Archive for category Linux Trick

Linux Important System Files

File Purpose Where to Find More Information
User and Security Files
/etc/group User group information group, chmod
/etc/npasswd npasswd configuration file npasswd
/etc/shadow shadow password file password, npasswd
/etc/passwd User account information password, chmod
Networking Files
/etc/bootptab Internet Bootstrap Protocol server database bootptab
/etc/exports Directories to export to NFS clients exports
/etc/gateways List of gateways routed
/etc/hosts Hostname to IP address mapping file route
/etc/hosts.equiv Lists of trusted hosts and remote users hosts.equiv
/etc/inetd.conf inetd configuration file inetd
/etc/named.conf named default initialization file named
/etc/networks Known networks route
/usr/lib/named or/etc/named.d named configuration files named
/etc/smb.conf or /etc/samba/smb.conf SAMBA configuration file smb.conf
/etc/snmpd.conf SNMP daemon configuration file snmpd.conf
/etc/ftpaccess FTP configuration file ftpaccess
/etc/httpd/access.conf HTTP access configuration file
/etc/httpd/httpd.conf HTTP daemon configuration file
/etc/httpd/srm.conf HTTP server resource management configuration file
/etc/services Network services list services(5)
X-Windows Files
/etc/XF86Config or /etc/X11/XF86Config X-Server configuration file XF86Config, xf86config
/etc/X11/xinit/xinitrc xinit configuration file xinit
$HOME/.xinitrc User-specific xinit configuration file xinit
$HOME/.fvwmrc fvwm configuration file fvwm, X
/usr/lib/X11/system.fvwmrc System default MWM configuration file fvwm, X
/usr/lib/X11/app-defaults Application-specific defaults X
$HOME/.Xdefaults-hostname Host-specific defaults X
System Start-Up Files
/etc/inittab init configuration file inittab
/etc/lilo.conf Lilo configuration file lilo.conf, lilo
/etc/rc* System start-up scripts init, initscript
System Log Files
/etc/syslog.conf System login configuration file syslog.conf
/var/log/message General system log file syslogd
Miscellaneous Files
/etc/profile
/etc/bashrc
/etc/cshrc
Systemwide shell configuration files man-page for respective shell
$HOME/.bashrc
$HOME/.chsrc
$HOME/.kshrc
User-specifc shell configuration files man-page for respective shell
/etc/sysconfig Miscellaneous configuration files
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FTP Bash script for auto file transfer and backup

#!/bin/bash

ftp_site=ftp://yoursite.com
username=roger
passwd=abc123
backupdir=$HOME
filename="backup-$(date '+%F-%H%M').tar.gz"

echo "Creating a backup file $filename of $backupdir."

# Make a tar gzipped backup file
tar -cvzf  "$filename" "$backupdir"

ftp -in <<EOF
open $ftp_site
user $username $passwd
bin
put $filename 
close 
bye
EOF

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configure Kdump in centos (to recover data)

