Heckroth Industries

Redhat

Why I use cat even though there are more efficient methods

When using a series of commands tied together with pipes I usually start with the cat command. A lot of times when I post a one liner solution on a forum someone will reply that there was no point in starting with cat as it is inefficient. So I decided to put a quick post about why I use cat rather than one of the other methods.

The main reason that I use cat at the start of most strings of pipes is that it is easier to maintain. The logical flow of the data is going from left to right and the files that go into the pipe is easy to spot e.g.

cat /etc/passwd | grep bash | grep -v :x:

We can see here that /etc/passwd gets pushed through grep first to find those lines containing bash. Then those lines are pushed through grep again looking for lines that don’t contain :x: (i.e. non shadowed passwords). This could have been written in a number of different ways.

grep bash /etc/passwd | grep -v :x:
</etc/passwd | grep bash | grep -v :x:

In these examples the first way would be reasonable, but the original file at the start of the pipe is a little hidden tucked away in the first grep command. The second way puts the original file at the start and is very clear, but a typo of > instead of < will destroy the file I am really wanting to read from.

So yes there are more efficient ways to start off a string of pipes, but I like to to use cat as it makes things a bit more obvious than some and less prone to destroying data with a simple typo than others.

Jason — 2011-01-10

Changing the hostname on Red Hat Enterprise Linux

So I have to change the hostname on a server running Red Hat Enterprise Linux (v5). As I have to do this quite often I figured that I would note down where I need to change it.

  • Update the HOSTNAME entry in /etc/sysconfig/network

  • Update the entry in /etc/hosts to have the correct new hostnames both with domains and without.

  • Update the list of hostnames that sendmail will accept emails for in /etc/mail/local-host-names.

  • run hostname <NEW_HOSTNAME>

  • This will update the hostname there and then, and avoids the need to restart the server to pick up the new hostname.

  • Finally, if your server has DNS entries pointing to it update these to include the new hostname.

Jason — 2010-11-01

RedHat Enterprise Linux YUM update glitch

Well I have just figured out why some of the machines that I use had stopped picking up updates. When I look at the list of systems on the RedHat Network they had a list of updates that they hadn’t picked up but when I logged into the machines and ran

yum update

it said there were no updates. After trying a lot of things on one of the systems that I was free to test with I still couldn’t find out what was going on. Eventually I discovered that yum had corrupted it’s cache and so it thought that its list of packages was up to date when it wasn’t. The solution was quite easy after that

yum clean all

yum update

I know, I should have tried that at the start.

Jason — 2010-06-18