Unix Interview Questions and Answers
Ques 1. How are devices represented in UNIX?
Ans. 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).
Ques 2. What is 'inode'?Ans. 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 access permissions
File access times
Number of links
Location of the file data
Ques 3. Brief about the directory representation in UNIXAns. 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).
Ques 4. What are the Unix system calls for I/O?Ans. 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.
Ques 5. What are links and symbolic links in UNIX file system?Ans. 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
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