unix interview questions

• The following command displays output only on the screen (stdout).

$ ls
The following command writes the output only to the file and not to the screen.
$ ls > file
The following command (with the help of tee command) writes the output both to the screen (stdout) and to the file.
$ ls | tee file
http://linux.101hacks.com/unix/tee-command-examples

• Syntax To redirect all error to file

command-name 2> errors.txt

syntax to redirect both output (stdout) and errors (stderr) to same file
command1 > everything.txt 2>&1

Syntax to redirect errors (stderr) to null or zero devices
Data written to a null or zero special file is discarded by your system. This is useful to silence out errors (also know as ‘error spam’):
command1 2> /dev/null
command1 2> /dev/zero

http://www.cyberciti.biz/faq/linux-redirect-error-output-to-file/

• /dev/null

On Unix/Linux system there is a special
file called /dev/null. It behaves like a black hole. Whatever is printed to that file will disappear without any trace. The main use of this is when there is a program and the user wants to throw away either the regular output or the error messages.
For example, you might have an application, one that you cannot change, that spit tons of messages to the standard error channel. If you don't want to see that on the screen you can redirect it to a file. But if you do that, it can fill your disk quickly. So instead, you would redirect the standard error to /dev/null and the operating system will help you disregard all the "garbage".
http://perlmaven.com/stdout-stderr-and-redirection

• Every process in Linux is provided with three open files( usually called file descriptor). These files are the standard input, output and error files. By default :

    Standard Input is the keyboard, abstracted as a file to make it easier to write shell scripts.
    Standard Output is the shell window or the terminal from which the script runs, abstracted as a file to again make writing scripts & program easier
    Standard error is the same as standard output:the shell window or terminal from which the script runs.

A file descriptor is simply a number that refers to an open file. By default , file descriptor 0 (zero) refers to the standard input & often abbreviated as stdin. File descriptor 1 refers to standard output (stdout) and file descriptor 2 refers to standard error (stderr)
http://www.linuxtechi.com/standard-input-output-error-in-linux

• BASH Shell: How To Redirect stderr To stdout ( redirect stderr to a File )

Bash and other modern shell provides I/O redirection facility. There are 3 default standard files (standard streams) open:
[a] stdin – Use to get input (keyboard) i.e. data going into a program.
[b] stdout – Use to write information (screen)
[c] stderr – Use to write error message (screen)

Understanding I/O streams numbers
The Unix / Linux standard I/O streams with numbers:

Handle Name Description
0 stdin Standard input
1 stdout Standard output
2 stderr Standard error

The following will redirect program error message to a file called error.log:
$ program-name 2> error.log

http://www.cyberciti.biz/faq/redirecting-stderr-to-stdout

• 25 Linux Shell Scripting interview Questions & Answers

What is Shell Script and why it is required ?
Ans: A Shell Script is a text file that contains one or more commands. As a system administrator we often need to issue number of commands to accomplish the task, we can add these all commands together in a text file (Shell Script) to complete daily routine task.

Q:2 What is the default login shell and how to change default login shell for a specific user ?
# chsh <username> -s <new_default_shell>
# chsh linuxtechi -s /bin/sh

Q:4 How to redirect both standard output and standard error to the same location ?
Ans: There two method to redirect std output and std error to the same location:
Method:1 2>&1 (# ls /usr/share/doc > out.txt 2>&1 )
Method:2 &> (# ls /usr/share/doc &> out.txt )

http://www.linuxtechi.com/linux-shell-scripting-interview-questions-answers/

•  What is the system command to find the current environment variables of the shell you’re running?

 The command is “env”. The output will depend on which system you’re running:

 What is swapping and paging?
 Consider a reference book, for example. You only open the page you need- you don’t need to have all the pages open at once. That is both almost impossible and extremely inefficient. A book is comparable to a process in UNIX. At any given time, there are several books that UNIX is reading. However, UNIX only takes a few pages from a book at one time, as needed. Once it is done with the page, or if it becomes “old”, it is sent back to storage. This is known as paging. Sometimes UNIX sends the entire book, with all its pages, back to the memory- this is known as swapping. This happens when the work load is really heavy.

