[Lxc-users] Executing a command inside a running container?

[Michael H. Warfield]

On Sun, 2010-01-31 at 13:38 -0600, Tony Risinger wrote: 
> On Sat, Jan 30, 2010 at 12:11 PM, Michael H. Warfield <mhw at wittsend.com> wrote:
> > On Sat, 2010-01-30 at 14:20 +0100, Dominik Schulz wrote:
> >> Hi,
> >>
> >> I'm fairly new to LXC and I am looking for a way to execute a command inside a
> >> running container (a full blow one with its own rootfs and full isolation).
> >> lxc-execute doesn't seem to do the trick and lxc-console requires credential
> >> to login. I'm looking for a way to execute command w/o having to login via ssh
> >> or the lxc-console, so I can execute command directly inside the containers to
> >> shut them down properly.
> >
> > This is being worked on and looked at now through the use of some sort
> > of daemon in the running container but is not currently possible.  More
> > over, cgroups in the kernel does not currently support joining an active
> > cgroup, and may never support it, so it would have to be done through
> > the mediation of some sort of daemon in the running container and
> > communicating with the host.  There's been discussion on the -devel list
> > over the optimal way to do it and there are some patches running around
> > but AFAICT nothing has really been committed or committed to at this
> > time.

> if your using the standard init program, and you are only trying to
> control stutdown/reboot, i use something like this in my container
> inittab:

> p6::ctrlaltdel:/sbin/init 6
> p0::powerfail:/sbin/init 0

> ctrlaltdel responds to a SIGINT, and powerfail responds to SIGPWR.
> this lets you send a:

> kill -INT <init pid>

> to reboot, and:

> kill -PWR <init pid>

Ah that's a nice trick.  I'll have to incorporate that into my scripts.

> to shutdown.  you will also need the help of a "monitor" process to
> overcome the fact that the init process will hang around, even though
> all other processes in the container are dead.  an earlier thread
> detailed a nice trick using inotifywait and the utmp file in the
> container, see this thread for ideas:

> http://www.mail-archive.com/lxc-users@lists.sourceforge.net/msg00040.html

> specifically this post on:

> http://www.mail-archive.com/lxc-users@lists.sourceforge.net/msg00049.html

> you basically monitor the utmp file in the container for a change,
> then run "runlevel <utmp file>" to get the runlevel in the container,
> and act appropriately.  here is an excerpt from my scripts (originally
> based off one provided in the previous thread), it has variables
> specific to my setup but it's fairly self explanatory:
> 
> while true; do
>         # time of 5 minutes on it JUST IN CASE...
>         vps_utmp=${VPS_DOM}/${vps}/rootfs/var/run/utmp
>         inotifywait -qqt 300 ${vps_utmp}
>         if [ $(wc -l < ${VPS_VAR}/cgroup/${vps}/tasks) -eq 1 ]; then
> 
>             runlevel="$(runlevel ${vps_utmp})"
> 
>             case $runlevel in
>             N*)
>                 # nothing for new boot state
>             ;;
>             ??0)
>                 # halted...  kill vps.
>                 lxc-stop -n "${vps}"
>                 break
>             ;;
>             ??6)
>                 # rebooting...  kill vps and start again...
>                 lxc-stop -n ${vps}
>                 lxc-wait -n ${vps} -s STOPPED
>                 lxc-start -d -n ${vps} -o ${VPS_LOG}/${vps}.log
>                 # loop again.
>             ;;
>             *)
>                 # make sure vps is still running
>                 state="$(lxc-info -n "${vps}" | sed -e 's/.* is //')"
>                 [ "$state" = "RUNNING" ] || break
>             ;;
>             esac
>         fi
>     done

Yeah, that's some of my stuff.  I've since rolled that into an lxc-run
script that does some initialization and then forks off a subprocess
that calls lxc_start as a daemon and then drops into a variation of that
loop.  I'll probably be posting that script in the next day or so.  I'm
almost done cleaning up some of the OpenVZ migration hacks that are
incorporated into it.

Mike
-- 
Michael H. Warfield (AI4NB) | (770) 985-6132 |  mhw at WittsEnd.com
   /\/\|=mhw=|\/\/          | (678) 463-0932 |  http://www.wittsend.com/mhw/
   NIC whois: MHW9          | An optimist believes we live in the best of all
 PGP Key: 0x674627FF        | possible worlds.  A pessimist is sure of it!
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