Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
gzip
The standard GNU/UNIX compression utility, replacing the inferior and proprietary compress. The
corresponding decompression command is gunzip, which is the equivalent of gzip −d.
The zcat filter decompresses a gzipped file to stdout, as possible input to a pipe or redirection. This
is, in effect, a cat command that works on compressed files (including files processed with the older
compress utility). The zcat command is equivalent to gzip −dc.
On some commercial UNIX systems, zcat is a synonym for uncompress −c, and will
not work on gzipped files.
bzip2
See also Example 7−7.
An alternate compression utility, usually more efficient (but slower) than gzip, especially on large
files. The corresponding decompression command is bunzip2.
Newer versions of tar have been patched with bzip2 support.
compress, uncompress
This is an older, proprietary compression utility found in commercial UNIX distributions. The more
efficient gzip has largely replaced it. Linux distributions generally include a compress workalike for
compatibility, although gunzip can unarchive files treated with compress.
sq
The znew command transforms compressed files into gzipped ones.
Yet another compression utility, a filter that works only on sorted ASCII word lists. It uses the
standard invocation syntax for a filter, sq < input−file > output−file. Fast, but not nearly as efficient
as gzip. The corresponding uncompression filter is unsq, invoked like sq.
The output of sq may be piped to gzip for further compression.
zip, unzip
Cross−platform file archiving and compression utility compatible with DOS pkzip.exe. "Zipped"
archives seem to be a more acceptable medium of exchange on the Internet than "tarballs".
unarc, unarj, unrar
These Linux utilities permit unpacking archives compressed with the DOS arc.exe, arj.exe, and
rar.exe programs.
File Information
file
A utility for identifying file types. The command file file−name will return a file specification
for file−name, such as ascii text or data. It references the magic numbers found in
/usr/share/magic, /etc/magic, or /usr/lib/magic, depending on the Linux/UNIX
distribution.
The −f option causes file to run in batch mode, to read from a designated file a list of filenames to
analyze. The −z option, when used on a compressed target file, forces an attempt to analyze the
uncompressed file type.
bash$ file test.tar.gz
Chapter 12. External Filters, Programs and Commands 179