Linux Importan Questions & Answers

1.How do u force a user to change password on next login in linux?

A.  chage -d 0 "User"

2.What command can be used to findout server architechure (x86 or x64) apart from 

    uname?

A. arch

3. How do u find out the users who are NOT logged in for more than 30 days? which file u will check?

A. last, lastlog,   /var/log/wtmp

4. whats is called 1.5 stage in boot process of linux?

A. Which contains extra code to allow cylinders above 1024, or LBA type drives, to be read. The  

    1.5 boot loader is stored (if needed) in the MBR or the boot partition.The great thing about  

    GRUB is that it includes knowledge of Linux file systems. Instead of using raw sectors on the 

    disk, as LILO does, GRUB can load a Linux kernel from an ext2 or ext3 file system. It does this 

    by making the two-stage boot loader into a three-stage boot loader. Stage 1 (MBR) boots a 

    stage 1.5 boot loader that understands the particular file system containing the Linux kernel 

    image. Examples include reiserfs_stage1_5 (to load from a Reiser journaling file system) or  

   e2fs_stage1_5 (to load from an ext2 or ext3 file system). When the stage 1.5 boot loader is   

    loaded and running, the stage 2 boot loader can be loaded."

    So Basically,

    Stage 1 Boot loader is MBR

    Stage 2 Boot loader is GRUB

    Stage 1.5 Boot loader is e2fs_stage1_5 

    (Basically this module will load the knowledge of Filesystem to Grub to read the kernel)

5. When u try to create a file, u got a error that "No space available". But actually space  available on volume? How do u resolve this issue?

A. Try this df -i   list inode information instead of block usage [Perhaps are you out of inodes on 

    this file system.], To "rectify it", remove unwanted files or move them somewhere else.]

What is LILO?

LILO stands for Linux boot loader. It will load the MBR, master boot record, into the memory, and tell the system which partition and hard drive to boot from.

What is the main advantage of creating links to a file instead of copies of the file?

A: The main advantage is not really that it saves disk space (though it does that too) but, rather, that a change of permissions on the file is applied to all the link access points. The link will show permissions of lrwxrwxrwx but that is for the link itself and not the access to the file to which the link points. Thus if you want to change the permissions for a command, such as su, you only have to do it on the original. With copies you have to find all of the copies and change permission on each of the copies.

Write a command to find all of the files which have been accessed within the last 30 days.

find / -type f -atime -30 > December.files

This command will find all the files under root, which is ‘/’, with file type is file. ‘-atime -30′ will give all the files accessed less than 30 days ago. And the output will put into a file call December.files.

What is the most graceful way to get to run level single user mode?

A: The most graceful way is to use the command init s.
If you want to shut everything down before going to single user mode then do init 0 first and from the ok prompt do a boot -s.

What does the following command line produce? Explain each aspect of this line.

$ (date ; ps -ef | awk ‘{print $1}’ | sort | uniq | wc -l ) >> Activity.log

A: First let’s dissect the line: The date gives the date and time as the first command of the line, this is followed by the a list of all running processes in long form with UIDs listed first, this is the ps -ef. These are fed into the awk which filters out all but the UIDs; these UIDs are piped into sort for no discernible reason and then onto uniq (now we see the reason for the sort - uniq only works on sorted data - if the list is A, B, A, then A, B, A will be the output of uniq, but if it’s A, A, B then A, B is the output) which produces only one copy of each UID.

These UIDs are fed into wc -l which counts the lines - in this case the number of distinct UIDs running processes on the system. Finally the results of these two commands, the date and the wc -l, are appended to the file "Activity.log". Now to answer the question as to what this command line produces. This writes the date and time into the file Activity.log together with the number of distinct users who have processes running on the system at that time. If the file already exists, then these items are appended to the file, otherwise the file is created.

You need to see the last fifteen lines of the files dog, cat and horse. What command should you use?
tail -15 dog cat horse 

The tail utility displays the end of a file. The -15 tells tail to display the last fifteen lines of each specified file.

Who owns the data dictionary? 
The SYS user owns the data dictionary. The SYS and SYSTEM users are created when the database is created.

