#!/bin/bash

# via this command, it is possible to specify the zstd binary to use, defaulting to zstd from PATH
ZSTD=${1:-zstd}

# generate our testfile
echo 123456789 > testfile
truncate -s 1M testfile

rm -f testfile.zst
"${ZSTD}" testfile -o testfile.zst

# generate a target file and loop-mount it
dd if=/dev/urandom of=target bs=1M count=1 2>/dev/null

LOOPDEV="$(losetup -f)"
losetup "$LOOPDEV" target

# these are the same, all other sha256 outputs should be the same as well
echo "expected sums:"
sha256sum testfile
"${ZSTD}" -d < testfile.zst | sha256sum

echo

"${ZSTD}" -d testfile.zst -o "$LOOPDEV"
# this one fails
echo
echo "using -o:"
sha256sum "$LOOPDEV"

echo
hexdump -C target | head -n 5
echo
"${ZSTD}" -d < testfile.zst > "$LOOPDEV"
# now it is correct:
echo "using pipe:"
sha256sum "$LOOPDEV"

echo
hexdump -C target | head -n 5

losetup -d "$LOOPDEV"

testfile             :  0.01%   ( 1.000 MiB =>     63 B, testfile.zst)         
expected sums:
a29e830fc0eede27937b8cb0d7336c2c3fcdeedd7c6207923c3e31dec0541537  testfile
a29e830fc0eede27937b8cb0d7336c2c3fcdeedd7c6207923c3e31dec0541537  -

testfile.zst        : 1048576 bytes                                            

using -o:
e074889058caa6266c163e27d1c5990d3810b39927616732102667876c7cefc1  /dev/loop0

00000000  31 32 33 34 35 36 37 38  39 0a 00 00 00 00 00 00  |123456789.......|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00008000  63 4e 0f 16 5c e4 09 2b  a8 0f 3a 55 1e 14 52 a5  |cN..\..+..:U..R.|
00008010  ed 86 59 95 d2 b1 66 93  a4 36 2d 2c 0e 97 b0 a7  |..Y...f..6-,....|

using pipe:
a29e830fc0eede27937b8cb0d7336c2c3fcdeedd7c6207923c3e31dec0541537  /dev/loop0

00000000  31 32 33 34 35 36 37 38  39 0a 00 00 00 00 00 00  |123456789.......|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00100000