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Linux interview questions

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Linux interview questions

What is Kernel? Explain the task it performs.

Kernel is used in UNIX like systems and is considered to be the heart of the operating system. It is responsible for communication between hardware and software components. It is primarily used for managing the systems resources as well.

Kernel Activities:

The Kernel task manager allows tasks to run concurrently.

Managing the computer resources: Kernel allows the other programs to run and use the resources

Resources include i/o devices, CPU, memory.

Kernel is responsible for Process management. It allows multiple processes to run simultaneously allowing user to multitask.

Kernel has an access to the systems memory and allows the processes to access the memory when required.

Processes may also need to access the devices attached to the system. Kernel assists the processes in doing so.

For the processes to access and make use of these services, system calls are used.

What is Linux Shell? What is Shell Script?

Linux shell is a user interface used for executing the commands. Shell is a program the user uses for executing the commands. In UNIX, any program can be the users shell. Shell categories in Linux are:

Bourne shell compatible, C shell compatible, nontraditional, and historical

A shell script, as the name suggests, is a script written for the shell. Script here means a programming language used to control the application. The shell script allows different commands entered in the shell to be executed. Shell script is easy to debug, quicker as compared to writing big programs. However the execution speed is slow because it launches a new process for every shell command executed. Examples of commands are cp, cn, cd.

What are Pipes? Explain uses of pipes.

A pipe is a chain of processes so that output of one process (stdout) is fed an input (stdin) to another. UNIX shell has a special syntax for creation of pipelines. The commands are written in sequence separated by |. Different filters are used for Pipes like AWK, GREP.

e.g. sort file | lpr ( sort the file and send it to printer)

Uses of Pipe

Several powerful functions can be in a single statement

Streams of processes can be redirected to user specified locations using >

Explain trap command; shift Command, getopts command of linux.

Trap command: controls the action to be taken by the shell when a signal is received.

Trap [OPTIONS] [ [arg] signspec..]

Arg is the action to be taken or executed on receiving a signal specified in signspec.

e.g. trap “rm $FILE; exit” // exit (signal) and remove file (action)

Shift Command: Using shift command, command line arguments can be accessed. The command causes the positional parameters shift to the left. Shift [n] where n defaults to 1. It is useful when several parameters need to be tested.

Getopts command: this command is used to parse arguments passed. It examines the next command line argument and determines whether it is a valid option

Getopts {optstring} {variable1}. Here, optsring contains letters to be recognized if a letter is followed by a colon, an argument should be specified. E.g (whether the argument begins with a minus sign and is followed by any single letter contained inside options ) If not, diagnostic messages are shown. It is usually executed inside a loop.

What Stateless Linux server? What feature it offers?

A stateless Linux server is a centralized server in which no state exists on the single workstations. There may be scenarios when a state of a partilcuar system is meaningful (A snap shot is taken then) and the user wants all the other machines to be in that state. This is where the stateless Linux server comes into picture.

Features:

It stores the prototypes of every machine

It stores snapshots taken for those systems

It stores home directories for those systems

Uses LDAP containing information of all systems to assist in finding out which snapshot (of state) should be running on which system.

What does nslookup do? Explain its two modes.

Nslookup is used to find details related to a Domain name server. Details like IP addresses of a machine, MX records, servers etc. It sends a domain name query packet to the corresponding DNS.

Nslookup has two modes. Interactive and non interactive. Interactive mode allows the user to interact by querying information about different hosts and domains.

Non interactive mode is used to fetch information about the specified host or domain.

Interactive mode:

Nslookup [options] [server]

What is Bash Shell?

Bash is a free shell for UNIX. It is the default shell for most UNIX systems. It has a combination of the C and Korn shell features. Bash shell is not portable. any Bash-specific feature will not function on a system using the Bourne shell or one of its replacements, unless bash is installed as a secondary shell and the script begins with #!/bin/bash. It supports regular and expressions. When bash script starts, it executes commands of different scripts.

Explain Some Network-Monitoring Tools in Linux: ping, traceroute, tcpdump, ntop

Network monitoring tools are used to monitor the network, systems present on the network, traffic etc.

