UNIX Filesystems: Evolution, Design, and Implementation

Traditional UNIX Tools

There are a number of tools that have been available on UNIX for many years that deal with making copies of files, file hierarchies, and filesystems. The following sections describe tar, cpio, and pax, the best understood utilities for archiving file hierarchies.

This is followed by a description of the dump and restore commands, which can be used for backing up and restoring whole filesystems.

The tar, cpio, and pax Commands

The tar and cpio commands are both used to construct an archive of files. The set of files can be a directory hierarchy of files and subdirectories. The tar command originated with BSD while the cpio command came from System V. Because tar is available on just about every platform, including non-UNIX operating systems, cpio will not be mentioned further.

The tar Archive Format

It is assumed that readers are familiar with operation of the tar command. As a quick refresher, consider the following 3 commands:

$ tar cvf files.tar /lhome/spate/*
$ tar tvf files.tar
$ tar xvf files.tar

The first command (c option) creates a tar archive consisting of all files under the directory /lhome/spate. The second command (t option) displays the contents of the archive. The last command (x option) extracts files from the archive.

There are two main tar formats, the original format that originated in BSD UNIX and is shown in Figure 12.1, and the USTAR format as defined by Posix.1. In both cases, the archive consists of a set of records. Each record ...