1	================================================================
2	Documentation for Kdump - The kexec-based Crash Dumping Solution
3	================================================================
4	
5	This document includes overview, setup and installation, and analysis
6	information.
7	
8	Overview
9	========
10	
11	Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
12	dump of the system kernel's memory needs to be taken (for example, when
13	the system panics). The system kernel's memory image is preserved across
14	the reboot and is accessible to the dump-capture kernel.
15	
16	You can use common commands, such as cp and scp, to copy the
17	memory image to a dump file on the local disk, or across the network to
18	a remote system.
19	
20	Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
21	and s390x architectures.
22	
23	When the system kernel boots, it reserves a small section of memory for
24	the dump-capture kernel. This ensures that ongoing Direct Memory Access
25	(DMA) from the system kernel does not corrupt the dump-capture kernel.
26	The kexec -p command loads the dump-capture kernel into this reserved
27	memory.
28	
29	On x86 machines, the first 640 KB of physical memory is needed to boot,
30	regardless of where the kernel loads. Therefore, kexec backs up this
31	region just before rebooting into the dump-capture kernel.
32	
33	Similarly on PPC64 machines first 32KB of physical memory is needed for
34	booting regardless of where the kernel is loaded and to support 64K page
35	size kexec backs up the first 64KB memory.
36	
37	For s390x, when kdump is triggered, the crashkernel region is exchanged
38	with the region [0, crashkernel region size] and then the kdump kernel
39	runs in [0, crashkernel region size]. Therefore no relocatable kernel is
40	needed for s390x.
41	
42	All of the necessary information about the system kernel's core image is
43	encoded in the ELF format, and stored in a reserved area of memory
44	before a crash. The physical address of the start of the ELF header is
45	passed to the dump-capture kernel through the elfcorehdr= boot
46	parameter. Optionally the size of the ELF header can also be passed
47	when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
48	
49	
50	With the dump-capture kernel, you can access the memory image, or "old
51	memory," in two ways:
52	
53	- Through a /dev/oldmem device interface. A capture utility can read the
54	  device file and write out the memory in raw format. This is a raw dump
55	  of memory. Analysis and capture tools must be intelligent enough to
56	  determine where to look for the right information.
57	
58	- Through /proc/vmcore. This exports the dump as an ELF-format file that
59	  you can write out using file copy commands such as cp or scp. Further,
60	  you can use analysis tools such as the GNU Debugger (GDB) and the Crash
61	  tool to debug the dump file. This method ensures that the dump pages are
62	  correctly ordered.
63	
64	
65	Setup and Installation
66	======================
67	
68	Install kexec-tools
69	-------------------
70	
71	1) Login as the root user.
72	
73	2) Download the kexec-tools user-space package from the following URL:
74	
75	http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz
76	
77	This is a symlink to the latest version.
78	
79	The latest kexec-tools git tree is available at:
80	
81	git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
82	and
83	http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
84	
85	There is also a gitweb interface available at
86	http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git
87	
88	More information about kexec-tools can be found at
89	http://www.kernel.org/pub/linux/utils/kernel/kexec/README.html
90	
91	3) Unpack the tarball with the tar command, as follows:
92	
93	   tar xvpzf kexec-tools.tar.gz
94	
95	4) Change to the kexec-tools directory, as follows:
96	
97	   cd kexec-tools-VERSION
98	
99	5) Configure the package, as follows:
100	
101	   ./configure
102	
103	6) Compile the package, as follows:
104	
105	   make
106	
107	7) Install the package, as follows:
108	
109	   make install
110	
111	
112	Build the system and dump-capture kernels
113	-----------------------------------------
114	There are two possible methods of using Kdump.
115	
116	1) Build a separate custom dump-capture kernel for capturing the
117	   kernel core dump.
118	
119	2) Or use the system kernel binary itself as dump-capture kernel and there is
120	   no need to build a separate dump-capture kernel. This is possible
121	   only with the architectures which support a relocatable kernel. As
122	   of today, i386, x86_64, ppc64 and ia64 architectures support relocatable
123	   kernel.
124	
125	Building a relocatable kernel is advantageous from the point of view that
126	one does not have to build a second kernel for capturing the dump. But
127	at the same time one might want to build a custom dump capture kernel
128	suitable to his needs.
129	
130	Following are the configuration setting required for system and
131	dump-capture kernels for enabling kdump support.
132	
133	System kernel config options
134	----------------------------
135	
136	1) Enable "kexec system call" in "Processor type and features."
137	
138	   CONFIG_KEXEC=y
139	
140	2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
141	   filesystems." This is usually enabled by default.
142	
143	   CONFIG_SYSFS=y
144	
145	   Note that "sysfs file system support" might not appear in the "Pseudo
146	   filesystems" menu if "Configure standard kernel features (for small
147	   systems)" is not enabled in "General Setup." In this case, check the
148	   .config file itself to ensure that sysfs is turned on, as follows:
149	
150	   grep 'CONFIG_SYSFS' .config
151	
152	3) Enable "Compile the kernel with debug info" in "Kernel hacking."
153	
154	   CONFIG_DEBUG_INFO=Y
155	
156	   This causes the kernel to be built with debug symbols. The dump
157	   analysis tools require a vmlinux with debug symbols in order to read
158	   and analyze a dump file.
159	
160	Dump-capture kernel config options (Arch Independent)
161	-----------------------------------------------------
162	
163	1) Enable "kernel crash dumps" support under "Processor type and
164	   features":
165	
166	   CONFIG_CRASH_DUMP=y
167	
168	2) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems".
169	
170	   CONFIG_PROC_VMCORE=y
171	   (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
172	
173	Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
174	--------------------------------------------------------------------
175	
176	1) On i386, enable high memory support under "Processor type and
177	   features":
178	
179	   CONFIG_HIGHMEM64G=y
180	   or
181	   CONFIG_HIGHMEM4G
182	
183	2) On i386 and x86_64, disable symmetric multi-processing support
184	   under "Processor type and features":
185	
186	   CONFIG_SMP=n
187	
188	   (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
189	   when loading the dump-capture kernel, see section "Load the Dump-capture
190	   Kernel".)
191	
192	3) If one wants to build and use a relocatable kernel,
193	   Enable "Build a relocatable kernel" support under "Processor type and
194	   features"
195	
196	   CONFIG_RELOCATABLE=y
197	
198	4) Use a suitable value for "Physical address where the kernel is
199	   loaded" (under "Processor type and features"). This only appears when
200	   "kernel crash dumps" is enabled. A suitable value depends upon
201	   whether kernel is relocatable or not.
202	
203	   If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
204	   This will compile the kernel for physical address 1MB, but given the fact
205	   kernel is relocatable, it can be run from any physical address hence
206	   kexec boot loader will load it in memory region reserved for dump-capture
207	   kernel.
208	
209	   Otherwise it should be the start of memory region reserved for
210	   second kernel using boot parameter "crashkernel=Y@X". Here X is
211	   start of memory region reserved for dump-capture kernel.
212	   Generally X is 16MB (0x1000000). So you can set
213	   CONFIG_PHYSICAL_START=0x1000000
214	
215	5) Make and install the kernel and its modules. DO NOT add this kernel
216	   to the boot loader configuration files.
217	
218	Dump-capture kernel config options (Arch Dependent, ppc64)
219	----------------------------------------------------------
220	
221	1) Enable "Build a kdump crash kernel" support under "Kernel" options:
222	
223	   CONFIG_CRASH_DUMP=y
224	
225	2)   Enable "Build a relocatable kernel" support
226	
227	   CONFIG_RELOCATABLE=y
228	
229	   Make and install the kernel and its modules.
230	
231	Dump-capture kernel config options (Arch Dependent, ia64)
232	----------------------------------------------------------
233	
234	- No specific options are required to create a dump-capture kernel
235	  for ia64, other than those specified in the arch independent section
236	  above. This means that it is possible to use the system kernel
237	  as a dump-capture kernel if desired.
238	
239	  The crashkernel region can be automatically placed by the system
240	  kernel at run time. This is done by specifying the base address as 0,
241	  or omitting it all together.
242	
243	  crashkernel=256M@0
244	  or
245	  crashkernel=256M
246	
247	  If the start address is specified, note that the start address of the
248	  kernel will be aligned to 64Mb, so if the start address is not then
249	  any space below the alignment point will be wasted.
250	
251	
252	Extended crashkernel syntax
253	===========================
254	
255	While the "crashkernel=size[@offset]" syntax is sufficient for most
256	configurations, sometimes it's handy to have the reserved memory dependent
257	on the value of System RAM -- that's mostly for distributors that pre-setup
258	the kernel command line to avoid a unbootable system after some memory has
259	been removed from the machine.
260	
261	The syntax is:
262	
263	    crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
264	    range=start-[end]
265	
266	    'start' is inclusive and 'end' is exclusive.
267	
268	For example:
269	
270	    crashkernel=512M-2G:64M,2G-:128M
271	
272	This would mean:
273	
274	    1) if the RAM is smaller than 512M, then don't reserve anything
275	       (this is the "rescue" case)
276	    2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
277	    3) if the RAM size is larger than 2G, then reserve 128M
278	
279	
280	
281	Boot into System Kernel
282	=======================
283	
284	1) Update the boot loader (such as grub, yaboot, or lilo) configuration
285	   files as necessary.
286	
287	2) Boot the system kernel with the boot parameter "crashkernel=Y@X",
288	   where Y specifies how much memory to reserve for the dump-capture kernel
289	   and X specifies the beginning of this reserved memory. For example,
290	   "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
291	   starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
292	
293	   On x86 and x86_64, use "crashkernel=64M@16M".
294	
295	   On ppc64, use "crashkernel=128M@32M".
296	
297	   On ia64, 256M@256M is a generous value that typically works.
298	   The region may be automatically placed on ia64, see the
299	   dump-capture kernel config option notes above.
300	
301	   On s390x, typically use "crashkernel=xxM". The value of xx is dependent
302	   on the memory consumption of the kdump system. In general this is not
303	   dependent on the memory size of the production system.
304	
305	Load the Dump-capture Kernel
306	============================
307	
308	After booting to the system kernel, dump-capture kernel needs to be
309	loaded.
310	
311	Based on the architecture and type of image (relocatable or not), one
312	can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
313	of dump-capture kernel. Following is the summary.
314	
315	For i386 and x86_64:
316		- Use vmlinux if kernel is not relocatable.
317		- Use bzImage/vmlinuz if kernel is relocatable.
318	For ppc64:
319		- Use vmlinux
320	For ia64:
321		- Use vmlinux or vmlinuz.gz
322	For s390x:
323		- Use image or bzImage
324	
325	
326	If you are using a uncompressed vmlinux image then use following command
327	to load dump-capture kernel.
328	
329	   kexec -p <dump-capture-kernel-vmlinux-image> \
330	   --initrd=<initrd-for-dump-capture-kernel> --args-linux \
331	   --append="root=<root-dev> <arch-specific-options>"
332	
333	If you are using a compressed bzImage/vmlinuz, then use following command
334	to load dump-capture kernel.
335	
336	   kexec -p <dump-capture-kernel-bzImage> \
337	   --initrd=<initrd-for-dump-capture-kernel> \
338	   --append="root=<root-dev> <arch-specific-options>"
339	
340	Please note, that --args-linux does not need to be specified for ia64.
341	It is planned to make this a no-op on that architecture, but for now
342	it should be omitted
343	
344	Following are the arch specific command line options to be used while
345	loading dump-capture kernel.
346	
347	For i386, x86_64 and ia64:
348		"1 irqpoll maxcpus=1 reset_devices"
349	
350	For ppc64:
351		"1 maxcpus=1 noirqdistrib reset_devices"
352	
353	For s390x:
354		"1 maxcpus=1 cgroup_disable=memory"
355	
356	Notes on loading the dump-capture kernel:
357	
358	* By default, the ELF headers are stored in ELF64 format to support
359	  systems with more than 4GB memory. On i386, kexec automatically checks if
360	  the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
361	  So, on non-PAE systems, ELF32 is always used.
362	
363	  The --elf32-core-headers option can be used to force the generation of ELF32
364	  headers. This is necessary because GDB currently cannot open vmcore files
365	  with ELF64 headers on 32-bit systems.
366	
367	* The "irqpoll" boot parameter reduces driver initialization failures
368	  due to shared interrupts in the dump-capture kernel.
369	
370	* You must specify <root-dev> in the format corresponding to the root
371	  device name in the output of mount command.
372	
373	* Boot parameter "1" boots the dump-capture kernel into single-user
374	  mode without networking. If you want networking, use "3".
375	
376	* We generally don' have to bring up a SMP kernel just to capture the
377	  dump. Hence generally it is useful either to build a UP dump-capture
378	  kernel or specify maxcpus=1 option while loading dump-capture kernel.
379	
380	* For s390x there are two kdump modes: If a ELF header is specified with
381	  the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
382	  is done on all other architectures. If no elfcorehdr= kernel parameter is
383	  specified, the s390x kdump kernel dynamically creates the header. The
384	  second mode has the advantage that for CPU and memory hotplug, kdump has
385	  not to be reloaded with kexec_load().
386	
387	* For s390x systems with many attached devices the "cio_ignore" kernel
388	  parameter should be used for the kdump kernel in order to prevent allocation
389	  of kernel memory for devices that are not relevant for kdump. The same
390	  applies to systems that use SCSI/FCP devices. In that case the
391	  "allow_lun_scan" zfcp module parameter should be set to zero before
392	  setting FCP devices online.
393	
394	Kernel Panic
395	============
396	
397	After successfully loading the dump-capture kernel as previously
398	described, the system will reboot into the dump-capture kernel if a
399	system crash is triggered.  Trigger points are located in panic(),
400	die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
401	
402	The following conditions will execute a crash trigger point:
403	
404	If a hard lockup is detected and "NMI watchdog" is configured, the system
405	will boot into the dump-capture kernel ( die_nmi() ).
406	
407	If die() is called, and it happens to be a thread with pid 0 or 1, or die()
408	is called inside interrupt context or die() is called and panic_on_oops is set,
409	the system will boot into the dump-capture kernel.
410	
411	On powerpc systems when a soft-reset is generated, die() is called by all cpus
412	and the system will boot into the dump-capture kernel.
413	
414	For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
415	"echo c > /proc/sysrq-trigger" or write a module to force the panic.
416	
417	Write Out the Dump File
418	=======================
419	
420	After the dump-capture kernel is booted, write out the dump file with
421	the following command:
422	
423	   cp /proc/vmcore <dump-file>
424	
425	You can also access dumped memory as a /dev/oldmem device for a linear
426	and raw view. To create the device, use the following command:
427	
428	    mknod /dev/oldmem c 1 12
429	
430	Use the dd command with suitable options for count, bs, and skip to
431	access specific portions of the dump.
432	
433	To see the entire memory, use the following command:
434	
435	   dd if=/dev/oldmem of=oldmem.001
436	
437	
438	Analysis
439	========
440	
441	Before analyzing the dump image, you should reboot into a stable kernel.
442	
443	You can do limited analysis using GDB on the dump file copied out of
444	/proc/vmcore. Use the debug vmlinux built with -g and run the following
445	command:
446	
447	   gdb vmlinux <dump-file>
448	
449	Stack trace for the task on processor 0, register display, and memory
450	display work fine.
451	
452	Note: GDB cannot analyze core files generated in ELF64 format for x86.
453	On systems with a maximum of 4GB of memory, you can generate
454	ELF32-format headers using the --elf32-core-headers kernel option on the
455	dump kernel.
456	
457	You can also use the Crash utility to analyze dump files in Kdump
458	format. Crash is available on Dave Anderson's site at the following URL:
459	
460	   http://people.redhat.com/~anderson/
461	
462	
463	To Do
464	=====
465	
466	1) Provide relocatable kernels for all architectures to help in maintaining
467	   multiple kernels for crash_dump, and the same kernel as the system kernel
468	   can be used to capture the dump.

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Open Source Software for linux

Proprietary Software Open Source Alternative
Operating Systems
Microsoft Windows
Mac OS X
Ubuntu Linux
Fedora Linux
Centos (Red Hat Linux without Red Hat’s logos and support.)
openSUSE
Debian Linux
Mandriva Linux
Xandros
K12LTSP K12 Linux Terminal Server Project
VectorLinux
PC-BSD
There are too many others to list, go to Linux Online! for more distros!
Office Productivity Software / Suites
Microsoft Office OpenOffice (cross platform)
LibreOffice (cross platform)
KOffice – Linux
HTML Editors
Adobe Dreamweaver
Microsoft FrontPage
BlueGriffon (cross platform)
Digital Image / Photo Editing / Raster Image Editing Software
Adobe Photoshop GIMP (cross platform)
Image Viewers with RAW Capability
Photo Mechanic (Win and Mac OSX) Xee (Mac OSX only)
Vector Drawing Programs
Adobe Illustrator
Macromedia FreeHand
CorelDraw
Inkscape (cross platform)
2D CAD
(Computer Aided Drawing)
AutoCAD QCad (Linux/UNIX * Mac OS X)
3D CAD
(3D Modeling)
SolidWorks
Pro/ENGINEER
AutoDesk Inventor
Blender (cross platform)
BRL-CAD 3D
Povray 3D
Tulip 3D
Screen Recording
(to capture what you are doing on your computer and save as a video file)
CamStudio (for Windows only)
Desktop Publishing
Adobe Indesign
QuarkExpress
Microsoft Publisher
Scribus (cross platform)
PDF Creation
Adobe Acrobat PDF Creator (Windows only)OpenOffice

* This capability is built into Mac OS X via the print function.

