The kill builtin command

Synopsis

kill [-s SIGNAL | -n SIGNALNUMBER | -SIGNAL] PID|JOB

kill -l|-L [SIGNAL...]

The kill command is used to send signals to processes specified by their PID or their JOB-specification.

The signal(s) to be specified can have the following formats:

Numerical: The signal is specified using its constant numeric value. Be aware that not all systems have identical numbers for the signals.
Symbolic (long): The signal is specified using the same name that is used for the constant/macro in the C API (SIG<name>)
Symbolic (short): The signal is specified using the name from the C API without the SIG-prefix (<name>)

Without any specified signal, the command sends the SIGTERM-signal.

The kill command is a Bash builtin command instead of relying on the external kill command of the operating system to

be able to use shell job specifications instead of Unix process IDs
be able to send signals ("kill something") also, when your process limit is reached

Option	Description
`-s SIGNAL`	specifies the signal to send
`-n SIGNALNUMBER`	specifies the signal to send
`-SIGNAL`	specifies the signal to send
`-l [SIGNAL…]`	Lists supported/known signal numbers and their symbolic name. If `SIGNAL` is given, only list this signal, translated (if a number is given the symbolic name is printed, and vice versa)
`-L [SIGNAL…]`	Same as `-l [SIGNAL]` (compatiblity option)

Status	Reason
0	no error/success
!=0	invalid option
!=0	invalid signal specification
!=0	error returned by the system function (e.g. insufficient permissions to send to a specific process)

kill -l

kill -9 12345

kill -KILL 12345

kill -SIGKILL 12345

POSIX(R) and ISO C only standardize symbolic signal names (no numbers) and a default action