✏ EDIT: zdiff

#!/bin/sh
# sh is buggy on RS/6000 AIX 3.2. Replace above line with #!/bin/ksh

# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 3 of the License, or
# (at your option) any later version.

# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.

# You should have received a copy of the GNU General Public License along
# with this program; if not, write to the Free Software Foundation, Inc.,
# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.

case $1 in
  --__cmp) shift
        prog=cmp;  cmp='${CMP-cmp}'  ;;
  *)    prog=diff; cmp='${DIFF-diff}';;
esac

version="z$prog (gzip) 1.9
Copyright (C) 2009, 2011-2017 Free Software Foundation, Inc.
This is free software.  You may redistribute copies of it under the terms of
the GNU General Public License <https://www.gnu.org/licenses/gpl.html>.
There is NO WARRANTY, to the extent permitted by law.

Written by Jean-loup Gailly."

usage="Usage: $0 [OPTION]... FILE1 [FILE2]
Compare FILE1 to FILE2, using their uncompressed contents if they are
compressed.  If FILE2 is omitted, compare FILE1 to the uncompressed
contents of FILE1.gz.  Do comparisons like '$prog' does.

OPTIONs are the same as for '$prog'.

Report bugs to <bug-gzip@gnu.org>."

# sed script to escape all ' for the shell, and then (to handle trailing
# newlines correctly) turn trailing X on last line into '.
escape='
  s/'\''/'\''\\'\'''\''/g
  $s/X$/'\''/
'

while :; do
  case $1 in
  --h*) printf '%s\n' "$usage"   || exit 2; exit;;
  --v*) printf '%s\n' "$version" || exit 2; exit;;
  --) shift; break;;
  -*\'*) cmp="$cmp '"`printf '%sX\n' "$1" | sed "$escape"`;;
  -?*) cmp="$cmp '$1'";;
  *) break;;
  esac
  shift
done
cmp="$cmp --"

for file
do
  test "X$file" = X- || <"$file" || exit 2
done

gzip_status=0
exec 3>&1

if test $# -eq 1; then
  case $1 in
  *[-.]gz* | *[-.][zZ] | *.t[ga]z)
    FILE=`expr "X$1" : 'X$.*$[-.][zZtga]*$'`
    gzip_status=$(
      exec 4>&1
      (gzip -cd -- "$1" 4>&-; echo $? >&4) 3>&- | eval "$cmp" - '"$FILE"' >&3
    );;
  *)
    printf >&2 '%s\n' "$0: $1: unknown compressed file extension"
    exit 2;;
  esac
elif test $# -eq 2; then
        case "$1" in
        *[-.]gz* | *[-.][zZ] | *.t[ga]z | -)
                case "$2" in
                *[-.]gz* | *[-.][zZ] | *.t[ga]z | -)
                    if test "$1$2" = --; then
                        gzip_status=$(
                          exec 4>&1
                          (gzip -cdfq - 4>&-; echo $? >&4) 3>&- |
                            eval "$cmp" - - >&3
                        )
                    elif
                        # Reject Solaris 8's buggy /bin/bash 2.03.
                        echo X |
                         (echo X | eval "$cmp" /dev/fd/5 - >/dev/null 2>&1) \
                                5<&0
                    then
                        gzip_status=$(
                          exec 4>&1
                          (gzip -cdfq -- "$1" 4>&-; echo $? >&4) 3>&- |
                            ( (gzip -cdfq -- "$2" 4>&-; echo $? >&4) 3>&- 5<&- </dev/null |
                               eval "$cmp" /dev/fd/5 - >&3) 5<&0
                        )
                        cmp_status=$?
                        case $gzip_status in
                          *[1-9]*) gzip_status=1;;
                          *) gzip_status=0;;
                        esac
                        (exit $cmp_status)
                    else
                        tmp=
                        trap '
                          test -n "$tmp" && rm -f "$tmp"
                          (exit 2); exit 2
                        ' HUP INT PIPE TERM 0
                        case $TMPDIR in
                          / | /*/) ;;
                          /*) TMPDIR=$TMPDIR/;;
                          *) TMPDIR=/tmp/;;
                        esac
                        if type mktemp >/dev/null 2>&1; then
                          tmp=`mktemp "${TMPDIR}zdiffXXXXXXXXX"` ||
                            exit 2
                        else
                          set -C
                          tmp=${TMPDIR}zdiff$$
                        fi
                        gzip -cdfq -- "$2" > "$tmp" || exit 2
                        gzip_status=$(
                          exec 4>&1
                          (gzip -cdfq -- "$1" 4>&-; echo $? >&4) 3>&- |
                            eval "$cmp" - '"$tmp"' >&3
                        )
                        cmp_status=$?
                        rm -f "$tmp" || gzip_status=$?
                        trap - HUP INT PIPE TERM 0
                        (exit $cmp_status)
                    fi;;
                *)
                    gzip_status=$(
                      exec 4>&1
                      (gzip -cdfq -- "$1" 4>&-; echo $? >&4) 3>&- |
                        eval "$cmp" - '"$2"' >&3
                    );;
                esac;;
        *)      case "$2" in
                *[-.]gz* | *[-.][zZ] | *.t[ga]z | -)
                        gzip_status=$(
                          exec 4>&1
                          (gzip -cdfq -- "$2" 4>&-; echo $? >&4) 3>&- |
                            eval "$cmp" '"$1"' - >&3
                        );;
                *)	eval "$cmp" '"$1"' '"$2"';;
                esac;;
        esac
else
        printf >&2 '%s\n' \
          "$0: invalid number of operands; try \`$0 --help' for help"
        exit 2
fi

cmp_status=$?
test "$gzip_status" -eq 0 || exit 2
exit $cmp_status

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