NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/comp/unix/internal/1812

Internet Message Format | 1992-09-15 | 2.3 KB

Path: sparky!uunet!dove!cme!libes
From: [e-mail] (Don Libes)
Newsgroups: comp.unix.internals
Subject: Re: How can a Unix process put itself in background?
Message-ID: <[e-mail]>
Date: 15 Sep 92 18:37:44 GMT
References: <[e-mail]> <[e-mail]>
Organization: National Institute of Standards and Technology
Lines: 37

In article <[e-mail]> [e-mail] (Larry Augustin) writes:
}[e-mail] (Larry Augustin) writes:
}>A user invokes a program at the shell, and interacts with the program
}>for a short time.  Eventually the user selects some "quit" option of
}>the program.  Before the program really quits, it wants to go off and
}>spend a minute or two checking some database files on disk.  Is there
}>any way for the program to return control to the user's shell while it
}>does the background stuff?  In effect, the program wants to "detach"
}>itself from stdin/stdout, return control to the shell, and continue in
}>background for a while before exiting.
}
}>The only solutions we could come up with were based on forking a child
}>to do the background part.  is this the only way to do it?  Is there a
}>solution that does not need a fork?

}The consensus seems to be that it can't be done (without forking), as
}whatever shell exec'd the program will be stuck in a wait(), and could
}only continue upon death of the child (or some interaction by the
}user).  There would have to be some other convention for the running
}program to tell the shell to continue.

Here's a bit of Expect to do this.  It works as follows - when you are
ready to detach the program, you type "~detach".  Expect puts itself
into the background, tells the program to do the "longrunningcommand",
and when done, tells it to exit gracefully.  Obviously Expect is doing
the fork for you.  Nonetheless this is the easiest way to solve the
whole problem without hacking up the original program. - Don

spawn prog                      ;# start program
interact ~detach {
        if [fork] exit          ;# parent exits
        disconnect
        send "longrunningcommand\r"
        expect $prompt          ;# wait for prog to say it is ready to quit
        send "quit\r"           ;# tell program to quit
        expect eof
}