Paging is more efficient if UNIX is running a large number of processes at one time. Pages (memory segments) can be stored and retrieved as needed. Swapping is more demanding in terms of resources and memory, but it provides faster results, as UNIX doesn’t need to search and retrieve pages every single time while it is executing a process
 https://blog.udemy.com/unix-shell-scripting-interview-questions/

Specify the difference between absolute path and related path?
Absolute path refers to the exact path as defined from the root directory. Related path refers to the path related to the current locations.

What is the FIFO?
FIFO (First In First Out) is also called named pipes and it is a special file for date transient. Data is read-only in the written order. This is used to inter-process communications, where data write to one end and reads from another end of the pipe.

What is mean by Super User?
The user with access to all files and commands within the system is called a superuser. Generally, the superuser login is to root and the login is secured with the root password.

What is the process group?
A collection of one or more processes is called process group. There is a unique process id for each process group. The function “getpgrp” returns the process group ID for the calling process.

What are the different file types available with UNIX?
    Regular files
    Directory files
    Character special files
    Block special files
    FIFO
    Symbolic links
    Socket
   
https://www.softwaretestinghelp.com/unix-interview-questions/

• The mkfifo command basically lets you create FIFOs (a.k.a named pipes)

You'd have seen commands that contain a vertical bar (|) in them. This bar is called a pipe. What it does is, it creates a channel of communication between the two processes (when the complete command is executed).

ls | grep .txt
The command mentioned above consists of two programs: ls and grep.
Both these programs are separated by a pipe (|)
So what pipe does here is, it creates a channel of communication between these programs
when the aforementioned command is executed, the output of ls is fed as input to grep.
So finally, the output that gets displayed on the terminal consists of only those entries that have '.txt' string in them.

mkfifo pipe2

So 'pipe2' is now a named pipe. Now comes the question how named pipes are more useful? Well, consider the case where you have a process running in a terminal and producing output, and what you want is to channelize that output on to a different terminal. So here, a named pipe could of great help.


https://www.howtoforge.com/linux-mkfifo-command/

• Some of the frequently used shells in UNIX include the following:

    tcsh – enhanced C Shell
    zsh – Z SHell
    sh – Bourne shell
    csh – C SHell
    ksh – Korn SHell
    bash – Bourne Again Shell

Nohup is a distinctive command that is utilized to run the process in the background, yet it is marginally unique in relation to and which is typically utilized for putting a procedure in the background. A typical UNIX process that began with nohup will not stop regardless of whether the client has logged off from the framework. While the background process began with and will stop when the client logoff

https://www.onlineinterviewquestions.com/unix-interview-questions/   

• Explain the difference between soft and hard links?

The most common difference between the soft and hard link is, a hard link is a direct reference to the file in UNIX. However, on the other hand, the soft link is the name refers to the file which means they point out files using their names.

Can you link directories to soft links?
Yes, you can link directories to the soft links as the file system structure supports them.

How do you know if you have the soft or hard link?
The soft link commonly known as the symbolic is the original copy of the file while the hard link is the perfect replica of the original file. So, if you delete the original copy from the system then the soft link won’t have any value but the case is right opposite in hard link.

How hard links are used in the UNIX?
What hard links do on an immediate basis is to break down the file system structure to process the information. Unlike soft links, hard links cannot be spanned across the file system.

Do hard and soft link shares the same Inode?
Yes, both hard link and soft link share the same inode.

Do hard link work even after deleting the soft link?
Yes, the hard will work even after you end up deleting the soft file. As they are a perfect mirror copy of soft link, you can access the file until the links to the file are do not end up on zero.

Can you link directories to hard links?
directories don’t fit into the system and essentially break it. So, in order to protect the file system structure, directories are avoided link with hard links.
https://www.gangboard.com/blog/unix-shell-scripting-interview-questions-and-answers/

• Discuss the difference between swapping and paging?

Swapping – The complete process is moved to the main memory for execution. To provide the memory requirement, the process size must be less than the available main memory capacity. The implementation is easy but is an overhead to the system. Memory handling is not more flexible with swapping systems.
It the procedure of copying the entire process from main memory onto secondary memory.
For execution, the whole process is moved from swap device to the main memory.

