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I am new to Shell Scripting.

I saw this Command on a Book for Linux shell Scripting

$ kill -HUP 1234 >killout.txt 2>killerr.txt

I know what the command does, but i am not able to understand the flags -HUP

please shed some light on it.

I did a manual page search for the kill command but could not find any answers.

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man kill; apropos signal – Martin Schröder May 7 '13 at 20:57
up vote 6 down vote accepted

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.

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Note that the POSIX syntax is kill -s HUP. kill -HUP and kill -1 are Unix (XSI extension in the SUS) but not POSIX. Note that kill -s HUP and kill -1 are the same as per Unix, but the signal number for HUP is not guaranteed to be 1 (in case you expect a process killed with HUP to return an exit status of 129 or 257) – Stéphane Chazelas May 5 '13 at 20:38

-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.

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