Unix File Management question and answer for Interview

1. 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.

2. 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).

3. How do you change File Access Permissions?
Every file has following attributes:

  1. owner's user ID ( 16 bit integer )
  2. owner's group ID ( 16 bit integer )
  3. 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: 

4. 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).

5. 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).

6. What are the Unix system calls for I/O?

  1. open(pathname,flag,mode) - open file
  2. creat(pathname,mode) - create file
  3. close(filedes) - close an open file
  4. read(filedes,buffer,bytes) - read data from an open file
  5. write(filedes,buffer,bytes) - write data to an open file
  6. lseek(filedes,offset,from) - position an open file
  7. dup(filedes) - duplicate an existing file descriptor
  8. dup2(oldfd,newfd) - duplicate to a desired file descriptor
  9. fcntl(filedes,cmd,arg) - change properties of an open file
  10. ioctl(filedes,request,arg) - change the behaviour of an open file
  11. The difference between fcntl anf ioctl is that the former is intended for any open file, while the latter is for device-specific operations.

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Unix File Management question and answer for Interview