Destroy and Undefine KVM VMs in a single run#

virsh is a goto console utility for managing Qemu/KVM virtual machines. But when it comes to deletion of the VMs you better keep calm - there is no single command to destroy the VM, its definition XML file and disk image.

Probably not a big problem if you have a long-living VMs, but if you in a testing environment it is naturally to spawn and kill VMs quite often. Lets see how xargs can help us with that routine.

Right now my KVM hypervisor is busy with virtualizing small Nuage VNS environment where 4010_DEMO* VMs are virtual SDN gateways:

[root@srv601 ~]# virsh list
 Id    Name                           State
----------------------------------------------------
 1     1-vsc1.pod60.cats              running
 2     1-vsc2.pod60.cats              running
 3     g1pe.play.cats                 running
 4     jenkins                        running
 5     dns                            running
 6     1-es1.pod60.cats               running
 7     1-vsd1.pod60.cats              running
 8     1-util1.pod60.cats             running
 18    4010_DEMO_I2                   running
 19    4010_DEMO_I1                   running
 22    4010_DEMO2_NSGI1               running
 23    4010_DEMO2_NSGI2               running
 24    4010_DEMO_1upl_testNSGI1       running
 25    4010_DEMO_1upl_testNSGI2       running 

And I really need to get these last six boys gone for good. With virsh one need to perform the following set of commands to achieve the goal:

# removing the last VM with a virsh domain name 4010_DEMO_1upl_testNSGI2
# first destroy the domain (you even cant pass multiple names)
$ virsh destroy 4010_DEMO_1upl_testNSGI2
Domain 4010_DEMO_1upl_testNSGI2 destroyed

# now undefine the domain
$ virsh undefine 4010_DEMO_1upl_testNSGI2
Domain 4010_DEMO_1upl_testNSGI2 has been undefined

# deleting the disk images
# assuming that all the VM-related data resides in that dir
$ rm -rf /var/lib/libvirt/images/4010_DEMO_1upl_testNSGI2

Too much typing for a simple task... Lets see how xargs comes into play!

grep and xargs#

What we need to do is to filter out the target domain names and pass these names to the virsh destroy && virsh undefine && rm -rf commands.

First things first, lets get the names of the domains. grep is the tool of choice.

# grep flags:
#   -o -- return only the matched group (not the whole line with match highlighted)
#   -E -- regular expression
virsh list --all | grep -o -E "(4010_DEMO\w*)"

# OUTPUT:
4010_DEMO_I2
4010_DEMO_I1
4010_DEMO2_NSGI1
4010_DEMO2_NSGI2
4010_DEMO_1upl_testNSGI1

Bingo, now its xargs time:

xargs reads items from the standard input, delimited by blanks or newlines, and executes the command (default is /bin/echo) one or more times with any initial-arguments followed by items read from standard input. Blank lines on the standard input are ignored.

virsh list --all | grep -o -E "(4010_DEMO\w*)" | \
xargs -I % sh -c 'virsh destroy % && virsh undefine % && rm -rf /var/lib/libvirt/images/%;'

The xargs flag -I % here allows us to substitute each % sign in the command with the xargs input argument. This effectively destroys the virsh domain along with its definition and disk image.

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