View Source: SIGHUP - Waikato Linux Users Group

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!!! Signal: Hangup

This signal is generated by the [Kernel] when your controlling terminal goes away to the terminals ProcessGroup. Or, in simplier terms, when you close the Xterm, or hang up a modem.  Since daemons run in the background and don't have a controlling terminal, they often use [SIGHUP] to signal that they should reread their configuration files.  This can cause issues with some programs that work as both a daemon and an interactive program, such as fetchmail(1).

An example of a daemon that rereads its configuration file on [SIGHUP] is init(8), the first process created (which is responsible for creating all other processes, like getty for logging in). If you edit inittab(5), its configuration file, you can do

   <verbatim>
   kill -HUP 1
   </verbatim>

and it will re-read the config file (note that the correct way to do this is to use [telinit(8)]).

To restart an inetd(8) service, you find its [PID] and send a [SIGHUP] to it:

   <verbatim>
   kill -HUP $inetd_pid
   </verbatim>

You can prevent a process from recieving a [SIGHUP] signal by using the command nohup(1), for example:

   <verbatim>
   nohup wget http://www.example.com/ &
   </verbatim>

will run wget(1) detached from the terminal, so it won't recieve a [SIGHUP] when you disconnect. This is useful if a download is so large that you want to log out instead of waiting until it is complete. (The <tt>&</tt> is [Shell] lingo to put the command into the background so that you can do something else, such as, uh, logging out.)

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Last edited on Saturday, July 23, 2005 1:23:41 pm by AristotlePagaltzis

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