Tuesday, April 19, 2016

Oracle UNIX Interview Questions and Answers (FAQs)

1) What exactly is UNIX?
UNIX is written in C and it is portable. It runs on a range of computers from microprocessors to the largest mainframes. The source code is available and written in high level language which makes it easy to adapt for a particular requirement.

2) Explain about the return key?
Return Key signifies the end of a line of input; it must be presses before the system will interpret the characters you have types. It serves as an example of control character return key can be typed by holding the control key and typing M.

3) Explain about DELETE and BREAK?
These keys have significant meaning sometimes DELETE is called as RUBOUT key. Break is sometimes called INTERRUPT. In most SYSTEMS the delete key stops a program immediately, without waiting for it to finish on some systems, Ctrl-C provides this service. Break is a synonym for DELETE or CTRL-C.

4) Explain about TYPE-ahead?
The kernel reads what you type as you type it, even if it’s busy with something else, so you can type as fast as you want, whenever you want, even when some command is printing at you. Your I/P characters will appear intermixed with the O/P characters but they will be stored away and interpreted in the correct order.

5) How to get a computer aided instruction?
Your system may have a command called learn which provides computer aided instruction on the file system and basic commands, the editor, document preparation and even “C” programming. IF $learn exists on your system, it will teach you what to do from there.

6) Explain about cat?
Cat is the simplest of all the printing commands. Cat prints the contents of all the files named by its arguments. The named file or files are catenated the terminal one after another with nothing between. You have to be quick with CTRL-S to stop O/P from cat before it flows off your screen.

7) How to search files for lines that match a pattern?
Grep command searches for files for lines which match a pattern.
E.g. g/regular expression/p.grep will also look for lines that don’t match the pattern, when the option-V is sued. grep can be used to search several files in that case it will prefix the filename.

8) Describe about the root file system?
The root file system has to be present for the system to execute /bin, /dev, and /etc are always kept on the root system because when the system starts only files in the root system are accessible and some files such as /bin/sh are needed to run at all. During the boot strap operation, all the file systems are checked for self consistency and attached to the root systems.

9) Explain about ZAP?
ZAP, which selectively kills processes, is another program. The main problem with that version is speed; it creates so many processes that it runs slowly, which is especially undesirable for a program that kills errant processes rewriting ZAP in C will make it faster.

10) Suppose that you are using a terminal in which the screen size is bigger than the normal 24 lines. If you want to use p and take full advantage of your terminal capabilities what choices are open to you?
You have to specify the screen size each time you use P. $P-36.
Also you could put a shell file in your bin.
Another solution is to modify p to use an environment variable that defines the properties of your terminal.

11) Explain abut low-level I/O?
The lowest of I/O is a direct entry into the O.S. your program reads or writes files in chunks of any convenient size. The kernel buffers your data into chunks that match the peripheral devices and schedule operations on the devices to optimize their performance over all users.

12) What are the special arrangements to make a terminal i/p and o/p?
When it is started by the shell, a program inherits three open files, with file descriptor 0, 1 and 2 called the standard i/p, the standard o/p, and the standard error. If the program reads “0” and writes descriptors 1 and 2, it can do I/O without opening files.

13) Explain about read slow?
One can call to read return 0 which signals the end of file life, if data is written on that file then a subsequent read will be able to find more bytes available. This observation is the basis of a program called readslow which continues to read its input, regardless of whether it got an end of file or not. Readslow is handy for watching the progress of the file.

14) What function does “errno” do?
Sometimes it is nice to know what specific error occurred; for this purpose all system calls, when appropriate, leave an error number in an external integer called calls, when appropriate, leave an error number in an external integer called errno. By using errno, your program can, for example, determine whether an attempt to open a file failed because it did not exist or because you lacked permission to read it.

15) Describe the process of “spname”?
The operation of spname is straightforward enough, although there are a lot of boundary conditions to get right. Suppose the file name is /d1/d2/f. The basic ideas is to peel off the first component (/), then search that directory for a name close to the nest component (dl), then search that directory for something near d2, and so on, until a match has been found for each component. If at any stage there isn’t a plausible candidate in the directory, the search is abandoned.

16) Explain about fork?
Splitting is done by a system call named fork. Proc_id = fork (); splits the program into two copies, both of which continue to run. The only difference between the two is the value returned by fork, the process-id. Two copies of the program are made by the fork. In the child, the value returned by fork is zero, so it calls execlp, which does the command line and then dies. In the parent, fork returns non-zero so it skips the execlp.

17) What is the easiest way to store variables and explain?
The easiest way to store the values of the variables is in a 26-element array; the single-letter variable name can be used to index the array. But if the grammar is to process both variable names and values in the same stack, yacc has to be told that its stack contains a union of a double and an int, not just a double.

1. 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).

2. 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

3. 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).

4. What are the Unix system calls for I/O?
* 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.

5. 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 chmod(filename,mode).
Example 1:
To change mode of myfile 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:

6. 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 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

7. 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 interprocess communication where a process writes to one end of the pipe (producer) and the other reads from the other end (consumer).

8. 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.

9. 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.

10. 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.

11. 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.

12. 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).

13. 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

14. 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.

15. Predict the output of the following program code
  printf("Hello World!");

Hello World!Hello World!

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.

16. Predict the output of the following program code
fork(); fork(); fork();
printf("Hello World!");

"Hello World" will be printed 8 times.

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

17. 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.

18. 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()'.

19. 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.

20. 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.)

21. What are the process states in Unix?
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.

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