Before the advent of a distributed domain name system; networked computers used local files to map hostnames to IP addresses. On Unix systems this file was named /etc/hosts or “the hosts file”. In those days, networks were small and managing a file with a handful of hosts was easy. However as the networks grew so did the methods of mapping hostnames and IP addresses.
In modern days with the internet totaling at somewhere around 246 million domain names (as of 2012) the hosts file has been replaced with a more scalable distributed DNS service.
Recently while I was scouring through a spreadsheet of “unallocated” IP addresses, I thought to myself. There has to be a better way to manage an inventory of IP addresses, and while I'm at it maybe even provide a list of provisioned servers.
After some googling I found myself on RackTables.org the home of an open source project that aims to provide just what I was looking for. An Inventory system that tracks and manages Servers, Racks, IP Addresses and just about everything else in a data center.
In a previous article I covered a little bit about Symlinks and Hardlinks but I never really explained what they are or how to create them. Today I am going to cover how to create both Symlinks and Hardlinks and what the difference is between the two.
What are Symlinks and Hardlinks Hard Links In Linux when you perform an listing in a directory the listing is actually is a list of references that map to an inode.
Recently I coveredhow to increase and decrease the CPU priority of processes using nice and renice. Today I am going to cover how to change the default niceness value for a user or group.
Why change the default CPU priority value? Before explaining how to change the default niceness value, let's cover why this could be useful.
Scenario #1 You have a system that has thousands of users that log in via SSH and could potentially run CPU intensive tasks.
A few weeks back I wrote an article Getting started with SaltStack; that article covered Configuration and Package Automation with Saltstack. In Today's article I am going to cover SaltStack's Remote Execution abilities, a feature that I feel Saltstack has implemented better than other automation tools.
Running a command in a State If you remember from the previous article SaltStack's states are permanent configurations. Adding a command in a Salt state is used when you want to have a command that is run after provisioning a server, run every time Salt manages the state of the system or run when certain conditions are true.
Recently I was working on an issue where an application was not retaining the umask setting set in the root users profile or /etc/profile. After looking into the issue a bit it seemed that the application in question only applied the umask setting that was set in /etc/bashrc and would not even accept the values being the applications own start scripts.
After doing a bit of researched I learned a little bit more about what exactly these files do, the differences between them and when they are executed.
Nice is a command in Unix and Linux operating systems that allows for the adjustment of the “Niceness” value of processes. Adjusting the “niceness” value of processes allows for setting an advised CPU priority that the kernel's scheduler will use to determine which processes get more or less CPU time. In Linux this niceness value can be ignored by the scheduler, however other Unix implementations can treat this differently.
Being able to adjust the niceness value comes in handy in two scenarios usually.
Today's article is going to cover a command that falls into the “I don't use this often, but when I do it's awesome” category.
The tac command is very similar to the cat command in that it is used to concatenate and print files. However there is one very large difference, the tac command does this in reverse, starting with the last line of the file and working its way up to the first line.
The grep command is a command that most Linux users learn early on, and many times they learn to use it via pipes (stdin). Because of this some Linux users just assume that grep can only be used with stdin; it's ok, I was one of those too!
Before I continue with some grep tricks I want to clarify the basic grep usage.
Stop Doing This:
$ cat file.log | grep "something" something Do This More:
Adding and Modifying Users and Groups is a core systems administration task. The act of adding a user and group is fairly easy however there are some tricks that help make the long-term management of users easier.
Tips for easier management Keep user attributes consistent amongst all systems A common mistake sysadmins make when building a new environment is they will allow uid's, gid's, home directories and other user attributes to be mis-matched from system to system.