Single-user mode is equivalent to
runlevel 1 on the system. If runlevel 1 is not configured properly, you will
not be able to boot into single-user mode. Rescue mode requires a boot media,
but single-user mode is specified as a kernel option using the installed boot
loader and does not require additional boot media. However, it does require
that the boot loader is working properly and that the filesystem be mounted. It
does not provide the ability to start a network connection.
Sallar
RedhatEnterpriseLinux Blog
Showing posts with label Introduction. Show all posts
Showing posts with label Introduction. Show all posts
What are Users and Groups in Linux
What Are Users and
Groups?
In addition to a Red Hat
Enterprise Linux system having a username for each user allowed access to a
system, each system has user groups. A user group is a group of one or more
users. A user can be a member of more than one group.
User groups can be any grouping
of users on which you decide: groups of users in a functional department,
groups in the same physical location, or groups based on security access. It is
important to plan the user groups for your company carefully before
implementing them because changing them means changing the groups associated
with files, which can sometimes lead to incorrect group permissions if they are
not changed correctly.
How to Detecting Hardware in Linux
After the installation program probes for hardware and maps the IDs to the appropriate kernel module (if available), the kernel module information is written to the /etc/ modprobe.conf file so it can be used on subsequent
boots.
But, what should you
do if
a device is not configured
by the
installation program? You can use tools to probe the hardware and discover information
that can be used to
manually configure the device. This section discusses two
tools for this purpose:
Kudzu and ddcprobe.
How to Find Commands
If you know a command exists on the system but keep getting the
error message command not found, check to make sure you are typing the command correctly.
Otherwise, it might not be in your PATH environment variable. To view the value of your PATH, execute the command echo $PATH from the command line. As you can see, your PATH
is a list of directories. When you execute a
command without providing its full path, it must be in one of the directories
listed in your PATH. Otherwise,
the command not found error is displayed. You
can provide the full path to the command if you know it, such as /sbin/lspci
to execute the command to list the PCI
devices. If you use the command often, but it is not in your PATH, you can add the directory to your PATH.
How to Read Text Files in linux
Sometimes you want to quickly read a text file such as a
configuration file without having to open a text editor. This is possible with
the less, more, and cat command-line
utilities. All three have the same basic syntax but work differently and have
different commandline options. To use them in their default modes, type the
command followed by the text file to read such as less
output.txt, more /var/loasg/messages, or cat
/etc/sysconfig/ network.
How to Become the Root User
It is important to log in with your user account instead of as the
root user when performing day-to- day tasks. Some of the graphical
administration tools will prompt you for the root password if you try to run
them as a regular user. But, what if you are logged in as a user and need to
perform an operation only the root user can do? It would be time consuming to
close all your open windows, log out of the graphical desktop, log back in as
root, execute root-only commands, log out again, and then log back in with your
user account.
How to Configure Service Configuration Tool
To immediately start, stop, or restart a service, select it from
the list and click the Start, Stop, or Restart button. This does not affect whether it is started or stopped at
boot time. If you do not have the graphical desktop installed or just prefer a
more simplistic interface, ntsysv can be used to configure runlevels. The program must be run as the
root user.
How to Configure the Runlevels In Redhat Enterprise linux
To configuring which services are started for a runlevel, use one
of three programs: chkconfig (command line
only), ntsysv (simple text-based application that doesn’t require
a graphical desktop), or the Service Configuration Tool (graphical
application).
The chkconfig command can
be used to configure runlevels and list the current runlevel configuration. It
must be run as root if modifying a runlevel. Otherwise commands such as listing
whether a service is started at boot time can be run as a non-root user.
To list the status of all services, execute the chkconfig --list
command. A line is output for each service
such as the following for the Apache HTTP Server:
How to Initialization Scripts in RHEL
Network services such as the Apache HTTP Server and DHCP along with
other programs such as cron and syslog require a daemon to be running at all
times. The daemon performs actions such as listening for connections to a
service on specific ports, making sure commands are executed at specific times,
and capturing data such as log messages when they are sent out by other
programs.Programs that require a
DIFFERENCES BETWEEN WINDOWS AND LINUX
As
you might imagine, the differences between Microsoft Windows and the Linux
operating system cannot be completely discussed in the confines of this
section. Throughout this topic, we’ll examine the specific contrasts between
the two systems.But
before we attack the details, let’s take a moment to discuss the primary architectural
differences between the two operating systems.
