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Unix OS Interview Questions

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The Open Group holds the definition of what a UNIX system is and its associated trademark in trust for the industry.

In 1994 Novell (who had acquired the UNIX systems business of AT&T/USL) decided to get out of that business. Rather than sell the business as a single entity, Novell transferred the rights to the UNIX trademark and the specification (that subsequently became the Single UNIX Specification) to The Open Group (at the time X/Open Company). Subsequently, it sold the source code and the product implementation (UNIXWARE) to SCO. The Open Group also owns the trademark UNIXWARE.

Today, the definition of UNIX takes the form of the worldwide Single UNIX Specification integrating X/Open Company's XPG4, IEEE's POSIX Standards and ISO C. Through continual evolution, the Single UNIX Specification is the defacto and dejure standard definition for the UNIX system application programming interfaces. As the owner of the UNIX trademark, The Open Group has separated the UNIX trademark from any actual code stream itself, thus allowing multiple implementations. Since the introduction of the Single UNIX Specification, there has been a single, open, consensus specification that defines the requirements for a conformant UNIX system.

There is also a mark, or brand, that is used to identify those products that have been certified as conforming to the Single UNIX Specification, initially UNIX 93, followed subsequently by UNIX 95, UNIX 98 and now UNIX 03.

The Open Group is committed to working with the community to further the development of standards conformant systems by evolving and maintaining the Single UNIX Specification and participation in other related standards efforts. Recent examples of this are making the standard freely available on the web, permitting reuse of the standard in open source documentation projects , providing test tools ,developing the POSIX and LSB certification programs.

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).

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:
1. File owner identifier
2. File type
3. File access permissions
4. File access times
5. Number of links
6. File size
7. 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).

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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