Packing, compressing and encoding files

It is very convenient and efficient for the storage and transfer of files via ftp to pack, compress and encode them. The reasons for so doing are:

• Packing: Application software very often covers a whole directory tree of files. If this whole tree structure is packed together into a single file, the task of transfering the software is made that much easier. Often the last modification date of the file is stored within the packed form.
• Compression can result in a 30 to 70% reduction in the size of a file depending on the tree structure and the algorithm used. This will save storage space on disk and floppy disks, and reduce the bandwidth necessary for the transfer of the file.
• Encoding is frequently used in the transfer of files by e-mail. E-mail systems are in many cases limited to 7-bit ASCII transfer making them per se unsuitable for binary programs. To get around this problem, the binary file is first encoded into a text format and then sent by e-mail. Moreover as the binary files tend to be rather long, the encoding algorithm also splits the encoded text file into several sections which are sent as separate e-mail messages.

Available Software

The most popular and widely-used programs for packing, compressing and encoding files are indicated in the list below.

  File     Program               Pack  Compress  Encode   File  Operating
Extension   Name                                          Type   System    

.arc       ARC                     X      X              binary  MS-DOS
.bin       MacBinary                               X     binary  Macintosh
.cpt       Compactor Pro           X      X              binary  Macintosh
.gz        GNU zip                        X              binary  UNIX
.hqx       BinHex4                 X               X     text    Macintosh
.lha,.lzh  Lempel-Ziv-Huffman      X      X              binary  Atari
.msa       Magic Shadow Archiver   X      X              binary  Atari
.sea       Self Extracting Archive X      X              binary  Macintosh
.shar      Shell Archive           X                     text    UNIX
.sit       Stuffit                 X      X              binary  Macintosh
.tar       tar                     X                     binary  UNIX
.uue       uuencode                                X     text    UNIX
.Z         compress                       X              binary  UNIX
.zip       PKZIP                   X      X              binary  MS-DOS
.zoo       Zoo                     X      X              binary  MS-DOS

In most cases one identifies the compression or packing algorithm from the file extension. A file may be packed by one program and then compressed by another. For example, .tar.Z means that file structure was first packed by tar and then compressed with compress. In some cases one see only one extension is appended, e.g. only .uue. But when such a file is decoded (with uuencode), a new file is produced with another extension, e.g. .Z.

The above software can be obtained from many anonymous ftp servers spread around the world. Locations can be identified by using an Archie server. On the anonymous ftp servers one frequently finds a FAQ document giving much fuller information on packing, compressing and encoding.

Image, video and audio

Other compression techniques exist for image, video and audio files. These can be both lossless and lossy algorithms. GIF, JPEG and MPEG are standards in this area.