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UNIX Process Management - Interview Questions and Answers

Here you will find all the possible answers of UNIX Process Management  Job Interview Questions and Answers that you might be asked by your Upwork employer:

1. 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).

2. 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.

3. 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.

4. 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()"

5. 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

6. 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.

7. 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

8. 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();

9. 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.

10. 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.

11. 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

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.)

17. 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)

18. 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.

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