Paging – Only the required memory pages are moved to the main memory for execution.

What do understand by Kernel?
Unix operating system is basically divided into three parts, namely, the kernel, the shell, and the commands and utilities

Explain Superblock in UNIX?
Each logical partitions in Unix are referred to as the File system and each file system contains, a ‘boot block’, a ‘superblock’, ‘inodes’, and ‘data blocks’.
https://www.softwaretestinghelp.com/unix-interview-questions/

VM Player vs VMware Workstation

• Workstation has some features that Player lacks, such as teams (groups of VMs connected by private LAN segments) and multi-level snapshot trees. It's aimed at power users and developers; they even have some hooks for using a debugger on the host to debug code in the VM (including kernel-level stuff). The core technology is the same, though.

VMware Workstation is much more advanced and comes with powerful features including snapshots, cloning, remote connections to vSphere, sharing VMs, advanced Virtual Machines settings and much more. Workstation is designed to be used by technical professionals such as developers, quality assurance engineers, systems engineers, IT administrators, technical support representatives, trainers, etc
https://stackoverflow.com/questions/4170600/vmware-player-vs-vmware-workstation

• VMware Player enables you to quickly and easily create and run virtual machines. However, VMware Player lacks many powerful features, remote connections to vSphere, drag and drop upload to vSphere, multiple Snapshots and Clones, and much more.

https://www.vmware.com/products/player/faqs.html

quantum computer

• Quantum computing is computing using quantum-mechanical phenomena, such as superposition and entanglement

A quantum computer is a device that performs quantum computing.
Such a computer is completely different from binary digital electronic computers based on transistors and capacitors.
common digital computing requires that the data be encoded into binary digits (bits), each of which is always in one of two definite states (0 or 1),
quantum computation uses quantum bits or qubits, which can be in superpositions of states.

https://en.wikipedia.org/wiki/Quantum_computing

• What is a quantum computer?

A quantum computer is a device able to manipulate delicate quantum states in a controlled fashion, not dissimilar from the way an ordinary computer manipulates its bits.

What does a quantum computer look like?
A quantum computer looks like nothing you have on your desk, or in your office, or in your pocket. It is housed in a large unit known as a dilution refrigerator and is supported by multiple racks of electronic pulse-generating equipment. However, you can access our quantum computer with very familiar personal computing devices, such as laptops, tablets, and smartphones.

What is a qubit?
A qubit (pronounced “cue-bit” and short for quantum bit) is the physical carrier of quantum information. It is the quantum version of a bit and its quantum state can take values of 0, 1, or both at once, which is a phenomenon known as superposition.

What is a superposition?
A superposition is a weighted sum or difference of two or more states; for example, the state of the air when two or more musical tones are sounding at once. Ordinary, or “classical,” superpositions commonly occur in macroscopic phenomena involving waves.

How is superposition different from probability?
A set of n coins, each of which might be heads or tails, can be described as a probabilistic mixture of states, but it actually is in only one of them—we just don’t know which. For this reason quantum superposition is more powerful than classical probabilism. Quantum computers capable of holding their data in superposition can solve some problems exponentially faster than any known deterministic or probabilistic classical algorithm. A more technical difference is that while probabilities must be positive (or zero), the weights in a superposition can be positive, negative, or even complex numbers.


http://www.research.ibm.com/quantum/

• Today, real quantum computers can be accessed through the cloud, and many thousands of people have used them to learn, conduct research, and tackle new problems.

Quantum computers could one day provide breakthroughs in many disciplines, including materials and drug discovery, the optimization of complex systems, and artificial intelligence.

Quantum and Chemistry
For challenges above a certain size and complexity, we don’t have enough computational power on Earth to tackle them. To stand a chance at solving some of these complex problems, we need a new kind of computing: one whose computational power also scales exponentially as the system size grows.

