FILE OPERATIONS

In order to store and retrieve the content of a file, the user manipulates it using various operations. 
These operations vary from system to system. 

Some of the commonly used operations are: 

• Create 
• Delete 
• Open 
• Close 
• Read 
• Write 
• Append
• Rename 
• Copy 
• Seek 
• Truncate 

Create:

Create Command creates a file with no data. Two steps are involved in creating a file:

First space is allocated to a file in a file system.

Second, an entry for the new file is made in the directory. Thus, a directory entry records the name of the file and its location in the file system.

Delete:

When a file is no longer needed, it has to be deleted to free up disk space.

There is always a system call for this purpose.

In addition, some operating systems automatically delete any file that has not been used for long days.

Open:

A process must open a file before using it.

Operating System opens a file from its location and loads it into memory for access by the user.

File can be opened implicitly by the system or explicitly by users.

Close:

When all the accesses are finished, file must be closed in order to store the changes made by different processes permanently.

Read:

To read from a file, a system call can be issued specifying the name of file and the address of memory block where the reading has to take place.

Files can be read sequentially or randomly.

Once the read has taken place, the read pointer is updated.

Write:

To write into a file, a system call can be issued specifying both the name of the file and the information to be written to the file.

The write operation starts from the current location of file pointer.

If the current position is the end of the file, the size of file increases.

If the current position is the middle of the file, existing data are overwritten and lost forever.

Append:

This call is restricted form of Write. It can only add data to the end of the file.

Rename:

It can be used to change the name of any existing file.

Copy:

It is used to copy the contents of the file into some other file or create another version of file with new name.

Seek:

For random access file, a method is needed to specify from where to take the data.

For this purpose Seek system call can be used.

It repositions the pointer from the current position to a specific place in the file.

After this call has completed, data can be read from or written to that position.

Truncate:

The user can erase the contents of a file. But can keep its attributes.

This operation allows all the attributes to remain unchanged except for file length.

Thus the length of file is reset to 0 and its space is released.