Well-behaved processes ultimately complete their work and
terminate with an
exit( ) system call.
Sometimes, however, it is necessary to terminate a process prematurely,
perhaps because it was started in error, requires more resources than
you care to spend, or is misbehaving.
The kill command does
the job, but it is misnamed. What it really does is send a
signal to a specified running process, and with
two exceptions noted later, signals can be caught by the process and
dealt with: it might simply choose to ignore them. Only the owner of a
root, or the kernel, or
the process itself, can send a signal to it. A process that receives a
signal cannot tell where it came from.
ISO Standard C defines only a half-dozen signal types. POSIX adds a couple of dozen others, and most systems add more, offering 30 to 50 different ones. You can list them like this example on an SGI IRIX system:
kill -lList supported signal names (option lowercase L) HUP INT QUIT ILL TRAP ABRT EMT FPE KILL BUS SEGV SYS PIPE ALRM TERM USR1 USR2 CHLD PWR WINCH URG POLL STOP TSTP CONT TTIN TTOU VTALRM PROF XCPU XFSZ UME RTMIN RTMIN+1 RTMIN+2 RTMIN+3 RTMAX-3 RTMAX-2 RTMAX-1 RTMAX
Most are rather specialized, but we've already used a few of the more common ones in trap commands in shell scripts elsewhere in this book.
Each program that handles signals is free to make its own interpretation of them. Signal names reflect conventions, not requirements, so there is some variation ...