What makes it ‘quantum’?
All computing systems rely on a fundamental ability to store and manipulate information. Current computers manipulate individual bits, which store information as binary 0 and 1 states.
Millions of bits work together to process and display information.
Quantum computers leverage different physical phenomena
superposition
entanglement
interference
to manipulate information.
To do this, we rely on different physical devices: quantum bits, or qubits

Superposition refers to a combination of states we would ordinarily describe independently. To make a classical analogy, if you play two musical notes at once, what you will hear is a superposition of the two notes
Entanglement is a famously counter-intuitive quantum phenomenon describing behavior we never see in the classical world. Entangled particles behave together as a system in ways that cannot be explained using classical logic
Quantum interference can be understood similarly to wave interference; when two waves are in phase, their amplitudes add, and when they are out of phase, their amplitudes cancel


In order to increase the computational power of quantum computing systems, improvements are needed along two dimensions. One is qubit count; the more qubits you have, the more states can in principle be manipulated and stored.
The second is to achieve lower error rates.
Combining these two concepts, we can create a single measure of a quantum computer’s power called quantum volume. Quantum volume measures the relationship between number and quality of qubits, circuit connectivity, and error rates of operations.

https://www.research.ibm.com/ibm-q/learn/what-is-quantum-computing/

• Quantum Computation

Rather than store information using bits represented by 0s or 1s as conventional digital computers do, quantum computers use quantum bits, or qubits, to encode information as 0s, 1s, or both at the same time.
Capabilities
Optimization
Machine learning
Sampling / Monte Carlo
Pattern recognition and anomaly detection
Cyber security
Image analysis
Financial analysis
Software / hardware verification and validation
Bioinformatics / cancer research
https://www.dwavesys.com/quantum-computing

Windows Analysis

• Windows Attack Surface

Attack Surface Analyzer takes a snapshot of your system state before and after the installation of product(s) and displays the changes to a number of key elements of the Windows attack surface.

This allows:
- Developers to view changes in the attack surface resulting from the introduction of their code on to the Windows platform
- IT Professionals to assess the aggregate Attack Surface change by the installation of an organization's line of business applications
- IT Security Auditors evaluate the risk of a particular piece of software installed on the Windows platform during threat risk reviews
- IT Security Incident Responders to gain a better understanding of the state of a systems security during investigations (if a baseline scan was taken of the system during the deployment phase)
https://blogs.microsoft.com/cybertrust/2012/08/02/microsofts-free-security-tools-attack-surface-analyzer/

Collecting attack surface information with .NET Framework 4 installed
C1. Download and install Attack Surface Analyzer on a machine with a freshly installed version of a supported operating system, as listed in the System Requirements section. Attack Surface Analyzer works best with a clean (freshly built) system. Not running the Attack Surface Analyzer on a freshly built system requires more time to perform scanning and analysis.
C2. Install any software prerequisite packages before the installation of your application.
C3. Run Attack Surface Analyzer from the Start menu or command-line. If Attack Surface Analyzer is launched from a non-elevated process, UAC will prompt you that Attack Surface Analyzer needs to elevate to Administrative privileges.
C4. When the Attack Surface Analyzer window is displayed, ensure the "Run new scan" action is selected, confirm the directory and filename you would like the Attack Surface data saved to and click Run Scan.
C5. Attack Surface Analyzer then takes a snapshot of your system state and stores this information in a Microsoft Cabinet (CAB) file. This scan is known as your baseline scan.
C6. Install your product(s), enabling as many options as possible and being sure to include options that you perceive may increase the attack surface of the machine. Examples include; if your product can install a Windows Service, includes the option to enable access through the Windows Firewall or install drivers.
C7. Run your application.
C8. Repeat steps C3 through C5. This scan will be known as your product scan.

Analyzing the Results
Note: You can either analyze the results on the computer you generated your scans from, or copy the CAB files to another computer for analysis. To perform analysis and report generation, a machine with .Net Framework 4 is required:
A1. Run Attack Surface Analyzer from the Start menu. If Attack Surface Analyzer is launched from a non-elevated process, UAC will prompt you that Attack Surface Analyzer needs to elevate to Administrative privileges. Note: To view the full list of command line options, including generating the report from the command line, execute the command: ““Attack Surface Analyzer.exe” /?” (without the surrounding quotation marks) from the console.
A2. Choose the "Generate Report" action and specify your baseline and product scan CAB files. Note: Make sure that you have the cab files selected for both baseline and product correctly, then generate report. Attack Surface Analyzer will inspect the contents of these files to identify changes in system state and if applicable important security issues that should be investigated. If a web browser is installed on the machine performing the analysis it should automatically load Attack Surface Analyzer's report - it is a HTML file.
A3. Review the report to ensure the changes are the minimum required for your product to function and are consistent with your threat model.

