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Unix Interview Questions and Answers
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).
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
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).
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.
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).
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
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).
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.
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.
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.
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