Text Editors
Notepad jEdit (cross platform)
Notepad++ (for Windows only)
Syn (for Windows only)
Smultron (for Mac OS X only)
Web Servers
Microsoft IIS (Internet Information Services) Apache (cross platform)
Web Browsers
Microsoft Internet Explorer
Safari
Mozilla Firefox (cross platform)
Flock
Email Software
Microsoft Outlook
Microsoft Entourage
QuickMail
Thunderbird (cross platform)
Evolution
Kmail
FTP Clients
(File Transfer Protocol)
Ipswitch WS_FTP Pro
Fetch
Filezilla – FTP and SFTP (cross platform)
Cyberduck – FTP and SFTP (for Mac OS X only)
SFTP Programs
(SSH Secure File Transfer)
Filezilla – FTP and SFTP (cross platform)
Cyberduck – FTP and SFTP (for Mac OS X only)
WinSCP – SFTP and SCP client for Windows
SSH
PuTTY – SFTP and Telnet client for Windows
VNC
Apple Remote Desktop (ARD) TightVNC – VNC server and client for Windows, Linux, and Unix
Chicken of the VNC – VNC client for Mac OS X
OSXvnc – VNC server for Mac OS X
Hard Disk Utilities / Partitioning Tools
Norton PartitionMagic GParted & GParted LiveCD (Gnome Partition Editor) – (cross platform)
Anti-Virus Software
McAfee
Norton Anti-virus
AVG
ClamAV
ClamWin – (for Windows only)
ClamXav – (for Mac OS X only)
Keyboarding / Typing Software
Tux Typing 2
Edutainment / Educational Software
The KDE Edutainment Project
FREEDUC – resource for FOSS educational software
A List of Open Source Software for Education
Gcompris – educational softare for kids ages 2 to 10
Childsplay – various games for ages 1-9
KStars – a simualted planetarium (astronomy)
Kalzium – periodic table
KWord Quiz – flash cards
KTouch – teaches typing
Tux Paint – vector and raster image editing program for young children
KTurtle – Logo programming for young children
Math Programs
Geometer’s Sketchpad or Cabri GeoGebra – joins geometry, algebra and calculus
Dr. Geo – interactive geometry software
TuxMath – math game for kids
Music Composition
Rosegarden – similar to Cubase
Noteedit
Solfege – ear training
Montessori Bells
Sound Editing Programs
Sound Forge Audacity (cross platform)
Video Editing
iMovie KINO
Brainstorming, Mind Mapping, Flowcharting, Planning
Inpsiration FreeMind
Kivio
View Your Mind
Compression Utilities
WinZip 7-Zip
The Unarchiver
Accounting / Personal Finance
Quicken GnuCash
Grisbi – (cross platform)
Course Management Systems (CMS) and Virtual Learning Environments (VLE)
Click here for a full list of non-open source CMS/VLEs
Blackboard
WebCT
Campus Pipeline
ClassCampus
CyberProf
eCollege
Virtuoso
TeleTOP cms
Virtual Campus
Sentient Discover
MindEdge CMS
MyEDUkit
Click here for a full list of Open Source CMS/VLEs
Also click here
Moodle – used in 74 countries in 34 diferent languages!
MyClassroom – in use by several middle schools and high schools in the US and around the world!
interact
OpenCourse
Boddington
.LRN
LogiCampus
ATutor
Wordcircle CMS
Sakai
Manhattan
ILIAS
CourseWork – used by Stanford University
COSE (Creation of Study Environments)
Claroline – used in 62 countries and 28 languages!
WebWork
Eledge
ClassWeb 2.0
ePICE
Whiteboard
Student Information Systems (SIS)
Powerschool
WebEIM
eSIS
SCHOOLMASTER
MMS
Centre
Open Admin for Schools
Emulators
Virtual PC for Mac QEMU
PearPC
Q – cocoa port of QEMU for Mac OS X
bochs
Virtualization
Virtual PC for Windows
VMware
VirtualBox (cross platform)
coLinux

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Linux some common things

1. In Linux systems, to document access to equipment.2. Linux kernel boot, from the file / etc / fstab to read the file system to load.

3. Linux file system, each file to identify with the i node.

4. All the disk block consists of four parts, namely, guide blocks, special blocks, i node table blocks and blocks of data storage

5. Links divided into: hard links and symbolic links.

6. Super-block contains the i node table and the free block table and other important file system information.

7. A certain file permissions: d-rw–r – -r -, with values expressed as to the authority, then the octal number: 644, the file attribute is a directory.

8. Foreground the process of starting to use Ctrl + c to terminate. 8. Foreground the process of starting to use Ctrl + c to terminate.

9. Static routing settings, if the network topology change is required by the system administrator to modify the routing settings.

10. The important task of network management: control and monitoring.

11. Install Linux system hard disk partition, there must be two kinds of partition types: file system partition and swap partition.

13. Prepared by Shell to run the script file must be given before the execute permission.

14. System management tasks is to be able to achieve in a distributed environment, programs and data security protection, backup, restore and update.

15. The system swap partition as a virtual memory system, a regional.

16. Kernel is divided into process management systems, memory management system, I / O management system and document management systems, four sub-systems.

17. Kernel configuration is the system administrator to change the system configuration hardware to conduct a major operation.

18. In the installation of Linux systems, use the netconfig program to configure the network, the installation program prompts the user for a step by step, type the host name, domain names, domain name server, IP address, gateway address and subnet mask and other necessary information.

19. That uniquely identifies each user is the user ID and user name.

20. RIP protocol is the most common is an internal agreement, commonly known as dynamic routing information protocol. Agreement is the most common is an internal agreement, commonly known as dynamic routing information protocol.

21. In the Linux system, all the contents are represented as files, organize files in various ways is called the file system.

22. DHCP dynamic IP address assignment can be achieved. 22. DHCP dynamic IP address assignment can be achieved.

23. System network management object is a server administrator, user and server processes and system resources.

24. Network management usually monitoring, transmission and management of three parts, of which management component of the entire network management center.

25. When you want to delete this system without access to the device driver must compile the kernel, when the kernel does not support the device driver on the system, you must upgrade to the kernel.

26 Ping command to test the network, the local systems are able to get to a remote host, so I often used to test network connectivity.

27. Vi editor has two operating modes: command mode and input mode.

28. Can use ls-al command to see the file permissions, the permissions of each file are in 10 bits and is divided into four sections, which accounted for a paragraph that file type, accounting for 3 second paragraph that the owner of the file permissions for the file.

29. The distinction between process and procedure lies in its dynamic nature, dynamic creation and termination of the process from creation to the termination of the basic state can have: Run state, ready state and wait state (blocked state).

30. DNS is actually distributed in the internet database of information on the host, its role is to achieve the IP address and host name conversion.

31. Apache is to achieve WWW server function of the application, known as the “Browse web server” in the server-side web services to provide users here is the apache application.

32. In the Linux system can have two types of backup: a backup system backup and user. Where the former refers to the operating system backup, which refers to applications and user files backed up. Where the former refers to the operating system backup, which refers to applications and user files backed up.

33. CD-ROM standard file system type is iso9660.

34. When the lilo.conf configuration has been completed, bring it into force, should run the command and parameters is lilo.

35. In using the ls command, use the octal display non-printing characters should be used to parameter-b.

36. Linux that supports Windows 9.x/2000 long file name file system type is vfat.

37. Set limits on the use of disk space a user command is quota.

38 In the Linux system, used to store system configuration files and subdirectories needed directory is /etc.

39. Hard link can only be built on the file link. Symbolic links across different file systems can be created. Symbolic links across different file systems can be created.

40. Socket file attribute bit is s.

41. The end of the background process command is kill.

42. There are two ways to run the process, that is independently run and use the parent process to run.

43. Links are divided into hard links and symbolic links.

44. In the super-user Linux system, under the display of all running processes, you should use the command and parameters are ps-aux.

45. Pipeline file attribute bit is p.

46. Will be the standard output of the previous command, after a command as standard input, called pipes.

47. For the implementation of the right of the specified script command and parameters are chmod a + x filename.

48. Remote login commands are telnet.

49. To send 10 packets packets abc.tuu.edu.cn test connectivity with the host, you should use the commands and parameters are: ping abc.tuu.edu.cn-c 10.

50. DNS server process named named, when it starts to automatically load / etc directory of the named.conf file partition defined in the DNS database file.

51. Apache server process configuration file is httpd.conf.

52. In the Linux system, the compressed files generated after the suffix. Gz file command is gzip.

53. Edit a file using vi, it will be put into the file test.txt file, should be the command mode, type: w test.txt.

54 can be displayed on standard output the whole year calendar orders and parameters are cal-y.

55. In shell programming, the use of square brackets indicate that the rules of test conditions are: there must be space on both sides of the square brackets.

56. Check the installed file system / dev/had5 is normal, if the check is wrong, then the auto-repair, the command and parameters are fsck-a / dev/had5.

57. Windows9.x environment shared Unix / Linux users in the directory tool is a Samba server.

58. System Administrator is responsible for system resource management, system performance management, device management, security management and system performance monitoring.

59 In the Linux system to test DNS server is able to correctly resolve the domain name of the client-side command, use the command nslookup.

60. In the Linux system, the second IDE channel hard disk (slave) was identified as hdb.

61. When the system administrator needs to upgrade the kernel version and change the system hardware configuration, should be re-compile the kernel.

62. If you just want to modify the system IP address, should be amended / etc/rc.d/rc.inet1 configuration file.

63. When the LAN there are no conditions for the establishment DNS server, but want to LAN users can use the computer name to visit each other, we should configure / etc / hosts file.

64. In the vi editor environment, using the Esc key to model.

65. Slackware Linux 9.0 typically use ext3 file system, the system’s total disk block consists of four parts.

66. To / home/stud1/naresh directory do archive compression, the compressed generated naresh.tar.gz file, and save this file to the / home directory, to achieve this task tar command format tar zcvf /home/naresh.tar.gz /home/stud1/naresh.

67. Pipeline from the former standard output as a command after a command standard input.

68. In the use of manual methods to configure the network, you can modify / etc / HOSTNAME file to change the host name, to configure the computer’s domain name resolution client, the need to configure / etc / resolv.conf file.

69. Startup process is to manually start and scheduling are two ways to start, where to start commonly used scheduling command at, batch, and crontab.

70. Test.bns.com.cn domain name is bns.com.cn, if you want to configure a domain name server should be defined in the named.conf file, DNS database, working directory.

71. Sendmail e-mail system uses two main protocols are: SMTP and POP, the former is used to send the message, which is used for receiving mail.

72. DHCP is short for Dynamic Host Configuration Protocol, its role is: To network host allocation of IP addresses.

73. At present the use of a proxy server software package there are a variety of teaching materials used in the squid.

74. Rm command to delete files or directories, the main difference is whether to use recursive switch-r or-R.

75. Mv command can move files and directories, you can also rename files and directories.

76. Routing Protocol (RIP) of the number of hops that must pass before the destination gateway number, RIP acceptable to jump the longest distance is 15.

77. Ping command is used to test network connectivity, ping command through the ICMP protocol (internet control message protocol) to achieve.

78. Nfs protocol used to implement Unix (/ linux) file system shared between hosts.
79. In the Linux operating system, devices are accessed through special files.