After addressing issues generated from the tool you should repeat the scanning process on a clean installation of Windows (that is, without the artifacts of your previous installation) and re-analyze the results. As you may need to repeat the process a number of times, we recommend using a virtual machine with "undo disks", differencing disks or the ability to revert to a prior virtual machine snapshot/configuration to perform your attack surface assessments.
https://www.microsoft.com/en-us/download/details.aspx?id=24487

Mind Mapping

FreeMind - free mind mapping software 

FreeMind is a premier free mind-mapping software written in Java. The recent development has hopefully turned it into high productivity tool
http://freemind.sourceforge.net/wiki/index.php/Main_Page

• Freemind Free Mind Mapping Software Tutorial Mind Map 

https://www.youtube.com/watch?v=vlUdQTeZiNo

• FreeMind Tutorial 

https://www.youtube.com/watch?v=grut_2cardM

Disaster Recovery-as-a-Service (DRaaS)

• An Information System Contingency Plan (ISCP) is a pre-established plan for restoration of the services of a given information system after a disruption.



• Stage 2: Disaster occurs

On a given point in time, disaster occurs and systems needs to be recovered. At this point the Recovery Point Objective (RPO) determines the maximum acceptable amount of data loss measured in time. For example, the maximum tolerable data loss is 15 minutes.

Stage 3: Recovery
At this stage the system are recovered and back online but not ready for production yet. The Recovery Time Objective (RTO) determines the maximum tolerable amount of time needed to bring all critical systems back online. This covers, for example, restore data from back-up or fix of a failure. In most cases this part is carried out by system administrator, network administrator, storage administrator etc.

Stage 4: Resume Production
At this stage all systems are recovered, integrity of the system or data is verified and all critical systems can resume normal operations. The Work Recovery Time (WRT) determines the maximum tolerable amount of time that is needed to verify the system and/or data integrity. This could be, for example, checking the databases and logs, making sure the applications or services are running and are available. In most cases those tasks are performed by application administrator, database administrator etc. When all systems affected by the disaster are verified and/or recovered, the environment is ready to resume the production again

The sum of RTO and WRT is defined as the Maximum Tolerable Downtime (MTD) which defines the total amount of time that a business process can be disrupted without causing any unacceptable consequences. This value should be defined by the business management team or someone like CTO, CIO or IT manager.
This is of course a simple example of a Business Continuity/Disaster Recovery plan and should be included in your Business Impact Analysis (BIA).

https://defaultreasoning.com/2013/12/10/rpo-rto-wrt-mtdwth/

• Recovery Point Objective (RPO)

Maximum Tolerable Downtime (MTD)
Recovery Time Objective (RTO)


Recovery Point Objective (RPO)
The RPO is defined by business and departmental managers, and any designated data owners. It is described as a period of time. For example, the recovery point may be "one hour", "end of previous business day", or "one week". It is derived based on:
How often data is updated;
How much expense and/or effort would be required of your users to reconstruct data created or updated since the last backup, if possible;
If reconstruction wouldn't be possible, how much recent data your company can tolerate losing permanently, considering the likelihood of a catastrophic data loss event.

Maximum Tolerable Downtime (MTD)
MTD is the maximum amount of time an application or data can be unavailable to users, as specified by business management. This is based on the impact on business functions, and analysis of anticipated lost revenue and other costs that are incurred for every hour, day, or week a given application or database might be unavailable.
https://www.jdfoxexec.com/resource-center/articles/mtd-rto-rpo/
https://en.wikipedia.org/wiki/Information_System_Contingency_Plan

• 5 WAYS TO TEST IT DISASTER RECOVERY PLANS

the five types of disaster recovery tests:

Paper test: Individuals read and annotate recovery plans.