You routinely compress old log files. You now need to examine a log from two months ago. In order to view its contents without first having to decompress it, use the _________ utility. 
zcat 

The zcat utility allows you to examine the contents of a compressed file much the same way that cat displays a file.

You suspect that you have two commands with the same name as the command is not producing the expected results. What command can you use to determine the location of the command being run? 
which 

The which command searches your path until it finds a command that matches the command you are looking for and displays its full path.

You locate a command in the /bin directory but do not know what it does. What command can you use to determine its purpose. 
whatis 

The whatis command displays a summary line from the man page for the specified command.

You wish to create a link to the /data directory in bob's home directory so you issue the command ln /data /home/bob/datalink but the command fails. What option should you use in this command line to be successful. 
Use the -F option 

In order to create a link to a directory you must use the -F option.

When you issue the command ls -l, the first character of the resulting display represents the file's ___________. 
type 

The first character of the permission block designates the type of file that is being displayed.

What utility can you use to show a dynamic listing of running processes? __________ 
top 

The top utility shows a listing of all running processes that is dynamically updated.

Where is standard output usually directed? 
to the screen or display 

By default, your shell directs standard output to your screen or display.

You wish to restore the file memo.ben which was backed up in the tarfile MyBackup.tar. What command should you type? 
tar xf MyBackup.tar memo.ben 

This command uses the x switch to extract a file. Here the file memo.ben will be restored from the tarfile MyBackup.tar.

You need to view the contents of the tarfile called MyBackup.tar. What command would you use?
tar tf MyBackup.tar 

The t switch tells tar to display the contents and the f modifier specifies which file to examine.

You want to create a compressed backup of the users' home directories. What utility should you use? 
tar 

You can use the z modifier with tar to compress your archive at the same time as creating it.

What daemon is responsible for tracking events on your system? 
syslogd 

The syslogd daemon is responsible for tracking system information and saving it to specified log files.

You have a file called phonenos that is almost 4,000 lines long. What text filter can you use to split it into four pieces each 1,000 lines long? 
split 

The split text filter will divide files into equally sized pieces. The default length of each piece is 1,000 lines.

You would like to temporarily change your command line editor to be vi. What command should you type to change it? 
set -o vi 

The set command is used to assign environment variables. In this case, you are instructing your shell to assign vi as your command line editor. However, once you log off and log back in you will return to the previously defined command line editor.

What account is created when you install Linux? 
root 

Whenever you install Linux, only one user account is created. This is the superuser account also known as root.

What command should you use to check the number of files and disk space used and each user's defined quotas? 

repquota 

The repquota command is used to get a report on the status of the quotas you have set including the amount of allocated space and amount of used space.

In order to run fsck on the root partition, the root partition must be mounted as 
readonly 

You cannot run fsck on a partition that is mounted as read-write.

In order to improve your system's security you decide to implement shadow passwords. What command should you use? 
pwconv 

The pwconv command creates the file /etc/shadow and changes all passwords to 'x' in the /etc/passwd file.

Bob Armstrong, who has a username of boba, calls to tell you he forgot his password. What command should you use to reset his command? 
passwd boba 

The passwd command is used to change your password. If you do not specify a username, your password will be changed.

The top utility can be used to change the priority of a running process? Another utility that can also be used to change priority is ___________? 
nice 

Both the top and nice utilities provide the capability to change the priority of a running process.

What command should you type to see all the files with an extension of 'mem' listed in reverse alphabetical order in the /home/ben/memos directory. 
ls -r /home/ben/memos/*.mem 

The -c option used with ls results in the files being listed in chronological order. You can use wildcards with the ls command to specify a pattern of filenames.

What file defines the levels of messages written to system log files? 
kernel.h 

To determine the various levels of messages that are defined on your system, examine the kernel.h file.

What command is used to remove the password assigned to a group? 
gpasswd -r 

The gpasswd command is used to change the password assigned to a group. Use the -r option to remove the password from the group.

What command would you type to use the cpio to create a backup called backup.cpio of all the users' home directories? 
find /home | cpio -o > backup.cpio 

The find command is used to create a list of the files and directories contained in home. This list is then piped to the cpio utility as a list of files to include and the output is saved to a file called backup.cpio.

