File deletion

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File deletion is a way of removing a file from a computer's file system.

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, delete/del in MS-DOS 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 removing

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 backup your files regularly. Erroneously deleted files may then be found in archives.

Another strategy is not to delete files instantly, but to move them to a temporary place where they are kept until one is really low on free space. This is how the "recycle bin" in Microsoft Windows and the "trash folder" in Mac OS (which orginated the idea) work.

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 will prevent undeletion, as the blocks used by deleted file will be overwritten since they are marked as "empty".

Another precautionary measure is to mark important files as read-only. Many operating systems will warn you if you are 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 peoples work or critical system files.

Problem with sensitive data

The common problem with sensitive data is that deleted files are not really deleted and so may be recovered by interested parties. Most file systems only remove the link to data (see undelete above). But even overwriting the disk with something else or formatting it does not guarantee that the sensitive data are completely unrecoverable. To deal with this, there are programs that write random data to the target regions on the disk many times over and over, so making data recovery unlikely (see shred). And the most reliable way of destroying data is, of course, destroying the physical media (e.g. throwing it in fire). See also data remanence.

Another problem is that there may exist several copies of the same file (perhaps representing different stages of editing). People delete one copy and forget about others.