Unix is a multi-user, multi-tasking operating system that was initially developed by Bell Labs in the 1970s. Various similar operating systems that are POSIX compliant (ie compatible with Unix) have since been developed (POSIX is an abbreviation for Portable Operating System Interface for UNIX). These include the following: the BSD family (OpenBSD, FreeBSD, NetBSD, Mac OS/X), Solaris, IRIX, AIX, SunOS, UnixWare, OpenServer and Linux.
Software written for POSIX compliant operating systems is generally able to be compiled for all POSIX platforms with little or no platform-specific changes. This makes the commands and utilities available for this extended family of operating systems very similar, and there are few differences between administering them.
The articles in this section are about useful command line utilities and commonly used application software on POSIX compliant operating systems. For Mac OSX specific articles see my OSX section.
I've just started using a VPS (Virtual Private Server) with CentOS 5.0 i386 as the virtual server operating system, but it has a pretty minimal operating system install. When I went to run the nslookup command to look up an IP address I got the error message nslookup: command not found.
Security-Enhanced Linux, also know as SELinux, implements various security policies on Linux and additional levels of access crontrol. It was originally developed by the U.S. National Security Agency to adhere to the "Orange Book" guidelines. On CentOS 5 it is enabled by default, but there may be circumstances where you don’t need SELinux’s additional security and may want to disable it.
From the BASH shell it’s possible to have auto-completion where you start to type in part of a command, and then use a keystroke sequence, such as PageUp or PageDown, to then cycle through the history for commands which started with the first text you have entered.
If the hostname setting is incorrect on your CentOS/RedHat/Fedora machine, it’s really easy to change the hostname from the command line. There are also GUI tools for doing this but we’ll just look at the CLI tools for doing this.
openSUSE has an excellent system management tool called Yast, but the software package management tool in Yast is really slow to run. It is possible to install and use Yum instead on openSUSE and all of a sudden the slowness of running Yast’s package management goes away. I have a pretty fast machine with a lot of memory, but it can take 5 to 10 minutes to open up the Yast software management application.
When trying to install something on a CentOS 5 machine using Yum today, I got the error message "Metadata file does not match checksum" when Yum was trying to update the repository metadata. My first instinct was to run the error message through Google but the first couple of answers looked a little complex. I then remembered I’d had issues with Yum in an earlier post when I got the database disk image is malformed error message, so decided to try out the same solution before following the advice from the other sites.