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.

Applications

Archive files are particularly useful in that they store file system data and metadata within the contents of a particular file, and thus can be stored on systems or sent over channels that do not support the file system in question, only file contents – examples include sending a directory structure over email.

Beyond archival purposes, archive files are frequently used for packaging software for distribution, as software contents are often naturally spread across several files; the archive is then known as a package. While the archival file format is the same, there are additional conventions about contents, such as requiring a manifest file, and the resulting format is known as a package format. Examples include deb for Debian, JAR for Java, and APK for Android.

Features

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. The file extension or file header of the archive file are indicators of the file format used. Computer archive files are created by file archiver software, optical disc authoring software, and disk image software.

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.

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). RAR archives may include redundant error correction data (called recovery records).

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

• Archive formats at DMOZ