UNIX - LINUX Interview Questions and Answers :
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
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
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
'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
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
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
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!");
}
{
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!");
}
{
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.
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.
* To display a list of all manual pages containing the keyword "date", what command would you type?
* What command will display the first several lines of a file called "junk"?
== Users and permissions practicum ==
* Rig it so everything in the folder gets deleted tonight at 10pm. Every night at 10pm.
== Local security ==
* How do you feel about `sudo`?
* What's the difference between `telnet` and `ssh`? What's a good use for each?
* How do you ensure your users have hard-to-guess passwords?
== Filesystem ==
* What is the difference between a symbolic and hard link? When would you use each?
* I have a file named `-fr`. How do I get rid of it?
* Why did I just ask that question?
* To partition or not? How?
* What are RAID 0, 1, 5, 0+1? What level would you use for a web server and why? A database server?
== `/etc` ==
* `ls -l /etc`. What is all this stuff?
* You added a line to `/etc/aliases`, but it doesn't seem to be working. Why?
* You've created a `zope` user to run Zope under. How do you secure it so someone doesn't guess its password, log in with it, and mess with stuff?
* Bring up `/etc/passwd`. What is all this junk?
* What are shadow passwords?
== Processes ==
* How many processes are running on your machine right now?
== Shells ==
* Name as many shells as you can.
* What's your favorite shell? Why?
* Write a shell script to append "snork" to the file "test" but only if "test" already exists.
* A user performed a `cd; chmod 644 .` before logging out. What problem occurs when he logs in the next time, and what level of privilege is required to correct the problem?
== Startup ==
* Describe the boot process of your favorite Linux in as much detail as you can.
* What are runlevels?
== Social ==
* Describe an experience you had with a difficult user.
* How do you keep up with current tools and practices?
* How did you document your work at your last job so someone else could pick up where you left off?
== Totally miscellaneous ==
* When debugging a core in gdb, what does the command `bt` give: core memory, heap usage, or calling stack?
* A user complains the web site is slow. What do you do?
== Apache ==
* How do you rig Apache to start up on boot?
* Apache doesn't start up on boot, and the thing above checks out okay. How do you track down the problem?
*To display a list of all manual pages containing the keyword "date", what command would you type?
Code:
man -k date man -f date
* What command will display the first several lines of a file called "junk"?
Code:
head junk man head
* Rig it so everything in the folder gets deleted tonight at 10pm. Every night at 10pm.
Set cronjob, see How do I add jobs to cron under Linux or UNIX oses?
== Local security ==
* How do you feel about `sudo`?
sudo allows a permitted user to execute a command as the superuser or another user. sudo is much better than su and you don't have to share root password with other users/admin.
Linux sudo Configuration
* What's the difference between `telnet` and `ssh`? What's a good use for each?
TELNET, by default, does not encrypt any data sent over the connection (including password, and so it is often practical to eavesdrop on the communications and use the password later for malicious purposes;
SSH by default encrypt password and traffic. SSH is recommended for all use.
* How do you ensure your users have hard-to-guess passwords?
Set password policy, see
Howto: Protect account against a password cracking attack
Linux check passwords against a dictionary attack
Linux Password Cracking: Explain unshadow and john commands ( john the ripper tool )
== Filesystem ==
* What is the difference between a symbolic and hard link? When would you use each?
How to: Linux / UNIX create soft link with ln command
Understanding UNIX / Linux symbolic (soft) and hard links
* I have a file named `-fr`. How do I get rid of it?
Code:
rm -- -fr rm \-rf
* Why did I just ask that question?
For testing UNIX concepts and command line args.
* To partition or not? How?
Sure. See The importance of Linux partitions
* What are RAID 0, 1, 5, 0+1? What level would you use for a web server and why? A database server?
See level @ What are the different RAID levels for Linux / UNIX and Windows Server?
More about RAID - Can RAID Act As The Reliable BACKUP Solution For Linux / UNIX / Windows Server?
