Can I nohup/screen an already-started process?

249

123

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 ;) )

ojrac

Posted 2009-06-11T22:53:44.700

Reputation: 1 348

4

Just saw an interesting blog post about disown. https://blogs.oracle.com/ksplice/entry/disown_zombie_children_and_the

– ojrac – 2011-03-17T15:46:39.590

Answers

201

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.

gharper

Posted 2009-06-11T22:53:44.700

Reputation: 4 576

4You are my hero – Thomas Dignan – 2010-07-09T01:42:28.220

Awesome; I was hoping something like this would turn up. – ojrac – 2009-06-12T00:13:24.640

7Life can be unfair. gharper and me were posting this at about the same time :) – serverhorror – 2009-06-12T07:02:20.833

Sorry for revoking the "answer" cred, but damn... a good one came up. I don't feel too bad, though; you're already at +12 from this ;) – ojrac – 2009-06-12T14:36:49.467

Totally agree - I'm going to have to play with that grab.c program a bit. Looks very cool! – gharper – 2009-06-12T17:03:48.447

9disown is not specific to bash. It's also in zsh, ksh93, ... – Phil P – 2011-01-17T10:18:54.243

thanks @gharper it works like a treat. I've got a process that was already running i estimate will take over 5 hours to complete and I can't be here to babysit it. – Matt Setter – 2012-04-18T15:36:49.547

Is the jobspec the same as the process pid? – Shoan – 2012-05-01T15:23:11.447

No, jobspec is different from proc id... there's a good overview of bash job control here: http://www.linuxselfhelp.com/gnu/bash/html_chapter/bashref_6.html

– gharper – 2012-05-01T16:14:22.910

2

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 http://www.serverwatch.com/tutorials/article.php/3935306/Detach-Processes-With-Disown-and-Nohup.htm

– mltsy – 2013-04-09T17:21:48.970

I tried this, but the program will die the first time it writes to stdout. You need to use reptyr - see Jonathan Tran's answer – Rich – 2013-05-23T16:34:08.867

@Rich, so http://stackoverflow.com/a/625436/819417 won't work?

– Cees Timmerman – 2015-06-17T15:16:57.937

@CeesTimmerman no, in the case where you start a process (without using screen) and then want to log out of SSH, disown won't cut it, and reptyr is needed. I tested that on RHEL in 2013; perhaps things are different now (seems unlikely though). – Rich – 2015-06-17T16:25:42.277

77

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:

Jonathan Tran

Posted 2009-06-11T22:53:44.700

Reputation: 891

I'm going to stick with built-in tools (i.e. disown), but it's not as flexible as reptyr. +1 – ojrac – 2011-05-23T15:19:36.813

22

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.

Juliano

Posted 2009-06-11T22:53:44.700

Reputation: 4 732

1Exceptionally cool. – ojrac – 2009-06-12T14:35:40.123

16

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.

freiheit

Posted 2009-06-11T22:53:44.700

Reputation: 12 419

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 – 2009-06-12T03:25:21.943

6You 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 – 2009-06-16T20:07:17.377

2yeah, you can change it. Involves pausing the process (SIGSTOP), and modifying the file descriptors for fd 0, 1, 2. Then restarting (SIGCONT). – Michael Martinez – 2013-10-16T00:53:29.580

12

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.

TRS-80

Posted 2009-06-11T22:53:44.700

Reputation: 2 429

1Nice! 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 – 2009-06-14T18:43:50.927

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 http://sharesource.org/project/cryopid/

– TRS-80 – 2009-06-14T19:10:09.827

11

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.

serverhorror

Posted 2009-06-11T22:53:44.700

Reputation: 5 833

9

nohup on Solaris/OpenSolaris has a -p flag to nohup a running process - for instance, see the Solaris 10 nohup man page.

alanc

Posted 2009-06-11T22:53:44.700

Reputation: 1 430

5

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.

Daniel Lawson

Posted 2009-06-11T22:53:44.700

Reputation: 4 950

2

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?

Chris_K

Posted 2009-06-11T22:53:44.700

Reputation: 2 704

1That 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 – serverhorror – 2009-06-11T23:11:47.940

OK I'll buy that. Thanks for the explanation. So the disown -h suggestions sure look a lot smarter than mine :-) – Chris_K – 2009-06-11T23:15:35.093

1

You could still do this, just after you'd have to disown -h: http://stackoverflow.com/a/625436/705198

– AndrewPK – 2012-01-26T20:47:48.753

2

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.

David Pashley

Posted 2009-06-11T22:53:44.700

Reputation: 20 392

1

I wanted to use nohup (or similar) to start the links command-line browser and to attach to it after to download a file from ASP.NET website with complicated authentication process and a lot of hidden view state that made it difficult to use do the job with curl/wget.

I finally ended up using tmux which solved the job just great:

  1. Run tmux
  2. Run your app (links in my case) and leave it running
  3. Close SSH session, the app will remain running
  4. Connect with SSH to the same machine later and run tmux attach to bring the app back to screen

Dmitry Gusev

Posted 2009-06-11T22:53:44.700

Reputation: 111

tmux is great -- but, like nohup or screen, it only works if you use it before starting your process. This question is about the times when you realize you need tmux after your process is already running. – ojrac – 2017-08-18T16:57:48.417

-1

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.

TrojanName

Posted 2009-06-11T22:53:44.700

Reputation: 135

It's the logout thing. – ojrac – 2009-06-12T00:10:00.787

1Another way to stop it disconnecting is to run top. THat's always shuffeling a few bytes around. – Rory – 2009-08-04T22:55:28.487