80. Shell is not only the user command interpreter, it is also a powerful programming language. Bash is the Linux default shell. bash is the Linux default shell.

81. Use “;”; symbol redirects the contents of the output appended to the back of the original.

82. To increase a user’s command is: adduser or useradd.

83 for string search using grep command.

84. Use * for each match the number of characters.

85. / Sbin directory is used to store the system administrator using the management procedure

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Linux interview questions and answers

What command is used to list the top 10 files / directories size wise?
for X in $(du -s * | sort -nr | cut -f 2); do du -hs $X ; done
I have a file named `-fr`. How do I get rid of it?
rm — -fr
rm \-rfWhat’s the difference between `telnet` and `ssh`? What’s a good use for each?
TELNET, by default, does not encrypt any data sent over the connection (including password, and so it is often practical to eavesdrop on the communications and use the password later for malicious purposes;
SSH by default encrypt password and traffic. SSH is recommended for all use.
What command will display the first several lines of a file called “junk”?
head junk
man head

To display a list of all manual pages containing the keyword “date”, what command would you type?
man -k date
man -f date

How can you find out how many users are currently logged into the database? How can you find their operating system id?
There are several ways. One is to look at the v$session or v$process views. Another way is to check the current_logins parameter in the v$sysstat view. Another if you are on UNIX is to do a “ps -ef|grep oracle|wc -l? command, but this only works against a single instance installation.

What are different categories of operating system?Answer:

OS Categories

Within the broad family of operating systems, there are generally four types, categorized based on the types of computers they control and the sort of applications they support. The broad categories are:
• Real-time operating system (RTOS) – Real-time operating systems are used to control machinery, scientific instruments and industrial systems. An RTOS typically has very little user-interface capability, and no end-user utilities, since the system will be a “sealed box” when delivered for use. A very important part of an RTOS is managing the resources of the computer so that a particular operation executes in precisely the same amount of time every time it occurs. In a complex machine, having a part move more quickly just because system resources are available may be just as catastrophic as having it not move at all because the system is busy.
• Single-user, single task – As the name implies, this operating system is designed to manage the computer so that one user can effectively do one thing at a time. The Palm OS for Palm handheld computers is a good example of a modern single-user, single-task operating system.
• Single-user, multi-tasking – This is the type of operating system most people use on their desktop and laptop computers today. Microsoft’s Windows and Apple’s MacOS platforms are both examples of operating systems that will let a single user have several programs in operation at the same time. For example, it’s entirely possible for a Windows user to be writing a note in a word processor while downloading a file from the Internet while printing the text of an e-mail message.
• Multi-user – A multi-user operating system allows many different users to take advantage of the computer’s resources simultaneously. The operating system must make sure that the requirements of the various users are balanced, and that each of the programs they are using has sufficient and separate resources so that a problem with one user doesn’t affect the entire community of users. Unix/LINUX , VMS and mainframe operating systems, such as MVS, are examples of multi-user operating systems.

Describe the golden history of LINUX OS.

Answer:

The roots of Linux can be traced back to the origins of Unix . In 1969, Ken Thompson of the Research Group at Bell Laboratories began experimenting on a multi-user, multi-tasking operating system using an otherwise idle PDP-7. He was soon joined by Dennis Richie and the two of them, along with other members of the Research Group produced the early versions of Unix. Richie was strongly influenced by an earlier project, MULTICS and the name Unix is itself a pun on the name MULTICS. Early versions were written in assembly code, but the third version was rewritten in a new programming language, C. C was designed and written by Richie expressly as a programming language for writing operating systems. This rewrite allowed Unix to move onto the more powerful PDP-11/45 and 11/70 computers then being produced by DIGITAL. The rest, as they say, is history. Unix moved out of the laboratory and into mainstream computing and soon most major computer manufacturers were producing their own versions.

Linux was the solution to a simple need. The only software that Linus Torvalds, Linux’s author and principle maintainer was able to afford was Minix. Minix is a simple, Unix like, operating system widely used as a teaching aid. Linus was less than impressed with its features, his solution was to write his own software. He took Unix as his model as that was an operating system that he was familiar with in his day to day student life. He started with an Intel 386 based PC and started to write. Progress was rapid and, excited by this, Linus offered his efforts to other students via the emerging world wide computer networks, then mainly used by the academic community. Others saw the software and started contributing. Much of this new software was itself the solution to a problem that one of the contributors had. Before long, Linux had become an operating system. It is important to note that Linux contains no Unix code, it is a rewrite based on published POSIX standards. Linux is built with and uses a lot of the GNU (GNU’s Not Unix ) software produced by the Free Software Foundation in Cambridge, Massachusetts.
What are different properties of Linux behind its popularity?

Answer:

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:
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. 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 teletransportation machines or time travel and sell your new code, as long as your customers can still have a copy of that code.

• Linux is portable to any hardware platform:
A vendor who wants to sell a new type of computer and who doesn’t know what kind of OS his new machine will run (say the CPU in your car or washing machine), can take a Linux kernel and make it work on his hardware, because documentation related to this activity is freely available.
• Linux was made to keep on running:
As with UNIX, a Linux system expects to run without rebooting all the time. That is why a lot of tasks are being executed at night or scheduled automatically for other calm moments, resulting in higher availability during busier periods and a more balanced use of the hardware. This property allows for Linux to be applicable also in environments where people don’t have the time or the possibility to control their systems night and day.
• Linux is secure and versatile:
The security model used in Linux is based on the UNIX idea of security, which is known to be robust and of proven quality. But Linux is not only fit for use as a fort against enemy attacks from the Internet: it will adapt equally to other situations, utilizing the same high standards for security. Your development machine or control station will be as secure as your firewall.
• Linux is scalable:
From a Palmtop with 2 MB of memory to a petabyte storage cluster with hundreds of nodes: add or remove the appropriate packages and Linux fits all. You don’t need a supercomputer anymore, because you can use Linux to do big things using the building blocks provided with the system. If you want to do little things, such as making an operating system for an embedded processor or just recycling your old 486, Linux will do that as well.

• The Linux OS and quite some Linux applications have very short debug-times:
Because Linux has been developed and tested by thousands of people, both errors and people to fix them are usually found rather quickly. It sometimes happens that there are only a couple of hours between discovery and fixing of a bug.

What are different popular Linux distributions?

Answer:

Linux distributions
• RedHat
• Fedora Core
• Debian
• SuSE Linux
• Mandriva (former MandrakeSoft)
• Knoppix: an operating system that runs from your CD-ROM, you don’t need to install anything.

How are devices represented in UNIX/Linux?

Answer:

All devices are represented by files called special files that are located in/dev directory. Thus, device files and other files are named and accessed in the same way. A ‘regular file’ is just an ordinary data file in the disk. A ‘block special file’ represents a device with characteristics similar to a disk (data transfer in terms of blocks). A ‘character special file’ represents a device with characteristics similar to a keyboard (data transfer is by stream of bits in sequential order).

What is ‘inode’?

Answer:

All UNIX files have its description stored in a structure called ‘inode’. The inode contains info about the file-size, its location, time of last access, time of last modification, permission and so on. Directories are also represented as files and have an associated inode. In addition to descriptions about the file, the inode contains pointers to the data blocks of the file. If the file is large, inode has indirect pointer to a block of pointers to additional data blocks (this further aggregates for larger files). A block is typically 8k.

Inode consists of the following fields:

* File owner identifier
* File type
* File access permissions
* File access times
* Number of links
* File size
* Location of the file data

Brief about the directory representation in UNIX

Answer:

A Unix directory is a file containing a correspondence between filenames and inodes. A directory is a special file that the kernel maintains. Only kernel modifies directories, but processes can read directories. The contents of a directory are a list of filename and inode number pairs. When new directories are created, kernel makes two entries named ‘.’ (refers to the directory itself) and ‘..’ (refers to parent directory).

System call for creating directory is mkdir (pathname, mode).

What are the Unix system calls for I/O?

Answer:

open(pathname,flag,mode) – open file
creat(pathname,mode) – create file
close(filedes) – close an open file
read(filedes,buffer,bytes) – read data from an open file
write(filedes,buffer,bytes) – write data to an open file
lseek(filedes,offset,from) – position an open file
dup(filedes) – duplicate an existing file descriptor
dup2(oldfd,newfd) – duplicate to a desired file descriptor
fcntl(filedes,cmd,arg) – change properties of an open file
ioctl(filedes,request,arg) – change the behaviour of an open file
The difference between fcntl anf ioctl is that the former is intended for any open file, while the latter is for device-specific operations.

How do you change File Access Permissions?

Answer:

Every file has following attributes:
owner’s user ID ( 16 bit integer )
owner’s group ID ( 16 bit integer )
File access mode word
‘r w x -r w x- r w x’
(user permission-group permission-others permission)
r-read, w-write, x-execute
To change the access mode, we use chmod(filename,mode).
Example 1:
To change mode of skpfile to ‘rw-rw-r–‘ (ie. read, write permission for user – read,write permission for group – only read permission for others) we give the args as:
chmod(skpfile,0664) .
Each operation is represented by discrete values
‘r’ is 4
‘w’ is 2
‘x’ is 1
Therefore, for ‘rw’ the value is 6(4+2).
Example 2:
To change mode of skpfile to ‘rwxr–r–‘ we give the args as:
chmod(skpfile,0744).

What are links and symbolic links in UNIX file system?

Answer:

A link is a second name (not a file) for a file. Links can be used to assign more than one name to a file, but cannot be used to assign a directory more than one name or link filenames on different computers.

Symbolic link ‘is’ a file that only contains the name of another file.Operation on the symbolic link is directed to the file pointed by the it.Both the limitations of links are eliminated in symbolic links.

Commands for linking files are:
Link: ln filename1 filename2
Symbolic link: ln -s filename1 filename2

What is a FIFO?

Answer:

FIFO are otherwise called as ‘named pipes’. FIFO (first-in-first-out) is a special file which is said to be data transient. Once data is read from named pipe, it cannot be read again. Also, data can be read only in the order written. It is used in interprocess communication where a process writes to one end of the pipe (producer) and the other reads from the other end (consumer).

How do you create special files like named pipes and device files?

Answer:
The system call mknod creates special files in the following sequence.

1. kernel assigns new inode,
2. sets the file type to indicate that the file is a pipe, directory or special file,
3. If it is a device file, it makes the other entries like major, minor device numbers.

For example:

If the device is a disk, major device number refers to the disk controller and minor device number is the disk.

