UNIX File Management

1. 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).

2. 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:

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

3. 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).

4. 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.

5. 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:

chmod(myfile,0744).