Computer file

Files can be shared with and transferred between computers and mobile devices via removable media, networks, or the Internet.

A file may be designed to store a written message, a document, a spreadsheet, an image, a video, a program, or any wide variety of other kinds of data.

Computer files may be reopened, modified, and copied an arbitrary number of times.

In Punched Card Methods in Scientific Computation,[2] W. J. Eckert stated, "The first extensive use of the early Hollerith Tabulator in astronomy was made by Comrie.

[3] He used it for building a table from successive differences, and for adding large numbers of harmonic terms".

In February 1950, in a Radio Corporation of America (RCA) advertisement in Popular Science magazine[4] describing a new "memory" vacuum tube it had developed, RCA stated: "the results of countless computations can be kept 'on file' and taken out again.

Electronically it retains figures fed into calculating machines, holds them in storage while it memorizes new ones – speeds intelligent solutions through mazes of mathematics."

On some platforms the format is indicated by its filename extension, specifying the rules for how the bytes must be organized and interpreted meaningfully.

In such systems, software employed other methods to track the exact byte count (e.g., CP/M used a special control character, Ctrl-Z, to signal the end of text files).

For example, the file to which the link /bin/ls points in a typical Unix-like system probably has a defined size that seldom changes.

This has led to a plethora of more or less standardized file structures for all imaginable purposes, from the simplest to the most complex.

In Unix-like systems, user space programs do not operate directly, at a low level, on a file.

The operating system provides a level of abstraction, which means that interaction with a file from user-space is simply through its filename (instead of its inode).

This free space is commonly considered a security risk (due to the existence of file recovery software).

Any secure-deletion program uses kernel-space (system) functions to wipe the file's data.

Microsoft Windows Explorer uses the former method for mass storage file moves, but the latter method using Media Transfer Protocol, as described in Media Transfer Protocol § File move behavior.

In modern computer systems, files are typically accessed using names (filenames).

Any string of characters may be a well-formed name for a file or a link depending upon the context of application.

Early computers permitted only a few letters or digits in the name of a file, but modern computers allow long names (some up to 255 characters) containing almost any combination of Unicode letters or Unicode digits, making it easier to understand the purpose of a file at a glance.

Microsoft Windows supports multiple file systems, each with different policies[which?]

Many modern computer systems provide methods for protecting files against accidental and deliberate damage.

This flag is useful for critical information that must not be modified or erased, such as special files that are used only by internal parts of the computer system.

In physical terms, most computer files are stored on some type of data storage device.

A file control block (FCB) is an area of memory which is manipulated to establish a filename etc.

When a file is said to be corrupted, it is because its contents have been saved to the computer in such a way that they cannot be properly read, either by a human or by software.

Depending on the extent of the damage, the original file can sometimes be recovered, or at least partially understood.

The program itself might warn the user that there was an error, allowing for another attempt at saving the file.

[20] In the event of an important file becoming corrupted, the user can simply replace it with the backed up version.

Most computer systems provide utility programs to assist in the back-up process, which can become very time-consuming if there are many files to safeguard.

Files are often copied to removable media such as writable CDs or cartridge tapes.