Getting The Maximum Of A Set Of Numbers

 Computing, Linux, Working Notes  No Responses »
Dec 102015
 

damascus-unix-prompt

You have a a column of numbers that you have produced in some manner such as :-

$ awk '/clean message/ {print $(NF-1)}' mail.info.log
...
100935
12197
3606
84653
4498
99110
4762
3001
10889
12611
12249
12245
136599
49097
6668

And you want a quick and dirty way of finding the largest number. Well there is a way but it is perhaps the least efficient way to do it, and that is to sort the numbers into numerical order and use “head” to display the first one :-

$ awk '/clean message/ {print $(NF-1)}' mail.info.log | sort -rn | head -1
5476168

But frankly there must be a better method. And yes there is if you happen to be using zsh (or possibly others, but this has been tested with zsh). Simply iterate over the values assigning the current value to the “max” variable if the current variable is larger :-

$ max=0; for x in $(awk '/clean message/ {print $(NF-1)}' mail.info.log); [[ $x -gt $max ]] && max=$x; echo $max
5476168

You may be wondering why I don’t simply use the ability of awk to perform calculations. Well that is certainly possible, but I may not always be using awk to produce the numbers in the first place, and this is supposed to be a generic recipe.

Linux Containers: IPv6 Behaving Non-Deterministically?

 Computing, Linux, Working Notes  No Responses »
Nov 142015
 

See an updated post here.

I am obviously doing something wrong because computers are not supposed to behave like this, but my Linux containers (despite previous attempts) are booting with IPv6 privacy addresses randomly :-

✓ root@pica» lxc-ls --fancy | grep chagers
chagers   RUNNING  10.0.0.32  2001:8b0:ca2c:dead::5e11, 2001:8b0:ca2c:dead:f42b:6dff:fe16:2f2d  YES        
✓ root@pica» lxc-stop --name chagers; lxc-start --daemon --name chagers
✓ root@pica» lxc-ls --fancy | grep chagers
chagers   RUNNING  10.0.0.32  2001:8b0:ca2c:dead:206b:70ff:fe45:7242, 2001:8b0:ca2c:dead::5e11  YES        
✓ root@pica» lxc-stop --name chagers; lxc-start --daemon --name chagers
✓ root@pica» lxc-ls --fancy | grep chagers
chagers   RUNNING  10.0.0.32  2001:8b0:ca2c:dead::5e11                                         YES        

That is not how computers are supposed to behave!damascus-unix-prompt

Linux Containers: Don’t Configure The Container Network

 Computing, Linux, Working Notes  1 Response »
Oct 032015
 

More up to date information can be found here.

One thing that has always puzzled me about Linux Containers was why it is necessary to configure the network address in two places – the container configuration, and the operating system configuration. The short answer is that it isn’t.

If you configure network addresses statically within the container configuration :-

» grep net /var/lib/lxc/mango/config 
# networking
lxc.network.type = veth
lxc.network.flags = up
lxc.network.link = br0
lxc.network.ipv4 = 10.0.0.35/16
lxc.network.ipv4.gateway = 10.0.0.1
lxc.network.ipv6 =         2001:0db8:ca2c:dead:0000:0000:0000:000a/64
lxc.network.ipv6.gateway = 2001:0db8:ca2c:dead:0000:0000:0000:0001

Then the configuration within the container’s operating system can simply be :-

» cat /var/lib/lxc/mango/rootfs/etc/network/interfaces
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual
iface eth0 inet6 manual

And that works fine.

Linux Containers: Disabling The Autoconfigured IPv6 Address

 Computing, Linux, Working Notes  No Responses »
Oct 032015
 

A newer post has more information (and more reliable information).

One of the things that has been mildly irritating me about my little collection of Linux containers has been that in addition to the statically defined IPv6 addresses, there is also an automatically defined IPv6 address :-

