What is correct or suggested approach to send the signal name through 'kill' command?

Question

I read many tutorials about the use of kill command, mostly 3 approaches

kill -15 <pid>
kill -SIGTERM <pid>
kill -TERM <pid>

For scripts purposes and for portability with macos too, the code numbers are not going to be used. Because kill -l in macos is different than Linux. So here enter in play the signal names.

Question

• What is the correct or suggested approach to send the signal name through the kill command?

I mean:

• SIGsomething or something?

And why? These 2 approaches exists for one reason, right? is there a mandatory reason to use an approach over the other?

Environment

This situation is for Ubuntu Desktop/Server and Fedora Workstation/Server

Answer 1

The most portable variant is

kill -s TERM -- …

That’s the form specified in POSIX with no extensions: -s followed by the signal’s name, as defined in signal.h, with no SIG prefix, or 0 for the “null” signal (used to check for the existence of a process with a given identifier).

SIGTERM is the default signal, so sending that to processes or process groups can be done using the following canonical forms:

kill <pid>
kill -- -<pgid>

On Linux in general, most implementations of kill (including shell builtins) accept signals as numbers or names with or without a SIG prefix; notable exceptions include the kill builtin of dash, which is the default /bin/sh in Debian-based distributions, and of the Schily Bourne Shell.

There’s no “mandatory” reason to use one form rather than another, among whatever forms are supported by the tools you use. I would personally avoid numeric forms because they can appear to be ambiguous: is kill -9 -15 intended to send SIGTERM to process groups 9 and 15, or SIGKILL to process group 15? (It’s the latter, but some readers may wonder.) I would also omit the SIG prefix since that’s not recognised everywhere.

Note that POSIX does specify a few numeric signal values:

Number	Signal
0	“Null” signal
1	HUP
2	INT
3	QUIT
6	ABRT
9	KILL
14	ALRM
15	TERM