Linux Check The Health of Adaptec RAID array
== `/etc` ==
* `ls -l /etc`. What is all this stuff?
See, Linux / UNIX - Display the permissions of a file
* You added a line to `/etc/aliases`, but it doesn't seem to be working. Why?
Restart sendmail so that file get updated. The program "newaliases" must be run after this file is updated for any changes to show through to sendmail / postfix.
Code:
newaliases
Deny login access to zope and set shell to /sbin/nologin. There are other ways too...
* Bring up `/etc/passwd`. What is all this junk?
See, Understanding /etc/passwd file format
* What are shadow passwords?
/etc/shadow file stores actual password in encrypted format for user's account with additional properties related to user password
Understanding /etc/ shadow file
== Processes ==
* How many processes are running on your machine right now?
Code:
top atop ps -e ps aux ps aux | wc -l man ps
* Name as many shells as you can.
Bourne shell (sh)
Almquist shell (ash)
Debian Almquist shell (dash)
Bourne-Again shell (bash)
Friendly interactive shell (fish)
Korn shell (ksh)
C shell (csh)
TENEX C shell (tcsh)
Es shell (e
esh (Unix) Easy Shell
rc shell (rc) - shell for Plan 9 and Unix
runscript The initial shell interpreter used to process startup scripts in Gentoo
scsh (Scheme Shell)
Stand-alone Shell (sash)
Z shell (zsh)
* What's your favorite shell? Why?
bash - it rocks and feature rich.
* Write a shell script to append "snork" to the file "test" but only if "test" already exists.
Code:
[ -f test ] && echo "snork" >> test ||:
User will not able to login. A root user can this problem by resting permission
cmod perm /home/user
. is current directory
.. parent directory
== Startup ==
* Describe the boot process of your favorite Linux in as much detail as you can.
See redhat or any other distro doc
* What are runlevels?
The term runlevel refers to a mode of operation in one of the computer operating systems that implement Unix System V-style initialization. Conventionally, seven runlevels exist, numbered from zero to six, though up to ten, from zero to nine, may be used.
Code:
man init man runlevel
* Describe an experience you had with a difficult user.
* How do you keep up with current tools and practices?
* How did you document your work at your last job so someone else could pick up where you left off?
Use our social skillz
== Totally miscellaneous ==
* When debugging a core in gdb, what does the command `bt` give: core memory, heap usage, or calling stack?
Code:
man gdb Read gdb page
Ask user to upgrade internet connection. If using windows ask to reboot windows .. LOL just kidding, google for slow apache problem. There could be zillions of causes
== Apache ==
* How do you rig Apache to start up on boot?
Code:
chkconfig httpd on
Code:
chkconfig httpd on httpd -t service httpd on netstat -tulpn | grep 80 tail -f /var/log/httpd/access_log tail -f /var/log/httpd/error_log
1. What is the transpernet proxy? 2.Can a squid proxy server is used as DNS, if yes how? 3.Is swap partisition mandetory in any installation, and why it is nessesary to define at the time of installation? 4.can apache be use as proxy server? how? wht is reverse proxy? 5.wht is the ssh version you are using. What are the drawbacks of ssh? What is the defference between ssh version 1 and 2?
- What are setuid/setgid in relation to file permissions?
- What are setuid/setgid in relation to directory permissions?
- What is an inode?
- What does init do? What does inetd do?
- What’s PGP/GPG; Public/Private Key cryptographic systems
- What’s ssh? Setting up trust between accounts.
- How does ssl work?
- What are different directories in / for?
- What to do , if the newly build kernel does not boot.?
- Do you know what source management is? have you used it? what software?
- How does the boot process (init levels) work on Linux
- What do you know about configuring and/or compiling Apache;
- Virtual Hosts?
- .htaccess files?
- mod_perl? mod_php?
- log files and log management
- What shell do you use?
- bash?
- do you know any other?
- Name some basic shell command like cut and explain what they do
- Sed/awk?
- Which editor? vim/emacs/something else
More.....>>
No comments:
Post a Comment