First of all, it is worth mentioning that Linux is not the only option available; other freely
available operating systems include the BSDs (FreeBSD, NetBSD, OpenBSD), Solaris Express,
Nexenta, and others. However, there are many GNU/Linux distributions available, and these
generally have support for the widest range of hardware and software. Most of these distributions
can be downloaded and used totally legally, even for production use. Of the Linux
distributions mentioned here, RedHat Enterprise Linux (RHEL) and SuSE Linux Enterprise
Server (SLES) have restricted availability and access to updates; Oracle Solaris is restricted to a
90-day trial period for production use.
gnu/linux
RHEL is the commercial distribution based on Fedora. It is particularly popular in North
America and much of Europe. Because the RHEL media includes RedHat trademarks and
some non-Free Software (such as the RedHat Cluster), distribution of the media is restricted
to licensed customers. However, the CentOS project rebuilds RHEL from source, removing
RedHat trademarks, providing a Linux distribution that is totally binary and source code–compatible
with RHEL. This can be very useful as a lot of commercial software for Linux is tested and
supported only on RHEL, but those vendors will often also support the application running on
CentOS, even if they do not support the OS itself.
RHEL itself is available by paid subscription only. However, CentOS and Oracle Enterprise Linux are
two clones built by stripping the RedHat trademarks from the source code and rebuilding in exactly
the same way as the RedHat binaries are built. CentOS is available from http://centos.org/, and
Oracle Enterprise Linux is available from http://edelivery.oracle.com/linux.
Fedora is the community-maintained distribution that feeds into RHEL. It has a highly active,
generally very technical user base, and a lot of developments tested in Fedora first are then pushed
upstream (to the relevant project, be it GNOME, KDE, the Linux kernel, and so on). Like Ubuntu,
it has six-month releases, but a much shorter one-year support cycle. The technologies that have
been proven in Fedora make their way into RedHat Enterprise Linux. As with Ubuntu, KDE,
XFCE, and LXDE respins are available as well as the main GNOME-based desktop. DVD images
can be obtained from http://fedoraproject.org/.
SLES is Novell’s enterprise Linux. It is based on OpenSUSE, which is the community edition. SLES
and OpenSUSE are particularly popular in Europe, partly due to SuSE’s roots as a German company
before Novell purchased it in 2004. SuSE’s biggest differentiator from other Linux distributions is
its YaST2 configuration tool. SLES has a fairly stable release cycle; with a new major release every
2–3 years, it is updated more frequently than RHEL but less frequently than most other Linux
distributions.
SLES is available for evaluation purposes from http://www.novell.com/products/server/. Like
RedHat Enterprise Linux, a support contract is required to use the full version.
OpenSUSE is to SLES as Fedora is to RHEL — a possibly less stable but more community-focused,
cutting-edge version of its Enterprise relative. Test versions are available before the official release.
OpenSUSE is available from http://software.opensuse.org/. The main OpenSUSE website is
http://www.opensuse.org/.
Ubuntu is based on the Debian “testing” branch, with additional features and customizations. It is
very easy to install and configure, has lots of Internet forums providing support, and is a polished
GNU/Linux distribution. Ubuntu offers a Long-Term Support (LTS) release once every 2 years,
which is supported for 2 years on the desktop and 5 years for servers. There are also regular releases
every 6 months, which are numbered as YY-MM, so the 10-10 release (Lucid Lynx) was released in
October 2010. Although widely known for its desktop OS, the server version, without the graphical
features, is growing in popularity.
Ubuntu can be installed in many ways — from a CD/DVD, a USB stick, or even from within an
existing Windows installation. Instructions and freely downloadable media and torrents are available
from http://ubuntu.com/. Many rebuilds of Ubuntu are also available: Kubuntu with KDE
instead of GNOME and Xubuntu with the XFCE window manager, as well Edubuntu, which
includes educational software, and the Netbook Edition tailored for netbook computers.