Single Users vs. Multiple Users vs. Network Users
Windows
was designed according to the “one computer, one desk, one user” vision of Microsoft’s
cofounder Bill Gates. For the sake of discussion, we’ll call this philosophy single-user.
In this arrangement, two people cannot work in parallel running
(for example) Microsoft Word on the same machine at the same time. (On the
other hand, one might question the wisdom of doing this with an overwhelmingly
weighty program like Word!) You can buy Windows and run what is known as
Terminal Server, but this requires huge computing power and extra costs in
licensing. Of course, with Linux, you don’t run into the cost problem, and
Linux will run fairly well on just about any hardware.
Linux
borrows its philosophy from UNIX. When UNIX was originally developed at Bell
Labs in the early 1970s, it existed on a PDP-7 computer that needed to be
shared by an entire department. It required a design that allowed for multiple
users to log into the central machine at the
same time. Various people could be editing documents, compiling programs, and
doing other work at the exact same time. The operating system on the central
machine took care of the “sharing” details so that each user seemed to have an individual
system. This multiuser tradition continues through today on other versions of UNIX
as well. And since Linux’s birth in the early 1990s, it has supported the
multiuser arrangement.
Characteristics or Properties of Linux
A lot of
the advantages of Linux are a consequence of Linux' origins, deeply rooted in
UNIX, except for the first advantage, of course:
Linux is free
As in free beer, they say. If you want to spend absolutely
nothing, you don't even have to pay the price of a CD. Linux can be downloaded
in its entirety from the Internet completely for free. No registration fees, no
costs per user, free updates, and freely available source code in case you want
to change the behavior of your system.
Most of all, Linux is free as in free speech
The license commonly used is the GNU Public License (GPL). The
license says that anybody who may want to do so, has the right to change Linux
and eventually to redistribute a changed version, on the one condition that the
code is still available after redistribution. In practice, you are free to grab
a kernel image, for instance to add support for tele transportation machines or
time travel and sell your new code, as long as your customers can still have a
copy of that code.
Is Linux difficult?
Whether
Linux is difficult to learn depends on the person you're asking. Experienced
UNIX users will say no, because Linux is an ideal operating system for
power-users and programmers, because it has been and is being developed by such
people.
Everything
a good programmer can wish for is available: compilers, libraries, development
and debugging tools. These packages come with every standard Linux
distribution. The C-compiler is included for free – as opposed to many UNIX
distributions demanding licensing fees for this tool. All the documentation and
manuals are there, and examples are often included to help you get started in
no time. It feels like UNIX and switching between UNIX and Linux is a natural
thing.
In the
early days of Linux, being an expert was kind of required to start using the
system. Those who mastered Linux felt better than the rest of the
"lusers" who hadn't seen the light yet. It was common practice to
tell a beginning user to "RTFM" (read the manuals). While the manuals
were on every system, it was difficult to find the documentation, and even if
someone did, explanations were in such technical terms that the new user became
easily discouraged from learning the system.
The
Linux-using community started to realize that if Linux was ever to be an
important player on the
Operating system market, there had to
be some serious changes in the accessibility of the system.
Linus and Linux
By the
beginning of the 90s home PCs were finally powerful enough to run a full blown
UNIX. Linus Torvalds, a young man studying computer science at the University
of Helsinki, thought it would be a good idea to have some sort of freely
available academic version of UNIX, and promptly started to code.