Walkthrough test: Groups walk through plans to identify issues and changes.

Simulation: Groups go through a simulated disaster to identify whether emergency response plans are adequate.

Parallel test: Recovery systems are built/set up and tested to see if they can perform actual business transactions to support key processes. Primary systems still carry the full production workload.

Cutover test: Recovery systems are built/set up to assume the full production workload. You disconnect primary systems

https://www.dummies.com/programming/networking/5-ways-to-test-it-disaster-recovery-plans/

• TESTING DISASTER RECOVERY PLANS

Structured Walk-Through Testing
During a structured walk-through test, disaster recovery team members meet to verbally walk through the specific steps of each component of the disaster recovery process as documented in the disaster recovery plan. The purpose of the structured walk-through test is to confirm the effectiveness of the plan and to identify gaps, bottlenecks or other weaknesses in the plan.
Checklist Testing
A checklist test determines if sufficient supplies are stored at the backup site, telephone number listings are current, quantities of forms are adequate, and a copy of the recovery plan and necessary operational manuals are available.
Simulation Testing
During this test, the organization simulates a disaster so normal operations will not be interrupted. A disaster scenario should take into consideration the purpose of the test, objectives, type of test, timing, scheduling, duration, test participants, assignments, constraints, assumptions, and test steps.
Parallel Testing
A parallel test can be performed in conjunction with the checklist test or simulation test. Under this scenario, historical transactions, such as yesterday’s transactions, are processed against the preceding day’s backup files at the contingency processing site or hot-site. All reports produced at the alternate site for the current business date should agree with those reports produced at the existing processing site.
Full-interruption Testing
A full-interruption test activates the total disaster recovery plan. This test is costly and could disrupt normal operations.
https://www.drj.com/drj-world-archives/dr-plan-testing/testing-disaster-recovery-plans.html

• Disaster Recovery as a Service (DRaaS) is the replication and hosting of physical or virtual servers by a third-party to provide failover in the event of a man-made or natural catastrophe.

 Typically, DRaaS requirements and expectations are documented in a service-level agreement (SLA) and the third-party vendor provides failover to a cloud computing environment, either through a contract or pay-per-use basis
 http://whatis.techtarget.com/definition/disaster-recovery-as-a-service-DRaaS

•  Veeam® enables Disaster Recovery-as-a-Service (DRaaS) as part of a comprehensive availability strategy, embracing investments made in your datacenter and extending them through the hybrid cloud.

 https://www.veeam.com/disaster-recovery-as-a-service-draas.html

• High Availability

High availability is a feature which provides redundancy and fault tolerance

What is Redundancy
Redundancy is basically extra hardware or software that can be used as backup if the main hardware or software fails. Redundancy can be achieved via load clustering, failover, RAID, load balancing, high availabiltiy in an automated fashion. A higher layer of redundancy is achieved when the backup device is completely separate from the primary device. For example a backup internet line is provided by another ISP provider, so a completely separate physical link and connection from the primary internet connection, or a redundant piece of hardware which resides in another building.
http://www.internet-computer-security.com/Firewall/Failover.html

• HIGH AVAILABILITY

A High Availability system is one that is designed to be available 99.999% of the time, or as close to it as possible. Usually this means configuring a failover system that can handle the same workloads as the primary system.
FAULT TOLERANCE
A Fault Tolerant system is extremely similar to HA, but goes one step further by guaranteeing zero downtime. HA still comes with a small portion of downtime, hence the ideal of a perfect HA strategy reaching “five nines” rather than 100% uptime. The time it takes for the intermediary layer, like the load balancer or hypervisor, to detect a problem and restart the VM can add up to minutes or even hours over the course of yearly runtime.
https://www.greenhousedata.com/blog/high-availability-vs-fault-tolerance-vs-disaster-recovery

•  Disaster Recovery as a Service (DRaaS) from Node4 is ideal for companies who need continuous protection of the data and applications that are essential for the operation of their critical business functions.DRaaS is delivered using award-winning software from Zerto to replicate your virtual machines and maintain standby copies on N4Compute, our highly resilient Cloud virtualisation platform

 http://www.node4.co.uk/cloud/draas/

• Fujitsu Backup as a Service (BaaS) provides a resilient, cloud-based backup and recovery service. Fujitsu Backup as a Service supports full system recovery, providing much more than folder and file backup and recovery. Delivered from FUJITSU Cloud, it offers the levels of speed, convenience and reliability demanded by organizations today. 

