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I'm doing some test-runs of long-running data migration scripts, over SSH. Let's say I start running a script around 4 PM; now, 6 PM rolls around, and I'm cursing myself for not doing this all in screen.

Is there any way to "retroactively" nohup a process, or do I need to leave my computer online all night? If it's not possible to attach screen to/nohup a process I've already started, then why? Something to do with how parent/child proceses interact? (I won't accept a "no" answer that doesn't at least address the question of 'why' -- sorry ;) )

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4  
Just saw an interesting blog post about disown. blog.ksplice.com/2011/03/… –  ojrac Mar 17 '11 at 15:46
    
happy 100 votes question! :-) –  dalloliogm Nov 28 '12 at 12:05

11 Answers 11

up vote 143 down vote accepted

If you're using Bash, you can run "disown -h job"

disown

disown [-ar] [-h] [jobspec ...]

Without options, each jobspec is removed from the table of active jobs.

If the -h' option is given, the job is not removed from the table, but is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP. If jobspec is not present, and neither the -a' nor -r' option is supplied, the current job is used. If no jobspec is supplied, the -a' option means to remove or mark all jobs; the `-r' option without a jobspec argument restricts operation to running jobs.

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Awesome; I was hoping something like this would turn up. –  ojrac Jun 12 '09 at 0:13
5  
Life can be unfair. gharper and me were posting this at about the same time :) –  Server Horror Jun 12 '09 at 7:02
2  
You are my hero –  Tom Dignan Jul 9 '10 at 1:42
8  
disown is not specific to bash. It's also in zsh, ksh93, ... –  Phil P Jan 17 '11 at 10:18
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I found that you actually have to use disown %1 if 1 is the jobspec, unlike fg or bg where you just use bg 1 serverwatch.com/tutorials/article.php/3935306/… –  mltsy Apr 9 '13 at 17:21

Use reptyr

From the README:

reptyr - A tool for "re-ptying" programs.
-----------------------------------------

reptyr is a utility for taking an existing running program and
attaching it to a new terminal. Started a long-running process over
ssh, but have to leave and don't want to interrupt it? Just start a
screen, use reptyr to grab it, and then kill the ssh session and head
on home.

USAGE
-----

  reptyr PID

"reptyr PID" will grab the process with id PID and attach it to your
current terminal.

After attaching, the process will take input from and write output to
the new terminal, including ^C and ^Z. (Unfortunately, if you
background it, you will still have to run "bg" or "fg" in the old
terminal. This is likely impossible to fix in a reasonable way without
patching your shell.)

A few blog posts by its author:

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I'm going to stick with built-in tools (i.e. disown), but it's not as flexible as reptyr. +1 –  ojrac May 23 '11 at 15:19

To steal a process from one tty to your current tty, you may want to try this hack:

http://www.ucc.asn.au/~dagobah/things/grab.c

It needs some reformatting in order to compile to current Linux/glibc versions, but still works.

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Exceptionally cool. –  ojrac Jun 12 '09 at 14:35

When a process starts, STDIN, STDOUT and STDERR are connected to something. Generally you can't change that once the command is started. In the case you're describing, that's probably a tty associated with the ssh session. nohup pretty much just does ...

command < /dev/null > nohup.out 2>&1

That is, sets STDIN to /dev/null, STDOUT to a file and STDERR to STDOUT. Screen does much more sophisticated things involving setting up ttys that direct to itself.

I don't know of any way to retroactively nohup or screenize a running process. If you cd to /proc/$pid/fd and see what 0, 1 and 2 point to.

You might have some luck with disown, but not if the process tries to do anything with STDIN, STDOUT or STDERR.

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2  
+1 for the good comments. st(din|out|err) is the other half of the problem, and I appreciate the advice on where to start looking, next time I'm in this jam. –  ojrac Jun 12 '09 at 3:25
5  
You actually can change it on most Unixes. It's a disgusting hack. I love it. :) What you do is, connect to the process using debug support like ptrace, then force the process to call dup2() to reconnect 0,1,2 to another filehandle. –  Zan Lynx Jun 16 '09 at 20:07
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yeah, you can change it. Involves pausing the process (SIGSTOP), and modifying the file descriptors for fd 0, 1, 2. Then restarting (SIGCONT). –  Michael Martinez Oct 16 '13 at 0:53

Cryopid is a further development from the author of grab.c that freezes a process to a file, which you then run (inside screen) to resume the process.

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1  
Nice! I tried to use cryopid in my master's dissertation about process migration, but it failed to work all the times, no matter what I did. In the end, I had to use dynckpt with an ancient version of Linux. Are you perhaps involved in the development of cryopid? I see that your name is similar to the author's domain. –  Juliano Jun 14 '09 at 18:43
1  
I'm not involved in the development, I just know the author from university. He doesn't have the time to maintain cryopid at the moment, so it looks like some people started working on it at sharesource.org/project/cryopid –  TRS-80 Jun 14 '09 at 19:10

I can only give you a simple "No" without the why for the screen part, I'd be interested in the reason myself thou.

However have you tried disown (a bash builtin)

~ $ echo $SHELL
/bin/bash
~ $ type disown
disown is a shell builtin
~ $ help disown
disown: disown [-h] [-ar] [jobspec ...]
     By default, removes each JOBSPEC argument from the table of active jobs.
    If the -h option is given, the job is not removed from the table, but is
    marked so that SIGHUP is not sent to the job if the shell receives a
    SIGHUP.  The -a option, when JOBSPEC is not supplied, means to remove all
    jobs from the job table; the -r option means to remove only running jobs.
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nohup on Solaris/OpenSolaris has a -p flag to nohup a running process - for instance, see the Solaris 10 nohup man page.

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I recently saw a link to neercs, which is a screen-like utility built using libcaca, a colour ascii-art library. Amongst other features, it boasts the ability to grab an existing process and re-parent it inside your neercs (screen) session.

I've not used it however, so I cannot comment on whether it works or not.

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I'm probably under-thinking this so feel free to correct me (I already learned about disown!)... Wouldn't a ctrl-Z and "bg" work to at least get the process running in the background? Or is the key issue that you'd still want to see STDOUT while it runs?

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1  
That would still kill the process when the owning tty dies so the OP needs to leave the box where he intiated the command running, which is what he wants to avoid –  Server Horror Jun 11 '09 at 23:11
    
OK I'll buy that. Thanks for the explanation. So the disown -h suggestions sure look a lot smarter than mine :-) –  Chris_K Jun 11 '09 at 23:15
    
You could still do this, just after you'd have to disown -h: stackoverflow.com/a/625436/705198 –  AndrewPK Jan 26 '12 at 20:47

If you can live with not being able to interact with the process and you don't object to loading random kernel modules, you could do worse than to look at Snoop. Alternatively, there are a couple of other projects. Here is one call injcode, which can mostly do what you want to do.

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Is it that you're worried about the session timing out? In that case you could Ctrl-z and bg the process, and then just put somthing to keep the session alive like a "ping -t localhost" or "top".

If it's that you want to logout, then I'm afraid I can't add to the other comments.

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It's the logout thing. –  ojrac Jun 12 '09 at 0:10
1  
Another way to stop it disconnecting is to run top. THat's always shuffeling a few bytes around. –  Rory Aug 4 '09 at 22:55

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