The reasons for deleting files are
- Freeing the disk space
- Removing duplicate or unnecessary data to avoid confusion
- Making sensitive information unavailable to others
All operating systems include commands for deleting files (rm on Unix, era in CP/M and DR-DOS, del/erase in MS-DOS/PC DOS, DR-DOS, Microsoft Windows etc.). File managers also provide a convenient way of deleting files. Files may be deleted one-by-one, or a whole directory tree may be deleted.
Problem with accidental removal
The common problem with deleting files is accidental removal of information that later proves to be important. One way to deal with this is to back up files regularly. Erroneously deleted files may then be found in archives.
Another technique often used is not to delete files instantly, but to move them to a temporary directory whose contents can then be deleted at will. This is how the "recycle bin" or "trash can" works. Microsoft Windows and Apple's Mac OS X both employ this strategy.
In MS-DOS, one can use the undelete command. In MS-DOS the "deleted" files are not really deleted, but only marked as deleted—so they could be undeleted during some time, until the disk blocks they used are eventually taken up by other files. Defragging a drive may prevent undeletion, as the blocks used by deleted file might be overwritten since they are marked as "empty".
Another precautionary measure is to mark important files as read-only. Many operating systems will warn the user trying to delete such files. Where file system permissions exist, "ordinary" users are only able to delete their own files, preventing the erasure of other people's work or critical system files.
Under Unix-like operating systems, in order to delete a file, one must usually have write permission to the parent directory of that file.
Problem with sensitive data
The common problem with sensitive data is that deleted files are not really erased and so may be recovered by interested parties. Most file systems only remove the link to data (see undelete, above). But even overwriting parts of the disk with something else or formatting it may not guarantee that the sensitive data is completely unrecoverable. Special software is available that overwrites data, and modern (post-2001) ATA drives include a secure erase command in firmware. However high security applications sometimes require that a disk drive be physically destroyed to ensure data is not recoverable, as microscopic changes in head alignment and other effects can mean even such measures are not guaranteed.
- Disk Drive Data Sanitization Gordon Hughes, UCSD Center for Magnetic Recording Research, Tom Coughlin, Coughlin Associates *LINK DOWN*