Decode Flags for Kill Command

Under Linux try man 7 signal.

kill -HUP 1234 means "send the SIGHUP signal (1) to process 1234", so it's equivalent to kill -1 1234. The default signal that is sent by kill is SIGTERM (15), so kill 1234 is equivalent to kill -TERM 1234 or kill -15 1234.

-HUP isn't the three flags H, U, P as in the common single-letter option syntax. For historical reasons, the kill command takes an optional signal name or signal number preceded by a dash (-).

kill -1 1234
kill -HUP 1234

(As opposed to kill 1 1234, which would send the default signal (SIGTERM) to processes 1 and 1234.)

Your man page probably lists something like this:

kill [ -signal | -s signal ] pid ...

That -signal means that you can use a dash followed by a signal designation (which can be a name or number). Alternatively, you can use the -s option followed by a signal name; these are two different syntaxes with the same meaning.

kill -l lists the available signal names and the corresponding numbers.

Most signals have a predefined meaning based on who sends them when and how processes are supposed and able to react to them.