He started
to ask questions, looking for answers and solutions that would help him get
UNIX on his PC. Below is
one of his first posts in comp.os.minix, dating from 1991:
From:
torvalds@klaava.Helsinki.FI (Linus Benedict Torvalds)
Newsgroups:
comp.os.minix
Subject:
Gcc-1.40 and a posix-question
Message-ID:
<1991Jul3.100050.9886@klaava.Helsinki.FI>
Date: 3 Jul
91 10:00:50 GMT
Hello
netlanders,
Due to a
project I'm working on (in minix), I'm interested in the posix standard
definition. Could somebody please point me to a (preferably) machine-readable
format of the latest posix rules? Ftp-sites would be nice.
Linux in The Business
In 2008, IDC analyst Al Gillen cited a nearly
24 percent annual growth rate for the Linux industry, which puts a $21 billion
2007 technology at $49 billion in 2011. The companies involved in Linux include
industry leaders such as IBM, Fujitsu, Hewlett-Packard, Oracle, Intel, Hitachi,
NEC, and Novell. All have all invested time, talent, and resources to bettering
Linux on their own and through the Linux Foundation.
If Linux is free of cost, how does a Linux
company generate revenue?
Linux Development
Linux is an operating system that is comprised
of many different development languages. A very large percentage of the
distributions' code is written in either the C (52.86%) or C++ (25.56%)
languages. All of the rest of the code falls into single-digit percentages,
with Java, Perl, and Lisp rounding out the rest of the top 5 languages.
The Linux kernel itself has an even more
dominant C presence, with over 95 percent of the kernel's code written in that
language. But other languages make up the kernel as well, making it more
heterogenous than other operating systems.
Linux in Community
Linux communities come in two basic forms:
developer and user communities.
One of the most compelling features of Linux
is that it is accessible to developers; anybody with the requisite skills can
improve Linux and influence the direction of its development. Proprietary
products cannot offer this kind of openness, which is a characteristic of the
free software process.
Developer communities can volunteer to
maintain and support whole distributions, such as the Debian or Gentoo
Projects. Novell and Red hat also support community-driven versions of their
products, openSUSE and Fedora, respectively. The improvements to these
community distros are then incorporated into the commercial server and desktop
products from these companies.
The Linux kernel itself is primarily supported
by its developer community as well and is one of the largest and most active
free software projects in existence. A typical three-month kernel development
cycle can involve over 1000 developers working for more than 100 different
companies (or for no company at all).
Redhat Licensing
Code is contributed to the Linux kernel under
a number of licenses, but all code must be compatible with version 2 of the GNU
General Public License (GPLv2), which is the license covering the kernel
distribution as a whole. In practice, that means that all code contributions
are covered either by GPLv2 (with, optionally, language allowing distribution
under later versions of the GPL) or the three-clause BSD license. Any
contributions which are not covered by a compatible license will not be
accepted into the kernel.
Copyright assignments are not required (or
requested) for code contributed to the kernel. All code merged into the
mainline kernel retains its original ownership; as a result, the kernel now has
thousands of owners.
Environments, Applications, Distributions
The Environments
The windows, menus, and dialog boxes most people think of as part of the operating system are actually separate layers, known as the windowing system and the desktop environment.
These layers provide the human-oriented graphical user interface (GUI) that enables users to easily work with applications in the operating system and third-party applications to be installed on the operating system.
In Linux, there a lot of choices for which windowing system and desktop environment can be used, something that Linux allows users to decide. This cannot be done in Windows and it's difficult to do in OS X.
Linux Operating System
Developers need special tools (like the compilers and command lines found in GNU) to write applications that can talk to the kernel. They also need tools and applications to make it easy for outside applications to access the kernel after the application is written and installed.
This collective set of tools, combined with a kernel, is known as the operating system. It is generally the lowest layer of the computer's software that is accessible by the average user. General users get to the operating system when they access the command line.
The Code & Kernel
The Code
Linux is also unique from other operating systems in that it has no single owner. Torvalds still manages the development of the Linux kernel, but commercial and private developers contribute other software to make the whole Linux operating system.
The Kernel
All operating systems have kernels, built around the architectural metaphor that there must be a central set of instructions to direct device hardware, surrounded by various modular layers of functionality. The Linux kernel is unique and flexible because it is also modular in nature.
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