07-29-2010
searching files for hex or oct values
I have a set of files without extensions. How can I programatically tell if a file is in gzip format? The gzip file format spec
RFC 1952 GZIP File Format Specification version 4.3
states that gzip files have certain hex/oct values at the beginning of the file.
1st byte = 0x1f in hex, \037 in octal
2nd byte = 0x8b in hex, \213 in octal
How can I search for these values to determine if a file is truly gzip format?
Thanks.
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LEARN ABOUT DEBIAN
perlio::gzip
gzip(3pm) User Contributed Perl Documentation gzip(3pm)
NAME
PerlIO::gzip - Perl extension to provide a PerlIO layer to gzip/gunzip
SYNOPSIS
use PerlIO::gzip;
open FOO, "<:gzip", "file.gz" or die $!;
print while <FOO>; # And it will be uncompressed...
binmode FOO, ":gzip(none)" # Starts reading deflate stream from here on
DESCRIPTION
PerlIO::gzip provides a PerlIO layer that manipulates files in the format used by the "gzip" program. Compression and Decompression are
implemented, but not together. If you attempt to open a file for reading and writing the open will fail.
EXPORT
PerlIO::gzip exports no subroutines or symbols, just a perl layer "gzip"
LAYER ARGUMENTS
The "gzip" layer takes a comma separated list of arguments. 4 exclusive options choose the header checking mode:
gzip
The default. Expects a standard gzip file header for reading, writes a standard gzip file header.
none
Expects or writes no file header; assumes the file handle is immediately a deflate stream (eg as would be found inside a "zip" file)
auto
Potentially dangerous. If the first two bytes match the "gzip" header "x1fx8b" then a gzip header is assumed (and checked) else a
deflate stream is assumed. No different from gzip on writing.
autopop
Potentially dangerous. If the first two bytes match the "gzip" header "x1fx8b" then a gzip header is assumed (and checked) else the
layer is silently popped. This results in gzip files being transparently decompressed, other files being treated normally. Of course,
this has sides effects such as File::Copy becoming gunzip, and File::Compare comparing the uncompressed contents of files.
In autopop mode Opening a handle for writing (or reading and writing) will cause the gzip layer to automatically be popped.
Optionally you can add this flag:
lazy
For reading, defer header checking until the first read. For writing, don't write a header until the first buffer empty of compressed
data to disk. (and don't write anything at all if no data was written to the handle)
By default, gzip header checking is done before the "open" (or "binmode") returns, so if an error is detected in the gzip header the
"open" or "binmode" will fail. However, this will require reading some data, or writing a header. With lazy set on a file opened for
reading the check is deferred until the first read so the "open" should always succeed, but any problems with the header will cause an
error on read.
open FOO, "<:gzip(lazy)", "file.gz" or die $!; # Dangerous.
while (<FOO>) {
print;
} # Whoa. Bad. You're not distinguishing between errors and EOF.
If you're not careful you won't spot the errors - like the example above you'll think you got end of file.
lazy is ignored if you are in autopop mode.
AUTHOR
Nicholas Clark, <nwc10+perlio-gzip@colon.colondot.net>
SEE ALSO
perl, gzip, rfc 1952 <http://www.ietf.org/rfc/rfc1952.txt> (the gzip file format specification), rfc 1951
<http://www.ietf.org/rfc/rfc1951.txt> (DEFLATE compressed data format specification)
perl v5.14.2 2006-10-01 gzip(3pm)