What can you type at a command line to determine which shell you are using? 
echo $SHELL 

The name and path to the shell you are using is saved to the SHELL environment variable. You can then use the echo command to print out the value of any variable by preceding the variable's name with $. Therefore, typing echo $SHELL will display the name of your shell.

What type of local file server can you use to provide the distribution installation materials to the new machine during a network installation?
A) Inetd
B) FSSTND
C) DNS
D) NNTP
E) NFS 
E - You can use an NFS server to provide the distribution installation materials to the machine on which you are performing the installation. Answers a, b, c, and d are all valid items but none of them are file servers. Inetd is the superdaemon which controls all intermittently used network services. The FSSTND is the Linux File System Standard. DNS provides domain name resolution, and NNTP is the transfer protocol for usenet news.

If you type the command cat dog & > cat what would you see on your display? Choose one: 
a. Any error messages only.
b. The contents of the file dog.
c. The contents of the file dog and any error messages.
d. Nothing as all output is saved to the file cat. 

d 

When you use & > for redirection, it redirects both the standard output and standard error. The output would be saved to the file cat.

You are covering for another system administrator and one of the users asks you to restore a file for him. You locate the correct tarfile by checking the backup log but do not know how the directory structure was stored. What command can you use to determine this? 
Choose one:
a. tar fx tarfile dirname
b. tar tvf tarfile filename
c. tar ctf tarfile
d. tar tvf tarfile 

d 

The t switch will list the files contained in the tarfile. Using the v modifier will display the stored directory structure.

You have the /var directory on its own partition. You have run out of space. What should you do? Choose one:
a. Reconfigure your system to not write to the log files.
b. Use fips to enlarge the partition.
c. Delete all the log files.
d. Delete the partition and recreate it with a larger size. 

d 

The only way to enlarge a partition is to delete it and recreate it. You will then have to restore the necessary files from backup.

You have a new application on a CD-ROM that you wish to install. What should your first step be? 
Choose one:
a. Read the installation instructions on the CD-ROM.
b. Use the mount command to mount your CD-ROM as read-write.
c. Use the umount command to access your CD-ROM.
d. Use the mount command to mount your CD-ROM as read-only. 

d 

Before you can read any of the files contained on the CD-ROM, you must first mount the CD-ROM.

When you create a new partition, you need to designate its size by defining the starting and ending _____________. 
cylinders 

When creating a new partition you must first specify its starting cylinder. You can then either specify its size or the ending cylinder.

What key combination can you press to suspend a running job and place it in the background?
ctrl-z 

Using ctrl-z will suspend a job and put it in the background.

The easiest, most basic form of backing up a file is to _____ it to another location. 
copy 

The easiest most basic form of backing up a file is to make a copy of that file to another location such as a floppy disk.

What type of server is used to remotely assign IP addresses to machines during the installation process? 
A) SMB
B) NFS
C) DHCP
D) FT
E) HTTP

C - You can use a DHCP server to assign IP addresses to individual machines during the installation process. Answers a, b, d, and e list legitimate Linux servers, but these servers do not provide IP addresses. The SMB, or Samba, tool is used for file and print sharing across multi-OS networks. An NFS server is for file sharing across Linux net-works. FTP is a file storage server that allows people to browse and retrieve information by logging in to it, and HTTP is for the Web.

Which password package should you install to ensure that the central password file couldn't be stolen easily? 
A) PAM
B) tcp_wrappers
C) shadow
D) securepass
E) ssh

C - The shadow password package moves the central password file to a more secure location. Answers a, b, and e all point to valid packages, but none of these places the password file in a more secure location. Answer d points to an invalid package.

When using useradd to create a new user account, which of the following tasks is not done automatically.
Choose one:
a. Assign a UID.
b. Assign a default shell.
c. Create the user's home directory.
d. Define the user's home directory.

c 

The useradd command will use the system default for the user's home directory. The home directory is not created, however, unless you use the -m option.