Debian is one of the older GNU/Linux distributions in mainstream use. It has a team of over 1,000
Debian developers, providing over 30,000 packages. The stable branch is generally released every
5 years or so, so the current stable release can be rather old, although plans are to increase the
frequency of stable releases. The testing branch is popular with many users, providing the latest
packages but without the unpredictability of the unstable branch. Debian CD/DVD images are
available for direct download, or via BitTorrent, from www.debian.org/CD/.
Many hundreds of GNU/Linux distributions are available. The website http://distrowatch.com/
is an excellent resource with information on just about every distribution that exists, as well as other
Unix and Unix-like software. Some other popular distributions worth highlighting include Gentoo,
Damn Small Linux, Knoppix, Slackware, and Mandriva.
The BSDs
Berkeley Software Distribution, or BSD, is one of the oldest Unix flavors. It has split into a number
of different developments, the main three of which are listed here. Each flavor of BSD has a different
focus which determines its development style.
FreeBSD is probably the most accessible of the BSDs, with support for a wider variety of hardware.
OpenBSD is a fork of NetBSD and is generally regarded as the most secure Unix system available,
and although its development is often slower, the resulting system is incredibly stable and secure.
OpenBSD is widely used as a router or firewall. As for version 4.9 which was released in May 2011,
only two remotely exploitable security holes have ever been found in a default install of OpenBSD.
Some operating systems find that many in one month.
NetBSD is the most portable of the BSDs, running on PC, Alpha, and PowerPC, as well as ARM,
HPPA, SPARC/SPARC64, Vax, and many others.
Proprietary Unix
Oracle Solaris traces its roots back to 1983, and is arguably the most feature-full and actively developed
enterprise OS on the market today. SunOS was originally based on BSD, but with the move to Solaris
switched to the System V flavor of Unix. Solaris today comes with the original Bourne shell as /bin/sh,
as well as ksh93, bash, csh, tcsh, and zsh shells. Solaris is available for SPARC and x86 architectures.
Oracle Solaris is available for download from http://www.oracle.com/technetwork/serverstorage/
solaris/downloads/index.html, which can be used for free in nonproduction use, or on a
90-day trial basis. Solaris Express is a technical preview of the version of Solaris currently in development.
There is also OpenIndiana, a fork of OpenSolaris available at http://openindiana.org/, and
Nexenta, another fork with a GNU user space, at http://nexenta.org/.
IBM AIX is IBM’s Unix for the Power architecture, based on System V Unix. It is available in an
Express edition (limited to four CPU cores and 8GB RAM), the Standard Edition (which does not
have the scalability limitations), and the Enterprise Edition (which adds extra monitoring tools and
features). At the time of this writing, the current version is AIX 7.1, released in September 2010.
HP-UX is HP’s Unix offering, based on System V Unix. It runs on PA-RISC and Intel Itanium systems.
At the time of this writing, the current version of HP-UX is 11iv3, released in April 2008.
Microsoft Windows
Cygwin is an environment that runs under Microsoft Windows, providing you with a fairly comprehensive
GNU toolset. If you can’t change to an OS that uses a shell natively, cygwin is a convenient way to
get a fully functioning bash shell and the core utilities (ls, dd, cat — just about everything you would
expect in your GNU/Linux distribution) without leaving Windows. This means that you have the GNU
tools such as grep, sed, awk, and sort working exactly as they do under Linux. Note that cygwin is
not an emulator — it provides a Windows DLL (cygwin1.dll) and a set of (mainly GNU) utilities
compiled as Microsoft Windows executables (.exe). These run natively under Windows; nothing is
emulated. Figure 2-1 shows cygwin in use. Notice that some of the binaries are named with the .exe
extension used by Microsoft DOS and Windows.
Shell Scripting Expert RECIP ES for Linux, Bash, and More
Seve Parker ISBN: 978-0-470-02448-5
Linux Shell Scrpting with Bash
Linux Shell Scripting Cookbook 4397758.8717885311