In this post, you will find all the possible and most recent Job Interview questions and answers of UNIX File Management for OS category which you might be asked by your employer. These interview questions and answers will help you to get job.
1. 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).
2. 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.
3. 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).
4. 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)
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).
5. How do you create special files like named pipes and device files?
The system call mknod creates special files in the following sequence.
• kernel assigns new inode,
• sets the file type to indicate that the file is a pipe, directory or special
file,
• 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.
6. 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.
7. 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"
8. 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.
9. 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).
10. 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.
11. 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