You want to enter a series of commands from the command-line. What would be the quickest way to do this? 
Choose One
a. Press enter after entering each command and its arguments
b. Put them in a script and execute the script
c. Separate each command with a semi-colon (;) and press enter after the last command
d. Separate each command with a / and press enter after the last command

c 

The semi-colon may be used to tell the shell that you are entering multiple commands that should be executed serially. If these were commands that you would frequently want to run, then a script might be more efficient. However, to run these commands only once, enter the commands directly at the command line.

You attempt to use shadow passwords but are unsuccessful. What characteristic of the /etc/passwd file may cause this? 
Choose one:
a. The login command is missing.
b. The username is too long.
c. The password field is blank.
d. The password field is prefaced by an asterisk. 

c 

The password field must not be blank before converting to shadow passwords.

When you install a new application, documentation on that application is also usually installed. Where would you look for the documentation after installing an application called MyApp?
Choose one:
a. /usr/MyApp
b. /lib/doc/MyApp
c. /usr/doc/MyApp
d. In the same directory where the application is installed.

c 

The default location for application documentation is in a directory named for the application in the /usr/doc directory.

What file would you edit in your home directory to change which window manager you want to use? 
A) Xinit
B) .xinitrc
C) XF86Setup
D) xstart
E) xf86init

Answer: B - The ~/.xinitrc file allows you to set which window man-ager you want to use when logging in to X from that account. 
Answers a, d, and e are all invalid files. Answer c is the main X server configuration file.

What command allows you to set a processor-intensive job to use less CPU time? 
A) ps
B) nice
C) chps
D) less
E) more

Answer: B - The nice command is used to change a job's priority level, so that it runs slower or faster. Answers a, d, and e are valid commands but are not used to change process information. Answer c is an invalid command.

While logged on as a regular user, your boss calls up and wants you to create a new user account immediately. How can you do this without first having to close your work, log off and logon as root? 
Choose one:
a. Issue the command rootlog.
b. Issue the command su and type exit when finished.
c. Issue the command su and type logoff when finished.
d. Issue the command logon root and type exit when finished. 

Answer: b 
You can use the su command to imitate any user including root. You will be prompted for the password for the root account. Once you have provided it you are logged in as root and can do any administrative duties.

There are seven fields in the /etc/passwd file. Which of the following lists all the fields in the correct order? 
Choose one:
a. username, UID, GID, home directory, command, comment
b. username, UID, GID, comment, home directory, command
c. UID, username, GID, home directory, comment, command
d. username, UID, group name, GID, home directory, comment
Answer: b 
The seven fields required for each line in the /etc/passwd file are username, UID, GID, comment, home directory, command. Each of these fields must be separated by a colon even if they are empty.

Which of the following commands will show a list of the files in your home directory including hidden files and the contents of all subdirectories?
Choose one:
a. ls -c home
b. ls -aR /home/username
c. ls -aF /home/username
d. ls -l /home/username

Answer: b 
The ls command is used to display a listing of files. The -a option will cause hidden files to be displayed as well. The -R option causes ls to recurse down the directory tree. All of this starts at your home directory.

In order to prevent a user from logging in, you can add a(n) ________at the beginning of the password field. 
Answer: asterick 

If you add an asterick at the beginning of the password field in the /etc/passwd file, that user will not be able to log in.

You have a directory called /home/ben/memos and want to move it to /home/bob/memos so you issue the command mv /home/ben/memos /home/bob. What is the results of this action?
Choose one:
a. The files contained in /home/ben/memos are moved to the directory /home/bob/memos/memos.
b. The files contained in /home/ben/memos are moved to the directory /home/bob/memos.
c. The files contained in /home/ben/memos are moved to the directory /home/bob/.
d. The command fails since a directory called memos already exists in the target directory. 

Answer: a 
When using the mv command to move a directory, if a directory of the same name exists then a subdirectory is created for the files to be moved.

Which of the following tasks is not necessary when creating a new user by editing the /etc/passwd file? 
Choose one:
a. Create a link from the user's home directory to the shell the user will use.
b. Create the user's home directory
c. Use the passwd command to assign a password to the account.
d. Add the user to the specified group.

