How to get the md5sum of a file on Amazon's S3

If I have existing files on Amazon's S3, what's the easiest way to get their md5sum without having to download the files?

AWS's documentation of ETag says:

The entity tag is a hash of the object. The ETag reflects changes only to the contents of an object, not its metadata. The ETag may or may not be an MD5 digest of the object data. Whether or not it is depends on how the object was created and how it is encrypted as described below:

• Objects created by the PUT Object, POST Object, or Copy operation, or through the AWS Management Console, and are encrypted by SSE-S3 or plaintext, have ETags that are an MD5 digest of their object data.
• Objects created by the PUT Object, POST Object, or Copy operation, or through the AWS Management Console, and are encrypted by SSE-C or SSE-KMS, have ETags that are not an MD5 digest of their object data.
• If an object is created by either the Multipart Upload or Part Copy operation, the ETag is not an MD5 digest, regardless of the method of encryption.

Reference: http://docs.aws.amazon.com/AmazonS3/latest/API/RESTCommonResponseHeaders.html

ETag does not seem to be MD5 for multipart uploads (as per Gael Fraiteur's comment). In these cases it contains a suffix of minus and a number. However, even the bit before the minus does not seem to be the MD5, even though it is the same length as an MD5. Possibly the suffix is the number of parts uploaded?

This is a very old question, but I had a hard time find the information below, and this is one of the first places I could find, so I wanted to detail it in case anyone needs.

ETag is a MD5. But for the Multipart uploaded files, the MD5 is computed from the concatenation of the MD5s of each uploaded part. So you don't need to compute the MD5 in the server. Just get the ETag and it's all.

As @EmersonFarrugia said in this answer:

Say you uploaded a 14MB file and your part size is 5MB. Calculate 3 MD5 checksums corresponding to each part, i.e. the checksum of the first 5MB, the second 5MB, and the last 4MB. Then take the checksum of their concatenation. Since MD5 checksums are hex representations of binary data, just make sure you take the MD5 of the decoded binary concatenation, not of the ASCII or UTF-8 encoded concatenation. When that's done, add a hyphen and the number of parts to get the ETag.

So the only other things you need is the ETag and the upload part size. But the ETag has a -NumberOfParts suffix. So you can divide the size by the suffix and get part size. 5Mb is the minimum part size and the default value. The part size has to be integer, so you can't get things like 7,25Mb each part size. So it should be easy get the part size information.

Here is a script to make this in osx, with a Linux version in comments: https://gist.github.com/emersonf/7413337

I'll leave both script here in case the page above is no longer accessible in the future:

Linux version:

#!/bin/bash
set -euo pipefail
if [ $# -ne 2 ]; then
    echo "Usage: $0 file partSizeInMb";
    exit 0;
fi
file=$1
if [ ! -f "$file" ]; then
    echo "Error: $file not found." 
    exit 1;
fi
partSizeInMb=$2
fileSizeInMb=$(du -m "$file" | cut -f 1)
parts=$((fileSizeInMb / partSizeInMb))
if [[ $((fileSizeInMb % partSizeInMb)) -gt 0 ]]; then
    parts=$((parts + 1));
fi
checksumFile=$(mktemp -t s3md5.XXXXXXXXXXXXX)
for (( part=0; part<$parts; part++ ))
do
    skip=$((partSizeInMb * part))
    $(dd bs=1M count=$partSizeInMb skip=$skip if="$file" 2> /dev/null | md5sum >> $checksumFile)
done
etag=$(echo $(xxd -r -p $checksumFile | md5sum)-$parts | sed 's/ --/-/')
echo -e "${1}\t${etag}"
rm $checksumFile

OSX version:

#!/bin/bash

if [ $# -ne 2 ]; then
    echo "Usage: $0 file partSizeInMb";
    exit 0;
fi

file=$1

if [ ! -f "$file" ]; then
    echo "Error: $file not found." 
    exit 1;
fi

partSizeInMb=$2
fileSizeInMb=$(du -m "$file" | cut -f 1)
parts=$((fileSizeInMb / partSizeInMb))
if [[ $((fileSizeInMb % partSizeInMb)) -gt 0 ]]; then
    parts=$((parts + 1));
fi

checksumFile=$(mktemp -t s3md5)

for (( part=0; part<$parts; part++ ))
do
    skip=$((partSizeInMb * part))
    $(dd bs=1m count=$partSizeInMb skip=$skip if="$file" 2>/dev/null | md5 >>$checksumFile)
done

echo $(xxd -r -p $checksumFile | md5)-$parts
rm $checksumFile