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Process signals as administration


 
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# 1  
Old 11-01-2009
Process signals as administration

Too generic to post elsewhere, too advanced for the newbie forums. There are some applications within the unix/linux milieu that understand signals such as SIGHUP, etc as instructions to perform administrative tasks (clearing information out of this, disconnect users, etc.) I was just wondering if there existed any general reference for the different applications that behave that way or if anyone just happens to know of applications that do.


TIA.
# 2  
Old 11-01-2009
it is application specific. if the application does not handle the signal the default action will be taken. in most cases this is terminating the process.
# 3  
Old 11-01-2009
Every application implements it on its own.
It is common to perform one of the following operations when a HUP is received:
- restart the application
- reload configuration
- quit gracefully
- messy quit
- to be unaware about the quit signal and die in an unpredicted way

In the shell scripts you have trap. Perl offers similar things (some of them are BEGIN and END blocks).
It is quite common to use nohup to run an application so that it won't die after disconnection from the console (I don't like that idea by the way): nohup my_app &. Sometime nohup tee my_app 1>log.log 2>log.log &. Crazy.
You have several signals. At least 1 of them cannot be handled by application (the app is not aware that it is killed). Other signals are defined in the source of your OS (if provided) or header files. If I remember it correctly then for linux you got signals.h or sys_signals.h... Try checking there.
HUP means HangUp - so that in the past user was connected using a slow-unreliable modem and was disconnected (caused by own choice or connection problems). This is HUP.

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