Archive file
An archive file is a file that is composed of one or more computer files along with metadata. Archive files are used to collect multiple data files together into a single file for easier portability and storage, or simply to compress files to use less storage space. Archive files often store directory structures, error detection and correction information, arbitrary comments, and sometimes use built-in encryption.
Computer archive files are created by file archiver software, optical disc authoring software, and disk image software. The file extension or file header of the archive file are indicators of the file format used.
Features supported by various kinds of archives include file concatenation, data compression, encryption, file spanning, checksums, self-extraction, self-installation, source volume and medium information, directory structure information, package notes and description, and other meta-data.
Archive formats
An archive format is the file format of an archive file. Some formats are well-defined by their authors and have become conventions supported by multiple vendors and communities.
Types
- Archiving only formats only concatenate files.
- Compression only formats only compress files.
- Multi-function formats can concatenate, compress, encrypt, create error detection and recovery information, and package the archive into self-extracting and self-expanding files.
- Software packaging formats are used to create software packages that may be self-installing files.
- Disk image formats are used to create disk images of mass storage volumes.
Examples
Filename extensions used to distinguish different types of archives include zip, rar, 7z, and tar.
Java also introduced a whole family of archive extensions such as jar and war (j is for Java and w is for web). They are used to exchange entire byte-code deployment. Sometimes they are also used to exchange source code and other text, HTML and XML files. By default they are all compressed.
By operating system
Unix operating systems use the tar file format, ar, and shar to concatenate files. These archive formats can then be compressed into gzip format.
On Windows platforms, the most widely used archive format is ZIP; other formats are CAB, RAR, and ACE. Windows Installer is a high-level archive format for distribution of software.
On Amiga computers the standard archive format is LHA.
On Apple Macintosh computers ZIP is now natively used in Mac OS X 10.3+, though StuffIt was the most common in previous versions.
Linux often uses TAR, gz, and RPM package manager, a package management system for distribution of software.
Error detection and recovery
Archive files often include parity checks and other checksums for error detection, for instance zip files use a cyclic redundancy check (CRC).
Archive files are sometimes accompanied by separate parity archive (PAR) files that allow for additional error detection and recovery, particularly in recovery of missing files in a multi-file archive.
See also
- File archiver
- Disk image
- Digital container format, a similar concept in media files
References
- "Application Note on the .ZIP file format"- official white paper published by PKWARE, Inc.
- Tape Archive (.TAR) file format specification- excerpt from File Format List 2.0 by Max Maischein
- "IBM 726 Magnetic tape reader/recorder from IBM Archives
- "1401 Data Processing System" from IBM Archives
External links
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