Discuss the mount and unmount system calls

Answer:
The privileged mount system call is used to attach a file system to a directory of another file system; the unmount system call detaches a file system. When you mount another file system on to your directory, you are essentially splicing one directory tree onto a branch in another directory tree. The first argument to mount call is the mount point, that is , a directory in the current file naming system. The second argument is the file system to mount to that point. When you insert a cdrom to your unix system’s drive, the file system in the cdrom automatically mounts to /dev/cdrom in your system.

How does the inode map to data block of a file?

Answer:

Inode has 13 block addresses. The first 10 are direct block addresses of the first 10 data blocks in the file. The 11th address points to a one-level index block. The 12th address points to a two-level (double in-direction) index block. The 13th address points to a three-level(triple in-direction)index block. This provides a very large maximum file size with efficient access to large files, but also small files are accessed directly in one disk read.

What is a shell?

Answer:

A shell is an interactive user interface to an operating system services that allows an user to enter commands as character strings or through a graphical user interface. The shell converts them to system calls to the OS or forks off a process to execute the command. System call results and other information from the OS are presented to the user through an interactive interface. Commonly used shells are sh,csh,ksh etc.

Brief about the initial process sequence while the system boots up.

Answer:

While booting, special process called the ‘swapper’ or ‘scheduler’ is created with Process-ID 0. The swapper manages memory allocation for processes and influences CPU allocation. The swapper inturn creates 3 children:

the process dispatcher,
vhand and
dbflush

with IDs 1,2 and 3 respectively.
This is done by executing the file /etc/init. Process dispatcher gives birth to the shell. Unix keeps track of all the processes in an internal data structure called the Process Table (listing command is ps -el).

What are various IDs associated with a process?

Answer:

Unix identifies each process with a unique integer called ProcessID. The process that executes the request for creation of a process is called the ‘parent process’ whose PID is ‘Parent Process ID’. Every process is associated with a particular user called the ‘owner’ who has privileges over the process. The identification for the user is ‘UserID’. Owner is the user who executes the process. Process also has ‘Effective User ID’ which determines the access privileges for accessing resources like files.

getpid() -process id
getppid() -parent process id
getuid() -user id
geteuid() -effective user id

Explain fork() system call.

Answer:

The `fork()’ used to create a new process from an existing process. The new process is called the child process, and the existing process is called the parent. We can tell which is which by checking the return value from `fork()’. The parent gets the child’s pid returned to him, but the child gets 0 returned to him.
Predict the output of the following program code

main()
{
fork();
printf(“Hello World!”);
}

Answer:

Hello World!Hello World!

Explanation:

The fork creates a child that is a duplicate of the parent process. The child begins from the fork().All the statements after the call to fork() will be executed twice.(once by the parent process and other by child). The statement before fork() is executed only by the parent process.

Predict the output of the following program code

main()
{
fork(); fork(); fork();
printf(“Hello World!”);
}

Answer:

“Hello World” will be printed 8 times.

Explanation:

2^n times where n is the number of calls to fork()

List the system calls used for process management:

Answer:

System calls Description

fork() To create a new process
exec() To execute a new program in a process
wait() To wait until a created process completes its execution
exit() To exit from a process execution
getpid() To get a process identifier of the current process
getppid() To get parent process identifier
nice() To bias the existing priority of a process
brk() To increase/decrease the data segment size of a process

How can you get/set an environment variable from a program?:

Answer:

Getting the value of an environment variable is done by using `getenv()’. Setting the value of an environment variable is done by using `putenv()’.

How can a parent and child process communicate?

Answer:

A parent and child can communicate through any of the normal inter-process communication schemes (pipes, sockets, message queues, shared memory), but also have some special ways to communicate that take advantage of their relationship as a parent and child. One of the most obvious is that the parent can get the exit status of the child.

What is a zombie?

Answer:

When a program forks and the child finishes before the parent, the kernel still keeps some of its information about the child in case the parent might need it – for example, the parent may need to check the child’s exit status. To be able to get this information, the parent calls `wait()’; In the interval between the child terminating and the parent calling `wait()’, the child is said to be a `zombie’ (If you do `ps’, the child will have a `Z’ in its status field to indicate this.)

What are the process states in Unix?

Answer:

As a process executes it changes state according to its circumstances. Unix processes have the following states:

Running : The process is either running or it is ready to run .
Waiting : The process is waiting for an event or for a resource.
Stopped : The process has been stopped, usually by receiving a signal.
Zombie : The process is dead but have not been removed from the process table.

What Happens when you execute a program?

Answer:

When you execute a program on your UNIX system, the system creates a special environment for that program. This environment contains everything needed for the system to run the program as if no other program were running on the system. Each process has process context, which is everything that is unique about the state of the program you are currently running. Every time you execute a program the UNIX system does a fork, which performs a series of operations to create a process context and then execute your program in that context. The steps include the following:

Allocate a slot in the process table, a list of currently running programs kept by UNIX.
Assign a unique process identifier (PID) to the process.
iCopy the context of the parent, the process that requested the spawning of the new process.
Return the new PID to the parent process. This enables the parent process to examine or control the process directly. After the fork is complete, UNIX runs your program.

What Happens when you execute a command?

Answer:

When you enter ‘ls’ command to look at the contents of your current working directory, UNIX does a series of things to create an environment for ls and the run it: The shell has UNIX perform a fork. This creates a new process that the shell will use to run the ls program. The shell has UNIX perform an exec of the ls program. This replaces the shell program and data with the program and data for ls and then starts running that new program. The ls program is loaded into the new process context, replacing the text and data of the shell. The ls program performs its task, listing the contents of the current directory.

What is a Daemon?

Answer:

A daemon is a process that detaches itself from the terminal and runs, disconnected, in the background, waiting for requests and responding to them. It can also be defined as the background process that does not belong to a terminal session. Many system functions are commonly performed by daemons, including the sendmail daemon, which handles mail, and the NNTP daemon, which handles USENET news. Many other daemons may exist. Some of the most common daemons are:

init: Takes over the basic running of the system when the kernel has finished the boot process.

inetd: Responsible for starting network services that do not have their own stand-alone daemons. For example, inetd usually takes care of incoming rlogin, telnet, and ftp connections.

cron: Responsible for running repetitive tasks on a regular schedule.

What is ‘ps’ command for?

Answer:

The ps command prints the process status for some or all of the running processes. The information given are the process identification number (PID),the amount of time that the process has taken to execute so far etc.

How would you kill a process?

Answer:

The kill command takes the PID as one argument; this identifies which process to terminate. The PID of a process can be got using ‘ps’ command.

What is an advantage of executing a process in background?

Answer:

The most common reason to put a process in the background is to allow you to do something else interactively without waiting for the process to complete. At the end of the command you add the special background symbol, &. This symbol tells your shell to execute the given command in the background.
Example: cp *.* ../backup& (cp is for copy)

How do you execute one program from within another?

Answer:

The system calls used for low-level process creation are execlp() and execvp(). The execlp call overlays the existing program with the new one , runs that and exits. The original program gets back control only when an error occurs. execlp(path,file_name,arguments..); //last argument must be NULL A variant of execlp called execvp is used when the number of arguments is not known in advance. execvp(path,argument_array); //argument array should be terminated by NULL

What is IPC? What are the various schemes available?

Answer:

The term IPC (Inter-Process Communication) describes various ways by which different process running on some operating system communicate between each other. Various schemes available are as follows: Pipes:
One-way communication scheme through which different process can communicate. The problem is that the two processes should have a common ancestor (parent-child relationship). However this problem was fixed with the introduction of named-pipes (FIFO).

Message Queues :

Message queues can be used between related and unrelated processes running on a machine.

Shared Memory:

This is the fastest of all IPC schemes. The memory to be shared is mapped into the address space of the processes (that are sharing). The speed achieved is attributed to the fact that there is no kernel involvement. But this scheme needs synchronization.
Various forms of synchronisation are mutexes, condition-variables, read-write locks, record-locks, and semaphores.

What is the difference between Swapping and Paging?

Answer:

Swapping: Whole process is moved from the swap device to the main memory for execution. Process size must be less than or equal to the available main memory. It is easier to implementation and overhead to the system. Swapping systems does not handle the memory more flexibly as compared to the paging systems.

Paging:

Only the required memory pages are moved to main memory from the swap device for execution. Process size does not matter. Gives the concept of the virtual memory.

It provides greater flexibility in mapping the virtual address space into the physical memory of the machine. Allows more number of processes to fit in the main memory simultaneously. Allows the greater process size than the available physical memory. Demand paging systems handle the memory more flexibly.

What is major difference between the Historic Unix and the new BSD release of Unix System V in terms of Memory Management?

Answer:
Historic Unix uses Swapping – entire process is transferred to the main memory from the swap device, whereas the Unix System V uses Demand Paging – only the part of the process is moved to the main memory. Historic Unix uses one Swap Device and Unix System V allow multiple Swap Devices.

What is the main goal of the Memory Management?

Answer:

It decides which process should reside in the main memory, Manages the parts of the virtual address space of a process which is non-core resident, Monitors the available main memory and periodically write the processes into the swap device to provide more processes fit in the main memory simultaneously.

What is a Map?

Answer:

A Map is an Array, which contains the addresses of the free space in the swap device that are allocatable resources, and the number of the resource units available there.

This allows First-Fit allocation of contiguous blocks of a resource. Initially the Map contains one entry – address (block offset from the starting of the swap area) and the total number of resources. Kernel treats each unit of Map as a group of disk blocks. On the allocation and freeing of the resources Kernel updates the Map for accurate information.

What scheme does the Kernel in Unix System V follow while choosing a swap device among the multiple swap devices?

Answer:

Kernel follows Round Robin scheme choosing a swap device among the multiple swap devices in Unix System V.

What is a Region?

Answer:

A Region is a continuous area of a process’s address space (such as text, data and stack). The kernel in a ‘Region Table’ that is local to the process maintains region. Regions are sharable among the process.

What are the events done by the Kernel after a process is being swapped out from the main memory?

Answer:

When Kernel swaps the process out of the primary memory, it performs the following:
Kernel decrements the Reference Count of each region of the process. If the reference count becomes zero, swaps the region out of the main memory,
Kernel allocates the space for the swapping process in the swap device,
Kernel locks the other swapping process while the current swapping operation is going on,
The Kernel saves the swap address of the region in the region table.

Is the Process before and after the swap are the same? Give reason.

Answer:

Process before swapping is residing in the primary memory in its original form. The regions (text, data and stack) may not be occupied fully by the process, there may be few empty slots in any of the regions and while swapping Kernel do not bother about the empty slots while swapping the process out. After swapping the process resides in the swap (secondary memory) device. The regions swapped out will be present but only the occupied region slots but not the empty slots that were present before assigning. While swapping the process once again into the main memory, the Kernel referring to the Process Memory Map, it assigns the main memory accordingly taking care of the empty slots in the regions.