 http://www.fujitsu.com/global/services/infrastructure/iaas/baas/

•  Backup as a Service (BaaS) provides backup and recovery operations from the cloud. The cloud-based BaaS provider maintains necessary backup equipment, applications, process and management in their data center. The customer will have some on-site installation – an appliance and backup agents are common – but there is no need to buy backup servers and software, run upgrades and patches, or purchase dedupe appliances.

•  DRaaS/RaaS. Disaster Recovery as a Service, or more simply Recovery as a Service, offers more recovery options than the backup recovery of BaaS. BaaS will recover your backed up files, and RaaS recovers your files and applications within contracted RTO and/or RPO periods. It is more costly than BaaS but can be a good option if you do not want to perform your own storage infrastructure recovery in case of disaster.

 http://www.datamation.com/cloud-computing/backup-as-a-service-to-baas-or-not-to-baas-1.html

•  Backup as a service (BaaS) is an approach to backing up data that involves purchasing backup and recovery services from an online data backup provider. Instead of performing backup with a centralized, on-premises IT department, BaaS connects systems to a private, public or hybrid cloud managed by the outside provider. Backup as a service is easier to manage than other offsite services. Instead of worrying about rotating and managing tapes or hard disks at an offsite location, data storage administrators can offload maintenance and management to the provider.

 http://searchdatabackup.techtarget.com/definition/backup-as-a-service-BaaS

• What is disaster recovery?

Disaster recovery (DR) consists of IT technologies and best practices designed to prevent or minimize data loss and business disruption resulting from catastrophic events—everything from equipment failures and localized power outages to cyberattacks, civil emergencies, criminal or military attacks, and natural disasters.

Business continuity planning
Business continuity planning creates systems and processes to ensure that all areas of your enterprise will be able to maintain essential operations or be able to resume them as quickly as possible in the event of a crisis or emergency. Disaster recovery planning is the subset of business continuity planning that focuses on recovering IT infrastructure and systems.

Disaster recovery planning
Business impact analysis
The creation of a comprehensive disaster recovery plan begins with business impact analysis. When performing this analysis, you’ll create a series of detailed disaster scenarios that can then be used to predict the size and scope of the losses you’d incur if certain business processes were disrupted.
Risk analysis
Assessing the likelihood and potential consequences of the risks your business faces is also an essential component of disaster recovery planning
Prioritizing applications
Separate your systems and applications into three tiers, depending on how long you could stand to have them be down and how serious the consequences of data loss would be.
Documenting dependencies
The next step in disaster recovery planning is creating a complete inventory of your hardware and software assets. It’s essential to understand critical application interdependencies at this stage
Establishing recovery time objectives, recovery point objectives, and recovery consistency objectives
By considering your risk and business impact analyses, you should be able to establish objectives for how long you’d need it to take to bring systems back up, how much data you could stand to use, and how much data corruption or deviation you could tolerate.
Your recovery time objective (RTO) is the maximum amount of time it should take to restore application or system functioning following a service disruption.
Your recovery point objective (RPO) is the maximum age of the data that must be recovered in order for your business to resume regular operations.
A recovery consistency objective (RCO) is established in the service-level agreement (SLA) for continuous data protection services. It is a metric that indicates how many inconsistent entries in business data from recovered processes or systems are tolerable in disaster recovery situations
Regulatory compliance issues
All disaster recovery software and solutions that your enterprise have established must satisfy any data protection and security requirements that you’re mandated to adhere to
Choosing technologies
Backups serve as the foundation upon which any solid disaster recovery plan is built.
Choosing recovery site locations
On the one hand, a copy of your data should be stored somewhere that’s geographically distant enough from your headquarters or office locations that it won’t be affected by the same seismic events, environmental threats, or other hazards as your main site. On the other hand, backups stored offsite always take longer to restore from than those located on-premises at the primary site, and network latency can be even greater across longer distances.
Continuous testing and review
Simply put, if your disaster recovery plan has not been tested, it cannot be relied upon
All employees with relevant responsibilities should participate in the disaster recovery test exercise, which may include maintaining operations from the failover site for a period of time.
Disaster Recovery-as-a-Service (DRaaS)
Disaster-Recovery-as-a-Service (DRaaS) is one of the most popular and fast-growing managed IT service offerings available today. Your vendor will document RTOs and RPOs in a service-level agreement (SLA) that outlines your downtime limits and application recovery expectations.
Cloud DR
Most on-premises DR solutions will incur costs for hardware, power, labor for maintenance and administration, software, and network connectivity. In addition to the upfront capital expenditures involved in the initial setup of your DR environment, you’ll need to budget for regular software upgrades. Because your DR solution must remain compatible with your primary production environment, you’ll want to ensure that your DR solution has the same software versions.Depending upon the specifics of your licensing agreement, this might effectively double your software costs.
moving to a DRaaS subscription reduce your hardware and software expenditures, it can lower your labor costs by moving the burden of maintaining the failover site to the vendor.
https://www.ibm.com/cloud/learn/disaster-recovery

