[linux-help] Re: Process persistence

[Tom Hull <thull2@xxxxxxx>]

Wayne White wrote:
> I am fooling around with some machines on a network. I am trying
> to telnet into one machine, from another, start a program and close
> the telnet connection. When I close the connection, the program
> dies.
> 
> I am trying to get the machines to run headless & without keyboards.
> To do so I need to be able to start programs remotely. I know that
> telnet isn't real secure, but this network isn't connected to the outside
> world.
> 
> So, how can I cause a program started over a telnet session to
> persist when the telnet connection is closed?

Run the program using nohup(1).

When you run a program, the process has a "controlling terminal",
and when you log out, Linux sends a signal (SIGHUP, hang-up) to
each process attached to that controlling terminal. All nohup
does is to arrange so that the program ignores the SIGHUP, and
redirects stdout/stderr to the file nohup.out, and stdin from
/dev/null. Make sure to use & to run the program as a background
job, otherwise nohup just waits on it:

   nohup PROGRAM ARGUMENTS ... &

There are other, fancier ways to do this: basically arrange for your
program to change its process group and session id, which has the
effect of changing the controlling terminal. Daemon programs are
written to do just this, because for them you want to break all
terminal-process associations (including SIGKILL, SIGSTOP, etc.).
But that only works if you write the program to do it; nohup works
for everything.

Also, if you don't want the nohup.out file left, redirect the
outputs yourself.

> Is there some other way to log into the headless machine to start
> a program? Perhaps just a console on the other machine? I wasn't
> going to mess with SSH, but would that do what I need? I could
> install it on both machines, I suppose.

You can use rsh(1), which is usually disabled due to security
considerations. If you set it up properly on a firewalled local
network, it should be ok. You can run programs on remote machines
through any network service that allows it; e.g., you can run
CGI programs through httpd (if you configure it to do so).

> Many thanks,
> 
> wayne
> 
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> 



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