What do you mean by u-area (user area) or u-block?

Answer:

This contains the private data that is manipulated only by the Kernel. This is local to the Process, i.e. each process is allocated a u-area.

What are the entities that are swapped out of the main memory while swapping the process out of the main memory ?

Answer:

All memory space occupied by the process, process’s u-area, and Kernel stack are swapped out, theoretically. Practically, if the process’s u-area contains the Address Translation Tables for the process then Kernel implementations do not swap the u-area.

What is Fork swap?

Answer:

fork() is a system call to create a child process. When the parent process calls fork() system call, the child process is created and if there is short of memory then the child process is sent to the read-to-run state in the swap device, and return to the user state without swapping the parent process. When the memory will be available the child process will be swapped into the main memory.

What is Expansion swap?

Answer:

At the time when any process requires more memory than it is currently allocated, the Kernel performs Expansion swap. To do this Kernel reserves enough space in the swap device. Then the address translation mapping is adjusted for the new virtual address space but the physical memory is not allocated. At last Kernel swaps the process into the assigned space in the swap device. Later when the Kernel swaps the process into the main memory this assigns memory according to the new address translation mapping.

How the Swapper works?

Answer:

The swapper is the only process that swaps the processes. The Swapper operates only in the Kernel mode and it does not uses System calls instead it uses internal Kernel functions for swapping. It is the archetype of all kernel process.

What are the processes that are not bothered by the swapper? Give Reason.

Answer:

Zombie process: They do not take any up physical memory.
Processes locked in memories that are updating the region of the process.
Kernel swaps only the sleeping processes rather than the ‘ready-to-run’ processes, as they have the higher probability of being scheduled than the Sleeping processes.

What are the requirements for a swapper to work?

Answer:

The swapper works on the highest scheduling priority. Firstly it will look for any sleeping process, if not found then it will look for the ready-to-run process for swapping. But the major requirement for the swapper to work the ready-to-run process must be core-resident for at least 2 seconds before swapping out. And for swapping in the process must have been resided in the swap device for at least 2 seconds. If the requirement is not satisfied then the swapper will go into the wait state on that event and it is awaken once in a second by the Kernel.

What are the criteria for choosing a process for swapping into memory from the swap device?

Answer:
The resident time of the processes in the swap device, the priority of the processes and the amount of time the processes had been swapped out.

What are the criteria for choosing a process for swapping out of the memory to the swap device?

Answer:

The process’s memory resident time,
Priority of the process and
The nice value.

What do you mean by nice value?

Answer:

Nice value is the value that controls {increments or decrements} the priority of the process. This value that is returned by the nice () system call. The equation for using nice value is: Priority = (“recent CPU usage”/constant) + (base- priority) + (nice value) Only the administrator can supply the nice value. The nice () system call works for the running process only. Nice value of one process cannot affect the nice value of the other process.

What are conditions on which deadlock can occur while swapping the processes?

Answer:

All processes in the main memory are asleep.
All ‘ready-to-run’ processes are swapped out.
There is no space in the swap device for the new incoming process that are swapped out of the main memory.
There is no space in the main memory for the new incoming process.

What are conditions for a machine to support Demand Paging?

Answer:

Memory architecture must based on Pages,
The machine must support the ‘restartable’ instructions.

What is ‘the principle of locality’?

Answer:

It’s the nature of the processes that they refer only to the small subset of the total data space of the process. i.e. the process frequently calls the same subroutines or executes the loop instructions.

What is the working set of a process?

Answer:
The set of pages that are referred by the process in the last ‘n’, references, where ‘n’ is called the window of the working set of the process.

What is the window of the working set of a process?

Answer:

The window of the working set of a process is the total number in which the process had referred the set of pages in the working set of the process.

What is called a page fault?

Answer:

Page fault is referred to the situation when the process addresses a page in the working set of the process but the process fails to locate the page in the working set. And on a page fault the kernel updates the working set by reading the page from the secondary device.

What are data structures that are used for Demand Paging?

Kernel contains 4 data structures for Demand paging. They are,
Page table entries,
Disk block descriptors,
Page frame data table (pfdata),
Swap-use table.

What are the bits that support the demand paging?

Answer:

Valid, Reference, Modify, Copy on write, Age. These bits are the part of the page table entry, which includes physical address of the page and protection bits.
Page address
Age
Copy on write
Modify
Reference
Valid
Protection

How the Kernel handles the fork() system call in traditional Unix and in the System V Unix, while swapping?

Answer:

Kernel in traditional Unix, makes the duplicate copy of the parent’s address space and attaches it to the child’s process, while swapping. Kernel in System V Unix, manipulates the region tables, page table, and pfdata table entries, by incrementing the reference count of the region table of shared regions.

Difference between the fork() and vfork() system call?

Answer:

During the fork() system call the Kernel makes a copy of the parent process’s address space and attaches it to the child process. But the vfork() system call do not makes any copy of the parent’s address space, so it is faster than the fork() system call. The child process as a result of the vfork() system call executes exec() system call. The child process from vfork() system call executes in the parent’s address space (this can overwrite the parent’s data and stack ) which suspends the parent process until the child process
exits.

What is BSS(Block Started by Symbol)?

Answer:

A data representation at the machine level, that has initial values when a program starts and tells about how much space the kernel allocates for the un-initialized data. Kernel initializes it to zero at run-time.

What is Page-Stealer process?

Answer:

This is the Kernel process that makes rooms for the incoming pages, by swapping the memory pages that are not the part of the working set of a process. Page-Stealer is created by the Kernel at the system initialization and invokes it throughout the lifetime of the system. Kernel locks a region when a process faults on a page in the region, so that page stealer cannot steal the page, which is being faulted in.

Name two paging states for a page in memory?

Answer:

The two paging states are:

The page is aging and is not yet eligible for swapping,
The page is eligible for swapping but not yet eligible for reassignment to other virtual address space.

What are the phases of swapping a page from the memory?

Answer:

Page stealer finds the page eligible for swapping and places the page number in the list of pages to be swapped. Kernel copies the page to a swap device when necessary and clears the valid bit in the page table entry, decrements the pfdata reference count, and places the pfdata table entry at the end of the free list if its reference count is 0.

What is page fault? Its types?

Answer:

Page fault refers to the situation of not having a page in the main memory when any process references it. There are two types of page fault :

Validity fault,
Protection fault.

In what way the Fault Handlers and the Interrupt handlers are different?

Answer:

Fault handlers are also an interrupt handler with an exception that the interrupt handlers cannot sleep. Fault handlers sleep in the context of the process that caused the memory fault. The fault refers to the running process and no arbitrary processes are put to sleep.

What is validity fault?

Answer:

If a process referring a page in the main memory whose valid bit is not set, it results in validity fault. The valid bit is not set for those pages:

that are outside the virtual address space of a process,
that are the part of the virtual address space of the process but no physical address is assigned to it.

What does the swapping system do if it identifies the illegal page for swapping?

Answer:

If the disk block descriptor does not contain any record of the faulted page, then this causes the attempted memory reference is invalid and the kernel sends a “Segmentation violation” signal to the offending process. This happens when the swapping system identifies any invalid memory reference.

What are states that the page can be in, after causing a page fault?

Answer:

On a swap device and not in memory,
On the free page list in the main memory,
In an executable file,
Marked “demand zero”,
Marked “demand fill”.

In what way the validity fault handler concludes?

Answer:

It sets the valid bit of the page by clearing the modify bit.
It recalculates the process priority.

At what mode the fault handler executes?

Answer:
At the Kernel Mode.

What do you mean by the protection fault?

Answer:

Protection fault refers to the process accessing the pages, which do not have the access permission. A process also incur the protection fault when it attempts to write a page whose copy on write bit was set during the fork() system call.

How the Kernel handles the copy on write bit of a page, when the bit is set?

Answer:

In situations like, where the copy on write bit of a page is set and that page is shared by more than one process, the Kernel allocates new page and copies the content to the new page and the other processes retain their references to the old page. After copying the Kernel updates the page table entry with the new page number. Then Kernel decrements the reference count of the old pfdata table entry. In cases like, where the copy on write bit is set and no processes are sharing the page, the Kernel allows the physical page to be reused by the processes. By doing so, it clears the copy on write bit and disassociates the page from its disk copy (if one exists), because other process may share the disk copy. Then it removes the pfdata table entry from the page-queue as the new copy of the virtual page is not on the swap device. It decrements the swap-use count for the page and if count drops to 0, frees the swap space.

For which kind of fault the page is checked first?

Answer:

The page is first checked for the validity fault, as soon as it is found that the page is invalid (valid bit is clear), the validity fault handler returns immediately, and the process incur the validity page fault. Kernel handles the validity fault and the process will incur the protection fault if any one is present.

In what way the protection fault handler concludes?

Answer:

After finishing the execution of the fault handler, it sets the modify and protection bits and clears the copy on write bit. It recalculates the process-priority and checks for signals.

How the Kernel handles both the page stealer and the fault handler?

Answer:

The page stealer and the fault handler thrash because of the shortage of the memory. If the sum of the working sets of all processes is greater that the physical memory then the fault handler will usually sleep because it cannot allocate pages for a process. This results in the reduction of the system throughput because Kernel spends too much time in overhead, rearranging the memory in the frantic pace.

Explain different types of Unix systems.

Answer:

The most widely used are: 1. System V (AT&T) 2. AIX (IBM) 3. BSD (Berkeley) 4. Solaris (Sun) 5. Xenix ( A PC version of Unix)

Explain kernel and shell.

Answer:

Kernal: It carries out basic operating system functions such as allocating memory, accessing files and handling communications.

Shell:A shell provides the user interface to the kernel.There are 3 major shells : C-shell, Bourne shell , Korn shell

What is ex and vi ?

Answer:
ex is Unix line editor and vi is the standard Unix screen editor.

Which are typical system directories below the root directory?

Answer:

(1)/bin: contains many programs which will be executed by users (2)/etc : files used by administrator (3)/dev: hardware devices (4)/lib: system libraries (5)/usr: application software (6)/home: home directories for different systems.

Construct pipes to execute the following jobs.

Answer:

1. Output of who should be displayed on the screen with value of total number of users who have logged in displayed at the bottom of the list.

2. Output of ls should be displayed on the screen and from this output the lines containing the word ‘poem’ should be counted and the count should be stored in a file.

3. Contents of file1 and file2 should be displayed on the screen and this output should be appended in a file
.From output of ls the lines containing ‘poem’ should be displayed on the screen along with the count.

