I recently upgraded the debian version from 8 to 9 and my surprise was to not being able to connect to the newly upgraded server with my usual ssh key.
It turns out that beyond OpenSSH version 6.6, the "unsafe algorithms" have been removed, including my kinda old dss key.

All you have to do is to create a new one. I choose ecdsa: $ ssh-keygen -t ecdsa
Do not forget to put the newly pub key (e.g. the content of ~/.ssh/id_ecdsa.pub) in ~/.ssh/authorized_keys of the upgraded server.

Well, that was kinda short. Enjoy! :)

How to remove the contents from a File:

Note: this does not removes the file. It only clears the content, which is very useful if you want to clear a log on which an application writes continuously. This will ensure the filesystem's space is cleared (especially when cleaning huge logs) and the log is still accessible by the application.
PS C:\> Clear-Content file

The error I got today was related to speedtest-cli:

florians-MacBook-Pro$ speedtest
Traceback (most recent call last):
  File "/opt/local/bin/speedtest", line 6, in <module>
    from pkg_resources import load_entry_point
ModuleNotFoundError: No module named 'pkg_resources'

The macports package speedtest-cli is a terminal port for the popular speedtest.net. The source is available here.

Found this error when attempting to clean all packages:

I had this problem the other day: my macbook pro was not sleeping when closing the lid.
I was 100% sure I have not modified any power settings nor installed new applications so I had to dig up a bit more.
Reading few websites about similar problems got me to a tool which helped me find the source of this problem.

The command pmset -g was showing me several applications that could prevent my system from going to sleep (line in red).

The OS X releases come only via App Store as an app package but sometimes there is need for a way to transform this into an iso image.
So without further writing, I present you two ways to do so.

1. Download the installer app from App Store or if not available there, use gibMacOS.

It is easy to use, via terminal of course. You clone it, run it and do your thing.

For several month I had this problem on one of my osx servers: the computer name was keep changing by adding a number at the end. Today was mmd (1), tomorrow mmd (2) and so on.
I have tried several things but nothing was successful until I finally gave up and looked up over the internet.
The solution was found here (link at the end of this article), among other methods. The one below was the most useful to me.

Php is able to read files and pass that information into a website. There are, of course, many ways to do this and below is just a tiny example.

Let's say you have in a text file, several lines in this format:

user@gzlinux $ cat files/servers-status.txt
server1#apache#10#up
server2#apache#20#down
server10#nfs#67#up
gzaix2#varnish#4#down
etc69#nginx#2#up

And you want to present this information in a php page with these tables: Hostname, Application, Number of Instances, Status.

Here is what your php page will need:

No matter the monitorization solution, CPU utilization is important but if you really want to know what is happening with your system, you should look at load average. I will explain below why.

The current CPU utilization does not reflect the actual load of the system because when a host is heavily loaded, its CPU usage doesn't have to be necessary close to or at 100%.
Furthermore, the CPU utilization tends to generate (in monitoring tools) a significant number of false alerts, even if the percentage alert is close to 100%. Explanation follows.

There are many tutorials over the web regarding this problem but unfortunately it is not always the same for everybody.

In my case I have tried all the links below unsuccessfully (for you it might work):

https://support.apple.com/en-us/HT201349
http://www.idownloadblog.com/2014/05/14/how-to-remove-a-phone-number-from-imessage/
http://www.igeeksblog.com/fix-imessage-waiting-for-activation/

also you can try more:
https://duckduckgo.com/?q=imessage+waiting+for+activation&t=osx