UNIX Interview Questions with Answers Page II

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1. Predict the output of the following program code

main()

{

fork(); fork(); fork();

printf("Hello World!");

}

Answer:

"Hello World" will be printed 8 times.

Explanation:

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

2. 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

3. 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()'.

4. 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.

5. 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.)

6. 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.

7. What Happens when you execute a program?

When you execute a program on your UNIX system, the system creates a special environment for that program. This environment contains everything needed

for the system to run the program as if no other program were running on the system. Each process has process context, which is everything that is unique

about the state of the program you are currently running. Every time you execute a program the UNIX system does a fork, which performs a series of

operations to create a process context and then execute your program in that context. The steps include the following:

Allocate a slot in the process table, a list of currently running programs kept by UNIX.

Assign a unique process identifier (PID) to the process.

iCopy the context of the parent, the process that requested the spawning of the new process.

Return the new PID to the parent process. This enables the parent process to examine or control the process directly. After the fork is complete, UNIX

runs your program.

8. What Happens when you execute a command?

When you enter 'ls' command to look at the contents of your current working directory, UNIX does a series of things to create an environment for ls and

the run it: The shell has UNIX perform a fork. This creates a new process that the shell will use to run the ls program. The shell has UNIX perform an

exec of the ls program. This replaces the shell program and data with the program and data for ls and then starts running that new program. The ls

program is loaded into the new process context, replacing the text and data of the shell. The ls program performs its task, listing the contents of the

current directory.

9. What is a Daemon?

A daemon is a process that detaches itself from the terminal and runs, disconnected, in the background, waiting for requests and responding to them. It

can also be defined as the background process that does not belong to a terminal session. Many system functions are commonly performed by daemons,

including the sendmail daemon, which handles mail, and the NNTP daemon, which handles USENET news. Many other daemons may exist. Some of the most common

daemons are:

init: Takes over the basic running of the system when the kernel has finished the boot process.

inetd: Responsible for starting network services that do not have their own stand-alone daemons. For example, inetd usually takes care of incoming

rlogin, telnet, and ftp connections.

cron: Responsible for running repetitive tasks on a regular schedule.

10. What is 'ps' command for?

The ps command prints the process status for some or all of the running processes. The information given are the process identification number (PID),the

amount of time that the process has taken to execute so far etc.

11. How would you kill a process?

The kill command takes the PID as one argument; this identifies which process to terminate. The PID of a process can be got using 'ps' command.

12. What is an advantage of executing a process in background?

The most common reason to put a process in the background is to allow you to do something else interactively without waiting for the process to complete.

At the end of the command you add the special background symbol, &. This symbol tells your shell to execute the given command in the background.

Example: cp *.* ../backup& (cp is for copy)

13. How do you execute one program from within another?

The system calls used for low-level process creation are execlp() and execvp(). The execlp call overlays the existing program with the new one , runs

that and exits. The original program gets back control only when an error occurs. execlp(path,file_name,arguments..); //last argument must be NULL A

variant of execlp called execvp is used when the number of arguments is not known in advance. execvp(path,argument_array); //argument array should be

terminated by NULL

14. What is IPC? What are the various schemes available?

The term IPC (Inter-Process Communication) describes various ways by which different process running on some operating system communicate between each

other. Various schemes available are as follows: Pipes:

One-way communication scheme through which different process can communicate. The problem is that the two processes should have a common ancestor

(parent-child relationship). However this problem was fixed with the introduction of named-pipes (FIFO).

Message Queues :

Message queues can be used between related and unrelated processes running on a machine.

Shared Memory:

This is the fastest of all IPC schemes. The memory to be shared is mapped into the address space of the processes (that are sharing). The speed achieved

is attributed to the fact that there is no kernel involvement. But this scheme needs synchronization.

Various forms of synchronisation are mutexes, condition-variables, read-write locks, record-locks, and semaphores.

15. What is the difference between Swapping and Paging?

Swapping: Whole process is moved from the swap device to the main memory for execution. Process size must be less than or equal to the available main

memory. It is easier to implementation and overhead to the system. Swapping systems does not handle the memory more flexibly as compared to the paging

systems.

Paging:

Only the required memory pages are moved to main memory from the swap device for execution. Process size does not matter. Gives the concept of the

virtual memory.

It provides greater flexibility in mapping the virtual address space into the physical memory of the machine. Allows more number of processes to fit in

the main memory simultaneously. Allows the greater process size than the available physical memory. Demand paging systems handle the memory more

flexibly.

16. What does the command “ $who | sort –logfile > newfile” do?

The input from a pipe can be combined with the input from a file . The trick is to use the special symbol “-“ (a hyphen) for those commands that

recognize the hyphen as std input.In the above command the output from who becomes the std input to sort , meanwhile sort opens the file logfile, the

contents of this file is sorted together with the output of who (rep by the hyphen) and the sorted output is redirected to the file newfile.

17. What does the command “$ls | wc –l > file1” do?

ls becomes the input to wc which counts the number of lines it receives as input and instead of displaying this count , the value is stored in file1.

18. Which of the following commands is not a filter (a) man , (b) cat , (c) pg , (d) head

Ans: man . A filter is a program which can receive a flow of data from std input, process (or filter) it and send the result to the std output.

19. Explain the steps that a shell follows while processing a command.

After the command line is terminated by the key, the Shel goes ahead with processing the command line in one or more passes. The sequence is well defined

and assumes the following order.Parsing: The shell first breaks up the command line into words, using spaces and the delimiters, unless quoted. All

consecutive occurrences of a space or tab are replaced here with a single space.Variable evaluation: All words preceded by a $ are evaluated as

variables, unless quoted or escaped.Command substitution: Any command surrounded by back quotes is executed by the shell which then replaces the standard

output of the command into the command line.Wild-card interpretation: The shell finally scans the command line for wild-cards (the characters *, ?, [,

]).Any word containing a wild-card is replaced by a sorted list offile names that match the pattern. The list of these filenames then forms the arguments

to the command.PATH evaluation: It finally looks for the PATH variable to determine the sequence of directories it has to search in order to hunt for the

command.

20. What difference between cmp and diff commands?

cmp - Compares two files byte by byte and displays the first mismatch diff - tells the changes to be made to make the files identical