•     Section 1. Example: Major goals of a disaster recovery plan

    Here are the major goals of a disaster recovery plan.
    Section 2. Example: Personnel

    You can use the tables in this topic to record your data processing personnel. You can include a copy of the organization chart with your plan.
    Section 3. Example: Application profile

    You can use the Display Software Resources (DSPSFWRSC) command to complete the table in this topic.
    Section 4. Example: Inventory profile

    You can use the Work with Hardware Products (WRKHDWPRD) command to complete the table in this topic.
    Section 5. Information services backup procedures

    Use these procedures for information services backup.
    Section 6. Disaster recovery procedures

    For any disaster recovery plan, these three elements should be addressed.
    Section 7. Recovery plan for mobile site

    This topic provides information about how to plan your recovery task at a mobile site.
    Section 8. Recovery plan for hot site

    An alternate hot site plan should provide for an alternative (backup) site. The alternate site has a backup system for temporary use while the home site is being reestablished.
    Section 9. Restoring the entire system

    You can learn how to restore the entire system.
    Section 10. Rebuilding process

    The management team must assess the damage and begin the reconstruction of a new data center.
    Section 11. Testing the disaster recovery plan

    In successful contingency planning, it is important to test and evaluate the plan regularly.
    Section 12. Disaster site rebuilding

    Use this information to do disaster site rebuilding.
    Section 13. Record of plan changes

    Keep your plan current, and keep records of changes to your configuration, your applications, and your backup schedules and procedures.

https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_73/rzarm/rzarmdisastr.htm

Microsoft Active Directory Topology Diagrammer

• ad topology diagrammer

https://www.youtube.com/watch?v=1hF9JJ6xHWI

• The Microsoft Active Directory Topology Diagrammer reads an Active Directory configuration using LDAP, and then automatically generates a Visio diagram of your Active Directory and /or your Exchange Server topology. The diagramms may include domains, sites, servers, organizational units, DFS-R, administrative groups, routing groups and connectors and can be changed manually in Visio if needed

https://www.microsoft.com/en-us/download/details.aspx?id=13380#Instructions

size on disk

• du gives two different results for the same file

When you run du without the --apparent-size you're getting the size based on the amount of disk's block space used, not the actual space consumed by the file(s).
This is a common problem when you put the same data on 2 different HDDs. You'll want to run the du command with and additional switch, assuming it has it - which it should given these are Linux nodes.
http://unix.stackexchange.com/questions/106275/du-gives-two-different-results-for-the-same-file

• What’s the difference between my file size and the size on disk?

When you right-click to view the properties of a folder, the property sheet includes two values: Size and Size on disk.
The values reported by Size and Size on disk aren’t meant to be a byte-for-byte accounting of the total impact of a directory on your disk free space. They’re just a rough estimate based on the assumption that most files are of the boring variety. By that, I mean no hard links and negligible use of alternate data streams. If you have a directory with numerous hard links—such as the Windows directory itself, for example—the values will be way off.