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).
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).
Use 'su' command. The system asks for password and when valid entry is made the user gains super user (admin) privileges.
Shell variables are special variables, a name-value pair created and maintained by the shell.
Example: PATH, HOME, MAIL and TERM
Directing the flow of data to the file or from the file for input or output.
Example : ls > wc