I'm trying to write a mail program which might contain data that a user wouldn't want to lose upon a broken SSH/telnet connection. Would SIGHUP be sent to the process? After all, it's the "hangup" signal...
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It depends - see for example [In which cases is SIGHUP not sent to a job when you log out?](84737).– Thomas DickeyDec 10, 2015 at 14:23
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@Thomas, ITYM In which cases is SIGHUP not sent to a job when you log out?. (yes, I agree it's a real pain to link to other questions in comments).– Stéphane ChazelasDec 10, 2015 at 14:33
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Depends if a pty is used. So with telnet yes, with ssh, only for interactive sessions or if passed -t/-tt– Stéphane ChazelasDec 10, 2015 at 14:35
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odd - it looked okay after I saved the edit.– Thomas DickeyDec 10, 2015 at 14:37
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@Thomas, just found SE Comment Link Helper, testing just now (edit: it works!)– Stéphane ChazelasDec 10, 2015 at 14:51
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1 Answer
By default the process will be sent a SIGHUP. The default signal handler (trap) will shutdown the program relatively gracefully, but won't save the users work.
In your case, I would have the program include a trap routine that saves the user's work when a SIGHUP is received. You can catch and handle all signals except SIGKILL. You may also want to run the same routine for a SIGTERM interupt.
There are other mechanisms that can be used to avoid getting a SIGHUP signal, but handling the signal is the simplest and safest. If the other approachs fail, or a SIGHUP is sent with the kill command your process would still shutdown without saving the work.
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ssh host 'cmd',cmdwon't get a SIGHUP if the connection drops.cmdcould get a SIGPIPE if it tries to write something on stdout/stderr after the connection has dropped (with openssh, there are issues with that when using connection sharing). Dec 10, 2015 at 14:55