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Why do people encode, compress, or archive files?


Binary files must be encoded into an ASCII format in order to transmit them through text-only media such as electronic mail or Usenet news.


Macintosh files must be encoded in order to preserve their resource forks and file-type and creator information when they are transferred to other systems. Similarly, some VMS files must be encoded to preserve file-type and record-structure information.


Encoding a binary file typically increases its size by 30 or 35 percent. To compensate for this, people usually compress files before encoding them. There's no point in compressing a file after it's encoded, of course, because that would undo the benefits of the encoding—the result would no longer be a plain-text file.


Groups of related files are often archived (combined into one file). This ensures that none of the parts gets lost or mislaid in transit, and also simplifies the downloading process.


Most archiving utilities (other than the venerable UNIX tar and shar programs) also compress files. Because tar doesn't, most tar archives are compressed or gzipped as well, and you see them on the Net as .tar.Z or .tar.gz files.


By and large, audio, image, and video files are quite portable, and, given the right hardware, can be displayed on practically anything.


Archived, encoded, and compressed files, though, tend to be much less portable. It's not that they are intrinsically less portable, but that they are more often intended for use on a single type of machine. With the exception of Zip and occasionally Zoo archives, these files are intended for use on a particular type of system, and unpacking them elsewhere is usually more trouble than it's worth.


Even if you manage to find software that can unpack a UNIX tar archive on a Macintosh, chances are it won't do you any good; most tar archives contain UNIX source code, which won't compile on a Mac. Similarly, unpacking a Macintosh Compactor Pro archive won't do you much good on a PC; more than likely, the archive contains Macintosh software, which won't run on your PC.


The following lists the most common encoded, compressed and archive file types.


Thge Common encoded, compressed, and archive file types.


File Extension
Description
Capabilities





.arc
The old DOS archive standard
Archiving and compression

.cpt
Compact Pro for Mac
Archiving and compression

.gz
GNU gzip
Compression only

.hqx
Macintosh BinHex
Encodes Mac files preserving resource fork, file type and creator

.lha, .lzh
LHarc
Archiving and compression

.sea
Macintosh
Self-extracting archive

.shar
UNIX shell archive
Archiving only

.sit
Stuffit for Macintosh
Compression and archiving

.tar
UNIX Tape Archive
Archiving only

.uue, .uu
UNIX UUEncoding
Simple binary-to-ASCII encoding

.z
pack or gzip (see .gz)
Compression only

.Z
UNIX compress
Compression only

.zip
Phil Katz's pkzip
Archiving and compression

.zoo
Rahul Dhesi's zoo
Archiving and compression