Linux Background Process – Will It Continue to Run After Logout?

backgroundbashlinuxprocess

Asking this after a prolonged discussion with a coworker, I'd really like a clarification here.

I launch a background process, either by appending "&" to the command line or by stopping it with CTRL-Z and resuming it in background with "bg". Then I log out.

What happens?

We were quite sure it should have been killed by a SIGHUP, but this didn't happen; upon logging in again, the process was happily running and pstree showed it was "adopted" by init.

Is this the expected behaviour?

But then, if it is, what's the nohup command's purpose? It just looks like the process isn't going to be killed anyway, with or without it…

Edit 1

Some more details:

  • The command was launched from a SSH session, not from the physical console.
  • The command was launched without nohup and/or &; it was then suspended with CTRL-Z and resumed in background with bg.
  • The ssh session did not drop. There was an actual logout ("exit" command).
  • The process was a scp file copy operation.
  • Upon logging in again, pstree showed the process running and being child of init.

Edit 2

To state the question more clearly: will putting a process in background (using & or bg) make it ignore SIGHUP, just like the nohup command does?

Edit 3

I tried manually sending a SIGHUP to scp: it exited, so it definitely doesn't ignore the signal.

Then I tried again launching it, putting it in the background and logging off: it got "adopted" by init and kept running, and I found it there when logging back on.

I'm quite puzzled now. Looks like no SIGHUP was sent at all upong logging off.

Best Answer

Answer found.

For BASH, this depends on the huponexit shell option, which can be viewed and/or set using the built-in shopt command.

Looks like this options is off by default, at least on RedHat-based systems.

More info on the BASH man page:

The shell exits by default upon receipt of a SIGHUP. Before exiting, an interactive shell resends the SIGHUP to all jobs, running or stopped. Stopped jobs are sent SIGCONT to ensure that they receive the SIGHUP. To prevent the shell from sending the signal to a particular job, it should be removed from the jobs table with the disown builtin (see SHELL BUILTIN COMMANDS below) or marked to not receive SIGHUP using disown -h.

If the huponexit shell option has been set with shopt, bash sends a SIGHUP to all jobs when an interactive login shell exits.

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