How to reverse the content of binary file?

With xxd (from vim) and tac (from GNU coreutils, also tail -r on some systems):

< file.gnp xxd -p -c1 | tac | xxd -p -r > file.png

In zsh (the only shell that can internally deal with binary data (unless you want to consider ksh93's base64 encoding approach)):

zmodload zsh/mapfile
(LC_ALL=C; printf %s ${(s::Oa)mapfile[file.gnp]} > file.png)

• LC_ALL=C: characters are bytes
• $mapfile[file.gnp]: content of file.gnp file
• s::: split the string into its byte constituents
• Oa: reverse Order on array subscript that array

Here is one way of reversing a binary file using ksh93. I have left the code "loose" to make it easier to understand.

#!/bin/ksh93

typeset -b byte

redirect 3< image.gpj || exit 1

eof=$(3<#((EOF)))

read -r -u 3 -N 1 byte
printf "%B" byte > image.jpg
3<#((CUR - 1))

while (( $(3<#) > 0 ))
do
    read -r -u 3 -N 1 byte
    printf "%B" byte >> image.jpg
    3<#((CUR - 2))
done

read -r -u 3 -N 1 byte
printf "%B" byte >> image.jpg

redirect 3<&- || echo 'cannot close FD 3'

exit 0