Parameter Expansion in Bash

It has been so long since I have scripted (back then I used Korn shell), that when I started looking at some advanced Bash, and started seeing some very weird syntax, I had to go research it.

It turns out that these "weird things", are a feature in Bash called Parameter Expansion.

http://wiki.bash-hackers.org/syntax/pe

Bash is "Borne Again Shell".

The Borne shell was the original shell, and then Korn shell became the shell of choice, replacing Borne in many cases, in the 90s.

Now, Bash has apparently re-taken Korn shell with these kinds of funky features.

They don't make Unix any easier to look at when you see this stuff. Although, power and flexibility is the tradeoff. Apparently. More power, less keystrokes.

Configuring KDump on CentOS 7

It used to be, back in my day, that a core file would just appear when the kernel decided to dump core.

Apparently that is not the case anymore. Now, you need to configure KDump, I have learned.

Here's an article on this:

https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Kernel_Crash_Dump_Guide/sect-kdump-config-cli.html

We had to do this recently because of an application that kept causing a kernel panic. We gave the dumps to developers who figured out the problem was related to an ioctl call being made in software.

UPDATE:
Here's another, perhaps better, article:
http://www.thegeekstuff.com/2014/05/kdump/

This one shows how to use the crash utility, as well as induce a system crash by sending signal 11 to a process.

Starting Gnome Automatically with SystemD

I used to start x by just "starting X" - meaning, running startx on the terminal, or putting a script in rc.local or init.d.

But now that SystemD is in play, here is how you do this:

systemctl set-default graphical.target

This is handy if you need to spin up VMs with minimal installs and then add a desktop manager later on.  I had to do this because I needed to access a KVM host with virt-manager. Yes, there's a way to do EVERYTHING on command line (or with CLI). But I like the GUI for this.

NOTE: To install Gnome on a box that did not even have X on it, I used:
yum groupinstall "GNOME-desktop"

VirtualBox Tips and Tricks - Installing Guest Additions

https://wiki.centos.org/HowTos/Virtualization/VirtualBox/CentOSguest

The trick here is in finding Devices.  Devices is actually in the VM window - not the host manager window! If you don't know this, you may find yourself tripping through all of the host menus and submenus trying to find the "Install Guest Additions" prompt.

Obviously you need to have the ISO mounted.

So again:

Copy Guest Additions iso to the VM
Go to the VM window, choose "Optical Drive" and mount that ISO.
Then go to Devices, Insert Guest Manager CD Image
Lastly, choose Install Guest Additions (once CD is loaded) - also from Devices on the VM.

Installing a KVM Host on CentOS 7

I am using another reference to do this, which can be found below.

http://www.server-world.info/en/note?os=CentOS_7&p=kvm

Ok. After I followed these instructions, I had to locate another site:

http://www.itzgeek.com/how-tos/linux/centos-how-tos/install-kvm-qemu-on-centos-7-rhel-7.html


So basically, step 1 is to install the necessary packages. Using BOTH of these sites, I came up with the following list:

• qemu-kvm =  QEMU emulator
• qemu-img = QEMU disk image manager
• virt-install =  Command line tool to create virtual machines.
• libvirt = Provides libvirtd daemon that manages virtual machines and controls hypervisor.
• libvirt-install = Provides libvirtd daemon that manages virtual machines and controls hypervisor.
• bridge-utils = creating bridges, et al.
• libvirt-client  = provides client side API’s for accessing servers and also provides virsh utility which provides command line tool to manage virtual machines.
• virt-viewer – Graphical console

So I installed these using yum -y (assume yes) ...packages...

Next, step 2, is to create a bridge that will be used to allow the VMs inside the KVM to reach the NIC on the box to communicate with outside networks.

Actually, because I already have a bridge interface (br0), I am going to try to use that rather than creating a bridge.

Next, I need to connect to the KVM host. I don't have X on this machine, so I will need to connect from another CentOS 7 box that does have X installed. On that box, which has X and Gnome Desktop, I installed libvirt, libvirt-client, and virt-viewer.

But - I ran into a problem connecting.

After debugging, I realized the KVM host does not accept inbound SSH connections as root. So I tried to connect as a different user that is in the wheel group, and this did not work either.

I think the answer to that is in this blog here, which involves policy set up.

http://wiki.libvirt.org/page/SSHPolicyKitSetup

This in fact did not work either, so I instead did this:

https://goldmann.pl/blog/2012/12/03/configuring-polkit-in-fedora-18-to-access-virt-manager/

polkit.addRule(function(action, subject) {
  if (action.id == "org.libvirt.unix.manage" && subject.local && subject.active && subject.isInGroup("wheel")) {
      return polkit.Result.YES;
  }
});

But I took the subject.local out, because I was connecting from a remote machine.

next, on the other CentOS 7 device I was connecting to the KVM host from (which is also a CentOS 7 VM on Virtual Box), we ran the following commands so that we wouldn't get repeatedly authenticated.

ssh-keygen (accept defaults)
ssh-copy-id [ kvm host address ]

UPDATE:
I finally got a server with 32 Gb RAM to use as a KVM host. I decided NOT to put XWindows on this host, and use it, truly, just as a host. I set up a virt-manager on another VirtualBox Linux VM, and figured I would use that to interact with the host.

I had a devil of a time getting the darn VMs to install.

Finally, I realized that the secret, was to pass in some extra-args on the command line. I realized this on a web discussion, which I would have liked to have posted here, but unfortunately I could not relocate that discussion. Also, I had to change cdrom to "location". Once I did this, I was able to see standard console output on the install.

#!/usr/bin/bash

virt-install \
-n CentOS7-KVM1 \
--description "KVM1 CentOS7" \
--os-type=Linux \
--os-variant=centOS7.0 \
--ram=2048 \
--vcpus=2 \
--disk path=/home/kvm/images/vDisk0.img,bus=virtio,size=10 \
--graphics none \
--location /var/tmp/CentOS-7-x86_64-Minimal-1511.iso \
--network bridge:virbr0 \
--extra-args "console=tty0 console=ttyS0,115200n8 serial"

Percona and MariaDB

Downloaded PerconaDB today.

Apparently it is re-packaged MariaDB, which in turn is re-packaged MySQL.

Why Percona? For Clustering.

More later.

CentOS7 Minimal Install - No ifconfig or earlier ip commands

On CentOS 7, they deprecate the older ip utilities in favor if iproute2.

There's a blog that discusses how to install the older utilities.

yum install net-tools

The blog also has some other information:

https://www.unixmen.com/ifconfig-command-found-centos-7-minimal-installation-quick-tip-fix/

TeamViewer

Someone today was discussing how TeamViewer has an uncanny ability to circumvent firewalls.

Does it punch out? Or other trick(s) are employed?

Need to look more into this.