» lxc-ls --fancy
NAME      STATE    IPV4       IPV6                                                              AUTOSTART  
---------------------------------------------------------------------------------------------------------
apricot   RUNNING  10.0.0.34  2001:db8:ca2c:dead:21e:a0ff:feb6:6a, 2001:db8:ca2c:dead::3eb      YES        
chagers   RUNNING  10.0.0.32  2001:db8:ca2c:dead:804a:bfff:fe83:f98d, 2001:db8:ca2c:dead::5e11  YES        
glanders  RUNNING  10.0.0.31  2001:db8:ca2c:dead:21e:a0ff:feb6:66, 2001:db8:ca2c:dead::ba11     YES        
lyme      RUNNING  10.0.0.30  2001:db8:ca2c:dead:21e:a0ff:feb6:65, 2001:db8:ca2c:dead::cafe     YES        
mango     RUNNING  10.0.0.35  2001:db8:ca2c:dead:6c42:24ff:fe7d:4e9, 2001:db8:ca2c:dead::a      YES        
peach     RUNNING  10.0.0.33  2001:db8:ca2c:dead:21e:a0ff:feb6:68, 2001:db8:ca2c:dead::3a11     YES        
rhubarb   RUNNING  10.0.0.40  2001:db8:ca2c:dead:21e:a0ff:feb6:69, 2001:db8:ca2c:dead::dead     YES

Now this is hardly the end of the world, but it is not tidy and it is the sort of thing that may lead to problems down the road if servers are communicating on an address that is not reverse DNS registered. Or indeed when someone contacts a server on an address such as 2001:db8:ca2c:dead::3eb and the reply comes from 2001:db8:ca2c:dead:21e:a0ff:feb6:6a.

After any number of false starts, the answer is quite simple – use sysctl to turn off autoconfigured address from within the container; which doesn’t make much sense logically – containers don’t have a kernel of their own, so the global kernel should be the one that is tuned. However :-

for container in $(lxc-ls)
do
  echo net.ipv6.conf.eth0.autoconf = 0 >> /var/lib/lxc/$container/rootfs/etc/sysctl.conf
done

Does the trick (after a reboot)  :-

» lxc-ls --fancy
NAME      STATE    IPV4       IPV6                                                              AUTOSTART  
---------------------------------------------------------------------------------------------------------
apricot   RUNNING  10.0.0.34  2001:db8:ca2c:dead:21e:a0ff:feb6:6a, 2001:db8:ca2c:dead::3eb      YES        
chagers   RUNNING  10.0.0.32  2001:db8:ca2c:dead:18d9:99ff:fe28:3591, 2001:db8:ca2c:dead::5e11  YES        
glanders  RUNNING  10.0.0.31  2001:db8:ca2c:dead:21e:a0ff:feb6:66, 2001:db8:ca2c:dead::ba11     YES        
lyme      RUNNING  10.0.0.30  2001:db8:ca2c:dead::cafe                                          YES        
mango     RUNNING  10.0.0.35  2001:db8:ca2c:dead:2411:80ff:feb9:6600, 2001:db8:ca2c:dead::a     YES        
peach     RUNNING  10.0.0.33  2001:db8:ca2c:dead::3a11                                          YES        
rhubarb   RUNNING  10.0.0.40  2001:db8:ca2c:dead::dead                                          YES        

Except for the older containers 🙁

I’ve obviously missed something, but fixing nearly half of the containers is a good start.

After attending to pending upgrades (some of my old containers were still running wheezy), and setting the network configuration to manual, one of the recalictrant containers (glanders) lost it’s autoconfigured address.

Two more containers lost their unwanted extra addresses after “fixing” their configuration. I’m not sure what was wrong with the old configuration, but after copying and modifying a recently created container configuration, they rebooted with just one IPv6 address. The last one was mango, but after an extra reboot, it also was fixed :-

» lxc-ls --fancy
NAME      STATE    IPV4       IPV6                      AUTOSTART  
-----------------------------------------------------------------
apricot   RUNNING  10.0.0.34  2001:db8:ca2c:dead::3eb   YES        
chagers   RUNNING  10.0.0.32  2001:db8:ca2c:dead::5e11  YES        
glanders  RUNNING  10.0.0.31  2001:db8:ca2c:dead::ba11  YES        
lyme      RUNNING  10.0.0.30  2001:db8:ca2c:dead::cafe  YES        
mango     RUNNING  10.0.0.35  2001:db8:ca2c:dead::a     YES        
peach     RUNNING  10.0.0.33  2001:db8:ca2c:dead::3a11  YES        
rhubarb   RUNNING  10.0.0.40  2001:db8:ca2c:dead::dead  YES        

Linux: Missing Mouse Pointer? Try This

 Computing, Working Notes  No Responses »
May 222015
 

So on my upgrade from Wheezy to Jessie, I found myself (amongst other issues) looking at a graphical interface where the mouse worked fine, but no mouse pointer was visible. After trying a few other things, it turned out that :-

gsettings set org.gnome.settings-daemon.plugins.cursor active false

Did the trick.

Of course that tip came from somewhere else, but as it worked for me, it’s worth making a note of.

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