1.      How do you change File Access Permissions?

Every file has following attributes:

Ø      owner’s user ID ( 16 bit integer )

Ø      owner’s group ID ( 16 bit integer )

Ø      File access mode word

‘r w x -r w x- r w x’ 

(user permission-group permission-others permission)

r-read, w-write, x-execute

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).

 

2.      What are links and symbolic links in UNIX file system?

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

 

3.      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).

 

4.      How do you create special files like named pipes and device files?

The system call mknod creates special files in the following sequence.

1.      kernel assigns new inode,

2.      sets the file type to indicate that the file is a pipe, directory or special file,

3.      If it is a device file, it makes the other entries like major, minor device numbers.

For example:

If the device is a disk, major device number refers to the disk controller and minor device number is the disk.

 

5.      Discuss the mount and unmount system calls

The privileged mount system call is used to attach a file system to a directory of another file system; the unmount system call detaches a file system. When you mount another file system on to your directory, you are essentially splicing one directory tree onto a branch in another directory tree. The first argument to mount call is the mount point, that is , a directory in the current file naming system. The second argument is the file system to mount to that point. When you insert a cdrom to your unix system’s drive, the file system in the cdrom automatically mounts to /dev/cdrom in your system.

 

6.      How does the inode map to data block of a file?

Inode has 13 block addresses. The first 10 are direct block addresses of the first 10 data blocks in the file. The 11th address points to a one-level index block. The 12th address points to a two-level (double in-direction) index block. The 13th address points to a three-level(triple in-direction)index block. This provides a very large maximum file size with efficient access to large files, but also small files are accessed directly in one disk read.

 

7.      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.

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