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What is Linux?
Linux is a reliable, secure, cross-platform, open-source,
multi-user multi-threaded operating system. And best of
all, it is free.
Linux has an excellent performance on any processor because
it is written in a fast programming language by the best
programmers.
Linux is based on Unix which goes back to the old mainframe
operating system called Multics. Multics was one of the
first multiuser operating systems. The history of Unix operating
system goes back to 1965 with the efforts and engineering
of developers at Bell Labs, MIT and General Electric.
Linux comes in different versions (called distributions).
They all use the same system core. Different Linux distribution
have slightly different file system configuration, command
line systems and programs. Everyone gets used to a particular
distribution and start liking and sticking to a one kind
of distribution.
I personally like CentOS.
CentOS is a derivitive of Red Hat Linux and
I like the long history, resources, commands and file system
configuration. But, you may like different distribution
once you start trying them.
Below is a list of a few of the popular distribution of
Linux:
Red Hat Linux
CentOS Linux (derivitive of Red Hat)
Fedora Linux
openSUSE Linux
Mandrake Linux
Ubuntu Linux
Debian Linux
Knoppix Linux
What is Open Source Software?
Open source software and applications are developed by the great programmers around the world who contribute their time in order to help everyone, rich or poor gain access to
the greatest software.
Open source softwares are usually written in such a manner that can provide every possible features and tools that we need to keep up with today's technology.
Most Linux Operating Systems are generally distributed under a licence created by Richard Stallman and the the Free Software Foundation
(http://www.fsf.org) called the GNU General Public License or GPL. Under the GPL Software license:
- The copyright for software distributed under the GPL is retained by the author of the software.
- GPL software may be sold with support services but there must be a free version of it available to the public.
- The source code for GPL software must be available to the public.
- If other developers make improvements to GPL software, they must distribute it under the same rules outlined by the GPL.
You can find the full GPL license information at the below link: GNU
General Public License
Linux File System
Here is a summary of how Linux or Unix filesystem is configured:
/bin - Programs available to all users - (/usr/bin & /usr/local/bin - programs available to local users)
/sbin - Binary Program Files primarily used by the root user. Examples: fdisk & ifconfig
/usr - Programs that are specific to a distribution or local to a specific machine. For example you can find these important open source programs at the following directories:
/usr/bin/mysql
/usr/bin/mysqladmin
/usr/bin/mysqld_safe
/usr/bin/perl
/usr/bin/php
/etc - System Configuration and Scripts files. These are mostly text files which may be edited to change the behavior of the system and its programs.
Few examples of system configuration files:
/etc/passwd - contains user account information
/etc/shadow - contains encrypted passwords for users
/etc/group - contains group information
/etc/crontab - contains schedule information
/etc/my.cnf - contains MySQL configuration
/etc/sysconfig/network-scripts/ifcfg-eth0 - contains IP configuration information
/etc/sysconfig/network - contains IP gateway configuration information
/etc/resolv.conf - contains DNS configuration information
/etc/sysconfig/iptables - contains Firewall configuration information
/etc/php.ini - contains PHP configuration information
/etc/httpd/conf/httpd.conf - contains Apache configuration information
/etc/inittab - contains system boot information
Few examples of Startup script files:
/etc/init.d/sshd start - starts SSH program
/etc/init.d/httpd start - Starts Apache Web Server
/etc/init.d/mysqld start - Starts MySQL Server
/home - Home directories for users specified in /etc/passwd.
/root - Home directories for root user.
/boot - Contains Linux Kernel and startup files needed to start the operating system.
/var - Contains logfiles, process ID files, incoming email, variable-lenght files, MySQL databases, etc..
/dev - Contain device files; in Linux/Unix system each hardware or device is treated as a file.
/proc - Contain information on running processes and installed hardware. for example: /proc/cpuinfo
/mnt - Contain mounting points for external drives/temporary file systems. for example: /mnt/cdrom
/lib - Contains shared library files used by several programs.
/tmp - Contain temporary files. Any user may write/create files on this folder; however, only the user who created a file may alter or delete it. (excluding root user who has the full control of the system.
What is the boot sequence for Linux?
When a Linux computer boots, it performs a power-on self-test
(POST) and checks for memory and key devices. Then
it looks for the boot devices (such as floppy, CD-ROM, first
hard drive) for bootable software. For bootable hard drive,
Linux system looks in a special sector called the Master
Boot Record (MBR) for the drives' partition and format
information and a boot loader which tells the location of
the bootable software.
Linux systems usually use LILO (the Linux Loader)
and GRUB (the Grand Unified Boot Loader). It
is possible to use the Windows boot loader or a 3rd party
application like SystemCommander. The boot
loader allows the user to select which operating system
will be used during the boot sequence.
What is the Linux Runlevel?
When a linux sytems boots, you have an option to select a
different modes of operation called "Runlevels". This is similar to Windows "Safe Mode" or "Notmal Mode"
There is a default Runlevel configured in the configuration
file: /etc/inittab. After the Linux kernel is loaded (/boot/vmlinuz)
the system reads the /etc/inittab file for key
settings and the default run levels (this is usually level
3 or level 5).
There are six different runlevels most Linux distributions:
| Run Level |
Description |
| 0 |
Halt. Shuts down system and turns off power. |
| 1 |
Single User. Starts system and runs as root user. |
| 2 |
Multi-user without Network File System. |
| 3 |
Full Multi-user. Use this most of the time. |
| 4 |
not defined |
| 5 |
Full Multi-user with X-window login. |
| 6 |
Reboot. Shut down system and restart. |
After the run level is identified, symbolic links to files
in /etc/rc.d/init.d from /etc/rc.d/rcX.d
(where X is the run level number) are executed in alphabetical
order to start or stop (kill) server programs. The ntsysv
program is helpful to specify which programs are turned on
at boot. The final file to be processed is /etc/rc.d/rc.local
which may contain statements to be executed before a login
screen is available at the console or remote connections are
permitted.
You can use the following command examples to switch between
RunLevels:
# telinit 5
To read additional information
on Linux common commands and linux permissions, user the
below resources:
File
Systems Comparison - Maximum Characters
UNIX
& OS X Commands References
UNIX
& OS X File Permissions
More
Unix Commands
Unix/Linux
IP & Firewall Configuration, etc.
MySQL
Administration
Linux
rsync
Linux
Intalling and Updating with YUM
Linux
backing up with Cron and rsync
MySQL
Backing Up using MySQLDump and rsync
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