Answer: a 
There is no need to link the user's home directory to the shell command. Rather, the specified shell must be present on your system.

You issue the following command useradd -m bobm But the user cannot logon. What is the problem?
Choose one:
a. You need to assign a password to bobm's account using the passwd command.
b. You need to create bobm's home directory and set the appropriate permissions.
c. You need to edit the /etc/passwd file and assign a shell for bobm's account.
d. The username must be at least five characters long.

Answer: a 
The useradd command does not assign a password to newly created accounts. You will still need to use the passwd command to assign a password.

You wish to print the file vacations with 60 lines to a page. Which of the following commands will accomplish this? Choose one:
a. pr -l60 vacations | lpr
b. pr -f vacations | lpr
c. pr -m vacations | lpr
d. pr -l vacations | lpr

Answer: a 
The default page length when using pr is 66 lines. The -l option is used to specify a different length.

Which file defines all users on your system? 
Choose one:
a. /etc/passwd
b. /etc/users
c. /etc/password
d. /etc/user.conf

Answer: a 
The /etc/passwd file contains all the information on users who may log into your system. If a user account is not contained in this file, then the user cannot log in.

Which two commands can you use to delete directories? 
A) rm
B) rm -rf
C) rmdir
D) rd
E) rd -rf

Answer(s): B, C - You can use rmdir or rm -rf to delete a directory. Answer a is incorrect, because the rm command without any specific flags will not delete a directory, it will only delete files. Answers d and e point to a non-existent command.

Which partitioning tool is available in all distributions? 
A) Disk Druid
B) fdisk
C) Partition Magic
D) FAT32
E) System Commander

Answer(s): B - The fdisk partitioning tool is available in all Linux distributions. Answers a, c, and e all handle partitioning, but do not come with all distributions. Disk Druid is made by Red Hat and used in its distribution along with some derivatives. Partition Magic and System Commander are tools made by third-party companies. Answer d is not a tool, but a file system type. Specifically, FAT32 is the file system type used in Windows 98.

Which partitions might you create on the mail server's hard drive(s) other than the root, swap, and boot partitions? 
[Choose all correct answers]
A) /var/spool
B) /tmp
C) /proc
D) /bin
E) /home

Answer(s): A, B, E - Separating /var/spool onto its own partition helps to ensure that if something goes wrong with the mail server or spool, the output cannot overrun the file system. Putting /tmp on its own partition prevents either software or user items in the /tmp directory from overrunning the file system. Placing /home off on its own is mostly useful for system re-installs or upgrades, allowing you to not have to wipe the /home hierarchy along with other areas. Answers c and d are not possible, as the /proc portion of the file system is virtual-held in RAM-not placed on the hard drives, and the /bin hierarchy is necessary for basic system functionality and, therefore, not one that you can place on a different partition.

When planning your backup strategy you need to consider how often you will perform a backup, how much time the backup takes and what media you will use. What other factor must you consider when planning your backup strategy? _________ 

what to backup 
Choosing which files to backup is the first step in planning your backup strategy.

What utility can you use to automate rotation of logs? 
Answer: logrotate 
The logrotate command can be used to automate the rotation of various logs.

In order to display the last five commands you have entered using the history command, you would type ___________ .

Answer: history 5 
The history command displays the commands you have previously entered. By passing it an argument of 5, only the last five commands will be displayed.

What command can you use to review boot messages? 
Answer: dmesg 
The dmesg command displays the system messages contained in the kernel ring buffer. By using this command immediately after booting your computer, you will see the boot messages.

What is the minimum number of partitions you need to install Linux? 
Answer: 2 
Linux can be installed on two partitions, one as / which will contain all files and a swap partition.

What is the name and path of the main system log? 
Answer: /var/log/messages 
By default, the main system log is /var/log/messages.

Of the following technologies, which is considered a client-side script? 
A) JavaScript
B) Java
C) ASP
D) C++

Answer: A - JavaScript is the only client-side script listed. Java and C++ are complete programming languages. Active Server Pages are parsed on the server with the results being sent to the client in HTML

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

'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. 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()

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.