTMAX-11	54) SIGRTMAX-10	55) SIGRTMAX-9	56) SIGRTMAX-8	57) SIGRTMAX-7
58) SIGRTMAX-6	59) SIGRTMAX-5	60) SIGRTMAX-4	61) SIGRTMAX-3	62) SIGRTMAX-2
63) SIGRTMAX-1	64) SIGRTMAX

Interesting Signals

Number	Symbol	Action	Meaning	Common Usage
1	SIGHUP	exit	Hangup	closing terminal session or Reload config
2	SIGINT	exit	Interrupt Process	Ctrl + C to stop process
3	SIGQUIT	core dump	Quit + dump info	Ctrl + \
8	SIGFPE	core dump	Arithmetic Exception	Divide by zero, Invalid math
9	SIGKILL	exit	Force kill process; signal is never actually sent to the target program rather kernel terminates the process	kill -9 <pid>, cannot be caught, blocked, or ignored
11	SIGSEGV	core dump	Segmentation Fault	Invalid memory access
13	SIGPIPE	exit	Broken Pipe	Writing to a closed pipe
14	SIGALRM	exit	Timer expired	Timeouts
15	SIGTERM	exit	Graceful Termination	kill <pid> (default signal)
18	SIGCONT	continue	Continue if stopped	fg / bg commands; resumes a SIGSTOP/SIGTSTP process
19	SIGSTOP	stop	Pause process; keeps process in memory (resume with SIGCONT), unlike SIGKILL which terminates	Cannot be caught, blocked, or ignored (unlike SIGTSTP)
20	SIGTSTP	stop	Terminal Stop; polite pause request — process can catch it to clean up before suspending	Ctrl + Z; resumed by SIGCONT (process can also handle SIGCONT for reinit)

Sending signals to process

• kill command can be used to send a signal to a process

man kill
 
# sending SIGKILL
 
# by number
kill -9 <pid>
 
# by short name
kill -KILL <pid>
 
# by full name
kill -SIGKILL <pid>

• killall can be used to send signals to multiple processes

# kill all processes with name xlogo
killall xlogo

Terminal Signals

• Terminal shortcuts can be used to send signals to process
• https://unix.stackexchange.com/questions/362559/list-of-terminal-generated-signals-eg-ctrl-c-sigint
• Ctrl + C —> SIGINT
• Ctrl + \ —> SIGQUIT
• Ctrl + Z —> SIGTSTP
• View terminal shortcuts to send various signals

# stty: set the options for a terminal device interface
# display all current settings
stty -a

When you cannot kill a process

• https://superuser.com/questions/243460/what-to-do-when-ctrl-c-cant-kill-a-process