4. Name of cities should be accepted from the keyboard . This list should be combined with the list present in a file. This combined list should be sorted and the sorted list should be stored in a file ‘newcity’.

5. All files present in a directory dir1 should be deleted any error while deleting should be stored in a file ‘errorlog’.

What is the significance of the “tee” command?

Answer:

It reads the standard input and sends it to the standard output while redirecting a copy of what it has read to the file specified by the user.

What does the command “ $who | sort –logfile > newfile” do?

Answer:

The input from a pipe can be combined with the input from a file . The trick is to use the special symbol “-“ (a hyphen) for those commands that recognize the hyphen as std input.
In the above command the output from who becomes the std input to sort , meanwhile sort opens the file logfile, the contents of this file is sorted together with the output of who (rep by the hyphen) and the sorted output is redirected to the file newfile.

What does the command “$ls | wc –l > file1” do?

Answer:

ls becomes the input to wc which counts the number of lines it receives as input and instead of displaying this count , the value is stored in file1.

Explain the steps that a shell follows while processing a command.

Answer:

After the command line is terminated by the key, the shell goes ahead with processing the command line in one or more passes. The sequence is well defined and assumes the following order.

Parsing: The shell first breaks up the command line into words, using spaces and the delimiters, unless quoted. All consecutive occurrences of a space or tab are replaced here with a single space.

Variable evaluation: All words preceded by a $ are evaluated as variables, unless quoted or escaped.

Command substitution: Any command surrounded by back quotes is executed by the shell which then replaces the standard output of the command into the command line.

Wild-card interpretation: The shell finally scans the command line for wild-cards (the characters *, ?, [, ]).

Any word containing a wild-card is replaced by a sorted list of
filenames that match the pattern. The list of these filenames then forms the arguments to the command.

PATH evaluation: It finally looks for the PATH variable to determine the sequence of directories it has to search in order to hunt for the command.

What difference between cmp and diff commands?

Answer:

cmp – Compares two files byte by byte and displays the first mismatch diff – tells the changes to be made to make the files identical

What is the use of ‘grep’ command?

Answer:

‘grep’ is a pattern search command. It searches for the pattern, specified in the command line with appropriate option, in a file(s).

Syntax : grep

Example : grep 99sk skpmcafile

What is the difference between cat and more command?

Answer:

Cat displays file contents. If the file is large the contents scroll off the screen before we view it. So command ‘more’ is like a pager which displays the contents page by page.

Write a command to kill the last background job?

Answer:

Kill $!

Which command is used to delete all files in the current directory and all its sub-directories?

Answer:

rm -r *

Write a command to display a file’s contents in various formats?

Answer:

$od -cbd file_name
c – character, b – binary (octal), d-decimal, od=Octal Dump.

What will the following command do?

Answer:

$ echo *
It is similar to ‘ls’ command and displays all the files in the current directory.

Is it possible to create new a file system in UNIX?

Yes, ‘mkfs’ is used to create a new file system.

Is it possible to restrict incoming message?

Answer:

Yes, using the ‘mesg’ command.

What is the use of the command “ls -x chapter[1-5]”

Answer:
ls stands for list; so it displays the list of the files that starts with ‘chapter’ with suffix ‘1’ to ‘5’, chapter1, chapter2, and so on.

Is ‘du’ a command? If so, what is its use?

Answer:

Yes, it stands for ‘disk usage’. With the help of this command you can find the disk capacity and free space of the disk.

Is it possible to count number char, line in a file; if so, How?

Answer:

Yes, wc-stands for word count.
wc -c for counting number of characters in a file.
wc -l for counting lines in a file.

Name the data structure used to maintain file identification?

Answer:

‘inode’, each file has a separate inode and a unique inode number.

How many prompts are available in a UNIX system?

Answer:

Two prompts, PS1 (Primary Prompt), PS2 (Secondary Prompt).

How does the kernel differentiate device files and ordinary files?

Answer:

Kernel checks ‘type’ field in the file’s inode structure.

How to switch to a super user status to gain privileges?

Answer:

Use ‘su’ command. The system asks for password and when valid entry is made the user gains super user (admin) privileges.

What are shell variables?

Answer:

Shell variables are special variables, a name-value pair created and maintained by the shell.
Example: PATH, HOME, MAIL and TERM

What is redirection?

Answer:

Directing the flow of data to the file or from the file for input or output.
Example : ls > wc

How to terminate a process which is running and the specialty on command kill 0?

Answer:

With the help of kill command we can terminate the process.

Syntax: kill pid

Kill 0 – kills all processes in your system except the login shell.

What is a pipe and give an example?

Answer:

A pipe is two or more commands separated by pipe char ‘|’. That tells the shell to arrange for the output of the preceding command to be passed as input to the following command.

Example : ls -l | pr

The output for a command ls is the standard input of pr.

When a sequence of commands are combined using pipe, then it is called pipeline.

Explain kill() and its possible return values.

Answer:

There are four possible results from this call:
‘kill()’ returns 0. This implies that a process exists with the given PID, and the system would allow you to send signals to it. It is system-dependent whether the process could be a zombie.

‘kill()’ returns -1, ‘errno == ESRCH’ either no process exists with the given PID, or security enhancements are causing the system to deny its existence. (On some systems, the process could be a zombie.)

‘kill()’ returns -1, ‘errno == EPERM’ the system would not allow you to kill the specified process. This means that either the process exists (again, it could be a zombie) or draconian security enhancements are present (e.g. your process is not allowed to send signals to *anybody*).

‘kill()’ returns -1, with some other value of ‘errno’ you are in trouble! The most-used technique is to assume that success or failure with ‘EPERM’ implies that the process exists, and any other error implies that it doesn’t.

An alternative exists, if you are writing specifically for a system (or all those systems) that provide a ‘/proc’ filesystem: checking for the existence of ‘/proc/PID’ may work.

What is LILO?
LILO stands for Linux boot loader. It will load the MBR, master boot record, into the memory, and tell the system which partition and hard drive to boot from.
What is the main advantage of creating links to a file instead of copies of the file?
A: The main advantage is not really that it saves disk space (though it does that too) but, rather, that a change of permissions on the file is applied to all the link access points. The link will show permissions of lrwxrwxrwx but that is for the link itself and not the access to the file to which the link points. Thus if you want to change the permissions for a command, such as su, you only have to do it on the original. With copies you have to find all of the copies and change permission on each of the copies.
Write a command to find all of the files which have been accessed within the last 30 days.

find / -type f -atime -30 > December.files

This command will find all the files under root, which is ‘/’, with file type is file. ‘-atime -30′ will give all the files accessed less than 30 days ago. And the output will put into a file call December.files.
What is the most graceful way to get to run level single user mode?
A: The most graceful way is to use the command init s.
If you want to shut everything down before going to single user mode then do init 0 first and from the ok prompt do a boot -s.
What does the following command line produce? Explain each aspect of this line.

$ (date ; ps -ef | awk ‘{print $1}’ | sort | uniq | wc -l ) >> Activity.log

A: First let’s dissect the line: The date gives the date and time as the first command of the line, this is followed by the a list of all running processes in long form with UIDs listed first, this is the ps -ef. These are fed into the awk which filters out all but the UIDs; these UIDs are piped into sort for no discernible reason and then onto uniq (now we see the reason for the sort – uniq only works on sorted data – if the list is A, B, A, then A, B, A will be the output of uniq, but if it’s A, A, B then A, B is the output) which produces only one copy of each UID. These UIDs are fed into wc -l which counts the lines – in this case the number of distinct UIDs running processes on the system. Finally the results of these two commands, the date and the wc -l, are appended to the file “Activity.log”. Now to answer the question as to what this command line produces. This writes the date and time into the file Activity.log together with the number of distinct users who have processes running on the system at that time. If the file already exists, then these items are appended to the file, otherwise the file is created.

How to Configure YUM repository in RHEL 5
Installing packages and configuring Servers we need to configure the YellowDog Update Manager (YUM)
First,

1 If its local repository then
——————————————————————————————Install Package createrepo ( this pacakge is present in RHEL 5 dvd in Server folder)

RPM -ivh createrepo

createrepo -v /var/ftp/pub/Server

/var/ftp/pub/Server => this is the directory where local copy of RPM packages are there

Edit the Server.repo file
# vi Server.repo
[Server]
name= redhat
baseurl=file:///var/ftp/pub/Server
[enable=1]
[gpgcheck=0]
Save the file and quit
Then,
#yum clean all
#yum update
Now you can install packages
#yum install package-name
2 If repository is present in server
———————————————————————————————–
#cd /etc/yum.repos.d
#ls
Then it will list the files. There is a file with name rhel-debuginfo.repo. Copy this file to a temp file with name Sever.repo. The starting letter of Server should be Capital.
Second step is given below

#cp rhel-debuginfo.repo Server.repo
The Third and final steps
Edit the Server.repo file
# vi Server.repo
[Server]
[name= redhat]
[baseurl=ftp://192.168.0.254/pub/Server] ( eg: IP address : 192.168.0.254 (the server IP)
[enable=1]
[gpgcheck=0]
Save the file and quit
Then,
#yum clean all
#yum update
Now you can install packages
#yum install package-name

Configuring YUM behind the HTTP proxy

Under Normal Conditions yum dont work if you are suing internet behind proxy

So in yum.conf add
#The proxy server or IP address:port number

proxy=http://proxy.server.com:8080

Below one if you use ( username/password)

proxy_username=userid
proxy_password=password
Mount windows partition in linux
First find out partition detail,

#fdisk -l

#mkdir /win

#mount -t msdos /dev/sda1 /win …..(If partition is FAT32)

-or-

#mount -t ntfs /dev/sda1 /win …..(If partition is NTFS)

#cd /win

Note: If your are using RHEL 3,4,5 then NTFS mounting will not be Supported. For that you need to complile a new kernel.
Difference between a database and an instance
The terms instance and database are closely related, but not the same thing.

The database is the set of files where application data (the reason for a database) and meta data is stored.

An instance is the software (and memory) that Oracle uses to manipulate the data in the database. In order for the instance to be able to manipulate that data, the instance must open the database. A database can be opened (or mounted) by more than one instance, however, an instance can open at most one database.
What is a instance
Database programs, with few exceptions, need to utilize both the computer’s memory and permanent storage space such as the hard drive to operate. The drives provide both long-term storage and the necessary room for millions of records and gigabytes worth of information.

However, accessing information from disks is much slower than retrieving the same information from memory. Therefore, database engines use memory to cache information, which speeds its retrieval.
The complexity of how the information is stored and where it is retrieved from is hidden from the casual user who queries the database. But if you plan on administering Oracle, you need to become familiar with how Oracle handles both resources.

