UNIX Interview Questions with Answers Page I

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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 behavior 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:

chmod(myfile,0744).

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

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.

16. What is ex and vi ?

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

17. Which are typical system directories below the root directory?

(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.

18. Construct pipes to execute the following jobs.

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’.

19. Explain the following commands.

$ ls > file1

$ banner hi-fi > message

$ cat par.3 par.4 par.5 >> report

$ cat file1>file1

$ date ; who

$ date ; who > logfile

$ (date ; who) > logfile

20 What is the significance of the “tee” command?

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.