Ce qui fait que si on 2 fichiers (non compressés) au contenu identique fichier1.txt dans fichier2.txt,

   cmp renvoit que les fichiers sont identiques (ouf).
   cmp fichier1.txt fichier2.txt
   echo $?
   0

Mais, si on les compressent avec gzip (pas en même temps), on aura deux fichiers compressés différents

   cmp fichier1.txt.gz fichier2.txt.gz
   fichier1.txt.gz fichier2.txt.gz differ: byte 5, line 1
   echo $?
   1

Heureusement, on peut alors utiliser zcmp pour comparer les contenus

   zcmp test_gzip.txt.gz test_gzip2.txt.gz
   echo $?
   0

http://tools.ietf.org/html/rfc1952

MTIME (Modification TIME)

           This gives the most recent modification time of the original
           file being compressed.  The time is in Unix format, i.e.,
           seconds since 00:00:00 GMT, Jan.  1, 1970.  (Note that this
           may cause problems for MS-DOS and other systems that use
           local rather than Universal time.)  If the compressed data
           did not come from a file, MTIME is set to the time at which
           compression started.  MTIME = 0 means no time stamp is
           available.

zgrep – search possibly compressed files for a regular expression

SYNOPSIS

      zgrep [ grep_options ] [ -e ] pattern filename...

DESCRIPTION

      Zgrep  invokes grep on compressed or gzipped files.  All options speci-
      fied are passed directly to grep.  If no file is  specified,  then  the
      standard input is decompressed if necessary and fed to grep.  Otherwise
      the given files are uncompressed if necessary and fed to grep.

      If the GREP environment variable is set, zgrep uses it as the grep pro-
      gram to be invoked.