Category Archives: I —:::— Early Bird

bzip2, bunzip2 – a block-sorting file compressor, v1.0.4

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Connected From This

A UNIX Command 

$wget http://www.kernel.org/pub/linux/kernel/v3.0/patch-3.0.4.bz2
--2011-09-03 19:22:43--  http://www.kernel.org/pub/linux/kernel/v3.0/patch-3.0.4.bz2
Resolving www.kernel.org... 130.239.17.5, 149.20.4.69, 199.6.1.165, ...
Connecting to www.kernel.org|130.239.17.5|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 96120 (94K) [application/x-bzip2]
Saving to: `patch-3.0.4.bz2'

100%[===================================================================================>] 96,120      85.3K/s   in 1.1s    

2011-09-03 19:22:45 (85.3 KB/s) - `patch-3.0.4.bz2' saved [96120/96120]

$bunzip2 patch-3.0.4.bz2 
$ls patch-3.0.4 
patch-3.0.4
$

UNIX Explanation 

bzip2  compresses files  using the  Burrows-Wheeler block
sorting text  compression algorithm, and  Huffman coding.
Compression  is generally  considerably better  than that
achieved    by    more    conventional    LZ77/LZ78-based
compressors,  and approaches the  performance of  the PPM
family of statistical compressors.

Related Source Code Exposition


for (t = 0; t < nGroups; t++) {
minLen = 32;
maxLen = 0;
for (i = 0; i < alphaSize; i++) {
if (s->len[t][i] > maxLen) maxLen = s->len[t][i];
if (s->len[t][i] < minLen) minLen = s->len[t][i];
}
BZ2_hbCreateDecodeTables (
&(s->limit[t][0]),
&(s->base[t][0]),
&(s->perm[t][0]),
&(s->len[t][0]),
minLen, maxLen, alphaSize
);
s->minLens[t] = minLen;
}

Source Code Highlight

Create the Huffman decoding tables

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Related Knowledge 

bzip2  expects a  list  of file  names  to accompany  the
command-line   flags.   Each  file   is  replaced   by  a
compressed   version    of   itself,   with    the   name
"original_name.bz2".  Each  compressed file has  the same
modification  date,   permissions,  and,  when  possible,
ownership  as the corresponding  original, so  that these
properties  can be  correctly  restored at  decompression
time.   File name  handling is  naive in  the  sense that
there is no mechanism for preserving original file names,
permissions,  ownerships or  dates  in filesystems  which
lack  these concepts,  or have  serious file  name length
restrictions, such as MS-DOS.

source : debian manual pages for bzip2
trueandfalse

true and false

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A UNIX Command 

$true
$echo $?
0
$false
$echo $?
1
$

UNIX Explanation 

true - do nothing, successfully
exit with a status code indicating success.
false - do nothing, unsuccessfully
exit with a status code indicating failure.

Related Source Code Exposition


int
main (int argc, char **argv)
{

if (argc == 2)
{
initialize_main (&argc, &argv);
set_program_name (argv[0]);
setlocale (LC_ALL, "");
bindtextdomain (PACKAGE, LOCALEDIR);
textdomain (PACKAGE);

atexit (close_stdout);

if (STREQ (argv[1], "--help"))
usage (EXIT_STATUS);

if (STREQ (argv[1], "--version"))
version_etc (stdout, PROGRAM_NAME, PACKAGE_NAME, Version, AUTHORS,
(char *) NULL);
}

exit (EXIT_STATUS);
}

Source Code Highlight

Recognize –help or –version only if it’s the only
command-line argument.

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Related Knowledge 

`true' does  nothing except return  an exit status  of 0,
meaning "success".  It  can be used as a  place holder in
shell  scripts  where  a  successful command  is  needed,
although the  shell built-in  command `:' (colon)  may do
the same thing faster.   In most modern shells, `true' is
a built-in command,  so when you use `true'  in a script,
you're probably  using the built-in command,  not the one
documented here.


Note,  however, that it  is possible  to cause  `true' to
exit   with  nonzero   status:  with   the   `--help'  or
`--version'  option,  and  with standard  output  already
closed or redirected to a  file that evokes an I/O error.
For example, using a Bourne-compatible shell:


 $ ./true --version >&-
     ./true: write error: Bad file number
     $ ./true --version > /dev/full
     ./true: write error: No space left on device


This version of `true' is implemented as a C program, and
is  thus  more secure  and  faster  than  a shell  script
implementation, and  may safely be used as  a dummy shell
for the purpose of disabling accounts.

source : info coreutils ‘true invocation’