Ping: Ping command is used to check if the system is in the network or not. To check if the host is operating.

e.g. ping ip_address

When the command is executed, it returns a detailed summary of the host. Packets sent, received, lost by estimating the round trip time.

Traceroute : the command is used to trace the path taken by the packet across a network. Tracing the path here means finding out the hosts visited by the packet to reach its destination. This information is useful in debugging. Roundtrip time in ms is shown for every visit to a host.

Tcpdump: commonly used to monitor network traffic. Tcdump captures and displays packet headers and matching them against criteria or all. It interprets Boolean operators and accepts host names, ip address, network names as arguments.

Ntop : Network top shows the network usage. It displays summary of network usage by machines on the network in a format as of UNIX top utility. It can also be run in web mode, which allows the display to be browsed with a web browser. It can display network traffic statistics, identify host etc. Interfaces are available to view such information.

Explain file system of linux. The root "/" filesystem, /usr filesystem, /var filesystem, /home filesystem, /proc filesystem.

Root "/" file system: The kernel needs a root file system to mount at start up. The root file system is generally small and should not be changed often as it may interrupt in booting. The root directory usually does not have the critical files. Instead sub directories are created. E.g. /bin (commands needed during bootup), /etc (config files) , /lib(shared libraries).

/usr filesystem : this file system is generally large as it contains the executable files to be shared amongst different machines. Files are usually the ones installed while installing Linux. This makes it possible to update the system from a new version of the distribution, or even a completely new distribution, without having to install all programs again. Sub directories include /bin, /include, /lib, /local (for local executables)

/var filesystem : this file system is specific to local systems. It is called as var because the data keeps changing. The sub directories include /cache/man (A cache for man pages), /games (any variable data belong to games), /lib (files that change), /log (log from different programs), /tmp (for temporary files)

/home filesystem: - this file system differs from host to host. User specific configuration files for applications are stored in the user's home directory in a file. UNIX creates directories for all users directory. E.g /home/my_name. Once the user is logged in ; he is placed in his home directory.

/proc filesystem :  this file system does  not exist on the hard disk. It is created by the kernel in its memory to provide information about the system. This information is usually about the processes. Contains a hierarchy of special files which represent the current state of the kernel .Few of the Directories include /1 (directory with information about process num 1, where 1 is the identification number), /cpuinfo (information about cpu), /devices (information about devices installed), /filesystem (file systems configured), /net (information about network protocols), /mem (memory usage)

What are the process states in Linux?

Process states in Linux:

Running: Process is either running or ready to run

Interruptible: a Blocked state of a process and waiting for an event or signal from another process

Uninterruptible:- a blocked state. Process waits for a hardware condition and cannot handle any signal

Stopped: Process is stopped or halted and can be restarted by some other process

Zombie: process terminated, but information is still there in the process table.

What is a zombie?

Zombie is a process state when the child dies before the parent process. In this case the structural information of the process is still in the process table. Since this process is not alive, it cannot react to signals. Zombie state can finish when the parent dies. All resources of the zombie state process are cleared by the kernel

Explain each system calls used for process management in linux.

System calls used for Process management:

Fork () :- Used to create a new process

Exec() :- Execute a new program

Wait():- wait until the process finishes execution

Exit():- Exit from the process

Getpid():- get the unique process id of the process

Getppid():- get the parent process unique id

Nice():- to bias the existing property of process

Oracle DBA interview questions

By karthick , in Oracle DBA interview questions , 0 Comments

Oracle DBA interview questions

Explain the difference between a hot backup and a cold backup.

A cold backup is done when there is no user activity going on with the system. Also called as offline backup, is taken when the database is not running and no users are logged in. all files of the database are copied and no changes during the copy are made.

A hot backup is taken when the database needs to run all the time. It is an online backup. All files of the database are copied and there may be changes to the database during the copy.

How many memory layers are in the oracle shared pool? Explain them

Oracles shared pool consists of two layers namely, Library cache and Data dictionary cache.

