this r som q/s wh may r may not serve ur surpose
unix interview questions:
1.Which is the command used to find out currently executing Process in UNIX?.
2.What is the output of : $ ps-e $ ps-a commands.
3.How to check whether some services are running in another server or not? Suppose i am currently in a servernamed A.I want to check whether some services are running in server B without logging into the server B.
4.By using which command we can find the CPU utilization time?
5.How to find the files that are accessed before 10 minutes?
UNIX – LINUX Interview Questions and Answers :
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
(user permission-group permission-others permission)
r-read, w-write, x-execute
To change the access mode, we use chmod(filename,mode).
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:
Each operation is represented by discrete values
‘w’ is 2
‘x’ is 1
Therefore, for ‘rw’ the value is 6(4+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:
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.
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
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
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();
“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.
some other important questions:
- What are the main differences between Apache 1.x and 2.x?
- What does the “route” command do?
- What are the read/write/execute bits on a directory mean?
- What does iostat do?what does vmstat do?
- What does netstat do?
- What is the most graceful way to bring a system into single user mode?
- How do you determine disk usage?
- What is AWK?
- What is SED?
- What is the difference between binaries in /bin, and /usr/bin?
- What is a dynamically linked file?
- What is a statically linked file?
challenging c coding problems:
1.write a program that prints itself
2.write a code for a program that takes 2 names as input and prints the relation between them.(flames)
3.write a code for proogram tha accepts 2 numbers as input and prints their result of division as p/q form.
4.write a code for program that prints the maximum subarray that has maximum sum in a given array.
5.write a code for a program that prints the nth prime number(n is given as input).
c interview questions:
1. What is data structure?
A data structure is a way of organizing data that considers not only the itemsstored, but also their relationship to each other. Advance knowledge about therelationship between data items allows designing of efficient algorithms for themanipulation of data.
2.List out the areas in which data structures are applied extensively?
Database Management System
Statistical analysis package
3.What are the major data structures used in the following areas : RDBMS, Networkdata model & Hierarchical data model.
RDBMS – Array (i.e. Array of structures)
Network data model – Graph
Hierarchical data model – Trees
4. If you are using C language to implement the heterogeneous linked list, what pointertype will you use?
The heterogeneous linked list contains different data types in its nodes and weneed a link, pointer to connect them. It is not possible to use ordinary pointers for this. Sowe go for void pointer. Void pointer is capable of storing pointer to any type as it is ageneric pointer type.
5. Minimum number of queues needed to implement the priority queue?
Two. One queue is used for actual storing of data and another for storingpriorities.
6. What is the data structures used to perform recursion?
Stack. Because of its LIFO (Last In First Out) property it remembers its ‘caller’ soknows whom to return when the function has to return. Recursion makes use of systemstack for storing the return addresses of the function calls.Every recursive function has its equivalent iterative (non-recursive) function.Even when such equivalent iterative procedures are written, explicit stack is to be used.
7.What are the notations used in Evaluation of Arithmetic Expressions using prefix andpostfix forms?
Polish and Reverse Polish notations.
8. Convert the expression ((A + B) * C – (D – E) ^ (F + G)) to equivalent Prefix andPostfix notations.
Prefix Notation:^ – * +ABC – DE + FG
Postfix Notation:AB + C * DE – - FG + ^
9. Sorting is not possible by using which of the following methods?
Using insertion we can perform insertion sort, using selection we can performselection sort, using exchange we can perform the bubble sort (and other similar sortingmethods). But no sorting method can be done just using deletion.
10.What is C language?
The C programming language is a standardized programming languagedeveloped in the early 1970s by Ken Thompson and Dennis Ritchie foruse on the UNIX operating system. It has since spread to many otheroperating systems, and is one of the most widely used programminglanguages. C is prized for its efficiency, and is the most popularprogramming language for writing system software, though it is alsoused for writing applications.
11.printf() FunctionWhat is the output of printf(“%d”)?
1). When we write printf(“%d”,x); this means compiler will print thevalue of x. But as here, there is nothing after %d so compiler will showin output window garbage value.
2). When we use %d the compiler internally uses it to access theargument in the stack (argument stack). Ideally compiler determinesthe offset of the data variable depending on the format specificationstring. Now when we write printf(“%d”,a) then compiler first accessesthe top most element in the argument stack of the printf which is %dand depending on the format string it calculated to offset to the actualdata variable in the memory which is to be printed. Now when only %dwill be present in the printf then compiler will calculate the correctoffset (which will be the offset to access the integer variable) but asthe actual data object is to be printed is not present at that memorylocation so it will print what ever will be the contents of that memorylocation.
3). Some compilers check the format string and will generate an errorwithout the proper number and type of arguments for things likeprintf(…) and scanf(…).
12.malloc() Function- What is the difference between “calloc(…)” and”malloc(…)”?
1. calloc(…) allocates a block of memory for an array of elements of acertain size. By default the block is initialized to 0. The total number ofmemory allocated will be (number_of_elements * size). malloc(…) takes in only a single argument which is the memoryrequired in bytes. malloc(…) allocated bytes of memory and not blocksof memory like calloc(…).
2. malloc(…) allocates memory blocks and returns a void pointer to theallocated space, or NULL if there is insufficient memory available. calloc(…) allocates an array in memory with elements initialized to 0and returns a pointer to the allocated space. calloc(…) calls malloc(…)in order to use the C++ _set_new_mode function to set the newhandler mode.