[ababab]

Shouldn't that explanation be by followed by 'in order to request the termination of the process'?

[habosa]

Not always. The kill system call can be used to send any signal, it's just got a name that implies you're sending something like SIGKILL or SIGTERM. I have written C programs that use kill for harmless inter-process communication.

[eru]

SIGUSER is quite common for that. Or SIGHUP.

[brokenparser]

By convention, SIGHUP is used by daemons to signal a change of configuration.

[beedogs]

Not necessarily. You can send the CONTinue signal to a process using the kill command. This will not cause the process to terminate.

[twinge]

Or use "kill -0 <pid>" to check if a signal can be sent/the pid exists.

[rahimnathwani]

Or use kill -USR1 <pid> to check the progress of dd

[DHowett]

Only if you've enabled it. If you've not, the process will terminate unceremoniously (in a mechanism borne entirely out of hatred for users.)

This is my favourite part of dd: The "will I receive a status report or will I terminate my long-running copy?" gamble.

[_ea1k]

Is that a BSD thing? I've never had to enable it on Linux to avoid killing the process.

[rahimnathwani]

Me neither, on debian.

[Freaky]

It's SIGINFO on the BSD's, and works with just about any process (though obviously not everything has a customized handler). Also handily mapped to Ctrl-T.

[brokenparser]

No because only SIGQUIT, SIGABRT, SIGKILL, SIGTERM and often SIGHUP are supposed to do that. All of the other signals have wildly varying meanings. See man 7 signal: http://unixhelp.ed.ac.uk/CGI/man-cgi?signal+7

[nailer]

You can also cancel (sigint, eg ctrl c) processes via kill.

Or use sigchld to a parent to get rid of zombie children.

[drivers99]

SIGHUP is also used to tell some certain daemons to re-read their configuration files without quitting.