An instance is the (executed) Oracle software (Oracle processes) and the memory they use (SGA). It is the instance that manipulates the data stored in the database.

It acts as a bridge between the database and the filesystem
Oracle Architecture simplified
Oracle = Instance + Database File

1A) Instance [ SGA + Background Services]
2A) database file [ control file, datafile , redolog file]

SGA
1)Share pool [ library cache , data dictonary cache]
2)database buffer cache
3)Redolog buffer

Background servies
1) Pmon
2) Smon
3) DBWR
4) LGWR
5) CKPT
Command that every Administrator shoud know [Dig command]
DIG(Domian information groper)

It is a DNS lookup utility.
It ping’s DNS name servers.It performs DNS lookups and displays the answers that are returned from the name Server(s).
It is a flexible command so many admin’s use it.

Unless explictly specified dig will try to get servers listed in

/etc/resolv.conf

Simple usage

dig @server name type

server:
It is name or ip address of the name server to query.
If no server argument is provided, dig consults /etc/resolv.conf and queries the name servers listed there

name:
It is the name of the resource record that is to be looked up.
type:
indicates what type of query is required – ANY, A, MX, SIG, etc. type can be any valid query type. If no type argument is supplied, dig will perform a lookup for an A record.
Name as many shells as you can .
Bourne shell (sh)
Almquist shell (ash)
Debian Almquist shell (dash)
Bourne-Again shell (bash)
Friendly interactive shell (fish)
Korn shell (ksh)
C shell (csh)
TENEX C shell (tcsh)
Es shell (e
esh (Unix) Easy Shell
rc shell (rc) – shell for Plan 9 and Unix
runscript The initial shell interpreter used to process startup scripts in Gentoo
scsh (Scheme Shell)
Stand-alone Shell (sash)
Z shell (zsh)
How many processes are running on your machine right now?
Code:
top
atop
ps -e
ps aux
ps aux | wc -l
man ps

What is a zombie?
When a program forks and the child finishes before the parent, the kernel still keeps some of its information about the child in case the parent might need it – for example, the parent may need to check the child’s exit status. To be able to get this information, the parent calls `wait()’; In the interval between the child terminating and the parent calling `wait()’, the child is said to be a `zombie’ (If you do `ps’, the child will have a `Z’ in its status field to indicate this.)
How can a parent and child process communicate?
A parent and child can communicate through any of the normal inter-process communication schemes (pipes, sockets, message queues, shared memory), but also have some special ways to communicate that take advantage of their relationship as a parent and child. One of the most obvious is that the parent can get the exit status of the child.
How can you get/set an environment variable from a program?
Getting the value of an environment variable is done by using `getenv()’. Setting the value of an environment variable is done by using `putenv()’.
List the system calls used for process management
System calls Description
• fork() To create a new process
• exec() To execute a new program in a process
• wait() To wait until a created process completes its execution
• exit() To exit from a process execution
• getpid() To get a process identifier of the current process
• getppid() To get parent process identifier
• nice() To bias the existing priority of a process
• brk() To increase/decrease the data segment size of a process.
Explain fork() system call.

The `fork()’ used to create a new process from an existing process. The new process is called the child process, and the existing process is called the parent. We can tell which is which by checking the return value from `fork()’. The parent gets the child’s pid returned to him, but the child gets 0 returned to him.

What are various IDs associated with a process?
Unix identifies each process with a unique integer called ProcessID. The process that executes the request for creation of a process is called the ‘parent process’ whose PID is ‘Parent Process ID’. Every process is associated with a particular user called the ‘owner’ who has privileges over the process. The identification for the user is ‘UserID’. Owner is the user who executes the process. Process also has ‘Effective User ID’ which determines the access privileges for accessing resources like files.
• getpid() -process id
• getppid() -parent process id
• getuid() -user id
• geteuid() -effective user id
Brief about the initial process sequence while the system boots up.
While booting, special process called the ‘swapper’ or ‘scheduler’ is created with Process-ID 0. The swapper manages memory allocation for processes and influences CPU allocation. The swapper inturn creates 3 children:
• the process dispatcher,
• vhand and
• dbflush
with IDs 1,2 and 3 respectively.
This is done by executing the file /etc/init. Process dispatcher gives birth to the shell. Unix keeps track of all the processes in an internal data structure called the Process Table (listing command is ps -el).
What is a shell?
A shell is an interactive user interface to an operating system services that allows an user to enter commands as character strings or through a graphical user interface. The shell converts them to system calls to the OS or forks off a process to execute the command. System call results and other information from the OS are presented to the user through an interactive interface. Commonly used shells are sh,csh,ks etc.
How does the inode map to data block of a file?
Inode has 13 block addresses. The first 10 are direct block addresses of the first 10 data blocks in the file. The 11th address points to a one-level index block. The 12th address points to a two-level (double in-direction) index block. The 13th address points to a three-level(triple in-direction)index block. This provides a very large maximum file size with efficient access to large files, but also small files are accessed directly in one disk read.
Discuss the mount and unmount system calls
The privileged mount system call is used to attach a file system to a directory of another file system; the unmount system call detaches a file system. When you mount another file system on to your directory, you are essentially splicing one directory tree onto a branch in another directory tree. The first argument to mount call is the mount point, that is , a directory in the current file naming system. The second argument is the file system to mount to that point. When you insert a cdrom to your unix system’s drive, the file system in the cdrom automatically mounts to /dev/cdrom in your system.
How do you create special files like named pipes and device files?
The system call mknod creates special files in the following sequence.
1. kernel assigns new inode,
2. sets the file type to indicate that the file is a pipe, directory or special file,
3. If it is a device file, it makes the other entries like major, minor device numbers.
For example:
If the device is a disk, major device number refers to the disk controller and minor device number is the disk.
What is a FIFO?
FIFO are otherwise called as ‘named pipes’. FIFO (first-in-first-out) is a special file which is said to be data transient. Once data is read from named pipe, it cannot be read again. Also, data can be read only in the order written. It is used in inter process communication where a process writes to one end of the pipe (producer) and the other reads from the other end (consumer).
What are links and symbolic links in UNIX file system?
A link is a second name (not a file) for a file. Links can be used to assign more than one name to a file, but cannot be used to assign a directory more than one name or link file names on different computers.
Symbolic link ‘is’ a file that only contains the name of another file.Operation on the symbolic link is directed to the file pointed by the it.Both the limitations of links are eliminated in symbolic links.
Commands for linking files are:
Link ln filename1 filename2
Symbolic link ln -s filename1 filename2
How do you change File Access Permissions?
Every file has following attributes:
owner’s user ID ( 16 bit integer )
owner’s group ID ( 16 bit integer )
File access mode word
‘r w x -r w x- r w x’

(user permission-group permission-others permission)
r-read, w-write, x-execute
To change the access mode, we use ch mod(file name,mode).
Example 1:
To change mode of my file to ‘rw-rw-r–’ (ie. read, write permission for user – read,write permission for group – only read permission for others) we give the args as:
chmod(myfile,0664) .
Each operation is represented by discrete values
‘r’ is 4
‘w’ is 2
‘x’ is 1

Therefore, for ‘rw’ the value is 6(4+2).
Example 2:
To change mode of myfile to ‘rwxr–r–’ we give the args as:
chmod(myfile,0744).
Brief about the directory representation in UNIX
A Unix directory is a file containing a correspondence between filenames and inodes. A directory is a special file that the kernel maintains. Only kernel modifies directories, but processes can read directories. The contents of a directory are a list of filename and inode number pairs. When new directories are created, kernel makes two entries named ‘.’ (refers to the directory itself) and ‘..’ (refers to parent directory).
System call for creating directory is mkdir (pathname, mode).
What is ‘inode’?
All UNIX files have its description stored in a structure called ‘inode’. The inode contains info about the file-size, its location, time of last access, time of last modification, permission and so on. Directories are also represented as files and have an associated inode. In addition to descriptions about the file, the inode contains pointers to the data blocks of the file. If the file is large, inode has indirect pointer to a block of pointers to additional data blocks (this further aggregates for larger files). A block is typically 8k.
Inode consists of the following fields:
• File owner identifier
• File type
• File access permissions
• File access times
• Number of links
• File size
• Location of the file data
How are devices represented in UNIX?
All devices are represented by files called special files that are located in/dev directory. Thus, device files and other files are named and accessed in the same way. A ‘regular file’ is just an ordinary data file in the disk. A ‘block special file’ represents a device with characteristics similar to a disk (data transfer in terms of blocks). A ‘character special file’ represents a device with characteristics similar to a keyboard (data transfer is by stream of bits in sequential order)

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Howto: Installing VLC Player on Linux Using Script

Following things must exist on your Computer/Linux :-

=> Direct Working Internet Connection or Yum Should run from Terminal. If you are using Internet connection with Proxy then it will be problematic for you.

=> RHEL 5.x must be installed

What is VLC Player ?

=> VLC media player is an open source, free software media player and multimedia framework written by the VideoLAN project.

There are so many peoples who is facing issue while installing VLC Player on Linux and CentOS, Most of the peoples suffer with dependencies problem if they are going to install VLC Player using RPM or YUM.

But Now you will find a very easy way to install VLC Player, I have created Bash Script to Install VLC Player with all Dependencies, So Forget about all problems Just Download and Run Bash Script which I have created for VLC Player Installation on RHEL 5.

Perform Following Steps to Install VLC Player on RHEL 5.x / Cent OS 5.x

1. You need to be root to run this script.

2. Download this VLC INSTALLATION SCRIPT.

3. Execute following Command where you have downloaded the script.
[root@ask4itsolutions ~]# chmod +x vlc-installation.sh

4. Now Execute your script by two ways mentioned below

[root@ask4itsolutions ~]# sh vlc-installation.sh
OR
[root@ask4itsolutions ~]# ./vlc-installation.sh

5. Now Just Site Back and Relax for Few Minutes as per your Internet Connection it will Install VLC Player Without any error.

6. Remember you have to be normal user to run VLC Player, You cannot Run VLC Player as root.

Hope you ENJOY THIS………

If this works for you then Show your support and encourage me to write some more howto’s by commenting on the same topic or by sending me mail on blog(at)ask4itsolutions(dot)com

If this doesn’t work for you then also please do let me know so i can helps you out to solve the issue, If you are facing the error then please send me mail on blog(at)ask4itsolutions(dot)com with Subject:- VLC Installation Issue

Enjoy Ubuntu 11.10 Oneiric Ocelot :) Enjoy Ubuntu  :) Enjoy Open Source :) Enjoy Linux :)

 

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