Library cache: This layer has information about SQL statements that were parsed, information about cursors and any plan data.

Data Dictionary cache: this Layer has information about the accounts of the users, their privileges and segments information.

What is the cache hit ratio, what impact does it have on performance of an Oracle database and what is involved in tuning it?

When the cache client like a CPU, web browser etc finds a successful entry in the cache memory, it is called as a cache hit. The percentage of these successful cache hits is called as cache hit ratio. Higher the cache hit ratio better will be the performance because data read from the cache is faster than the memory.

Explain the concept of the DUAL table.

On installing Oracle database, DUAL table Is present by default. It is a special table with just one row. It has a single column called DUMMY. The data type of this column in VARCHAR2(1). It has a value “X”. It is most commonly used to select pseudo columns in Oracle like sysdate.

Select sysdate from dual

What are the ways tablespaces can be managed and how do they differ?

Objects can be assigned to a table space. The related objects can be then grouped together. Table space can also be managed using extents. Extents consist of a specific number of contiguous data blocks. For the required extent, the free extent closest in size is determined.

Explain what are Oracle Catalog and Oracle Archive log?

Oracle catalog contains tables and views to get information about the database. It helps user to understand the available tables, attributes, constraints etc.

Oracle Archive log mode of a database in Oracle, ensures, the online redo logs are not overwritten before they are archived. This ensures that recovery is possible.

What are PCT Free and PCT Used? What is PCT increase parameter in segment?

PCTFREE is a parameter used to find how much space should be left in a database block for future updates. This means that if the PCTFREE = 20, new rows will be added in the block until it is 80% full.

PCTUSED is a parameter helps Oracle to find when it should consider a database block to be empty enough to be added to the freelist. This means that if the PCTFREE = 50, new rows will be not be added in the block until sufficient rows are deleted from the block so that it falls below 40% empty.

PCTINCREASE parameter is used to find how much will the each subsequent segment will grow. This value is in %.

What is dump destination? What are bdump, cdump and udump?

Trace files for Oracle processes are stored in dump destination.

Bdump- Oracle writes to the trace log and creates trace files for background processes in background dump destination. If this directory becomes full and more files cannot be written, debugging becomes difficult.

Cdump- Oracle writes core files and background processes in Core dump destination directory. If this directory becomes full and more files cannot be written, debugging becomes difficult.

Udump – Oracle creates trace files for the user processes in the User Dump directory if this directory becomes full and more files cannot be written, debugging becomes difficult.

How do you increase the performance of %LIKE operator?

LIKE% works the fastest because it uses the index to search on the column provided an index is specified on the column. Using % after LIKE, results in faster results.

Why use materialized view instead of a table

Materialized views are basically used to increase query performance since it contains results of a query. They should be used for reporting instead of a table for a faster execution.

Why and how the deadlock situation arises

A deadlock situation arises when two or more users wait for the same resource locked by one anther or two or more processes wait to update rows which are locked by other processes. Oracle if detects a deadlock, rolls back the session chosen by the deadlock victim.

What are standby databases? Difference between Physical and logical standby databases

A standby database is a replica of the original database. In order to keep both the database synchronized, archived redo logs can be used. It is mainly used in disaster protection. It can also be opened in read only mode which allows it to be used independently for reporting.

A logical standby database allows new database objects like tables, indexes to be added to the database. On the other hand, the physical standby database is a physical or structural copy of primary database. They can be opened in read only for disaster recovery.

What is Cache Fusion Technology?

In Cache fusion, multiple buffers join to act as one. It eliminates disk i/o operaions by making use of a scalable shared cache. It treats multiple buffer caches as one thereby resolving data consistency issues. Cash fusion technology can provide more resources and increases concurrency of users.

What is the difference between Cloning and Standby databases?

The clone database is a copy of the database which can be opened in read write mode. It is treated as a separate copy of the database that is functionally completely separate. The standby database is a copy of the production database used for disaster protection. In order to update the standby database; archived redo logs from the production database can be used. If the primary database is destroyed or its data becomes corrupted, one can perform a failover to the standby database, in which case the standby database becomes the new primary database.