Calculate checksum corresponding to the entity-tag hash (ETag) of Amazon S3 objects

compute_etag.sh

#!/bin/bash
#
# Calculate checksum corresponding to the entity-tag hash (ETag) of Amazon S3 objects
# 
# Usage: compute_etag.sh <filename> <part_size_mb>
#
# filename: file to process
# part_size_mb: chunk size in MiB used for multipart uploads.  
# This is 8M by default for the AWS CLI See:
# https://docs.aws.amazon.com/cli/latest/topic/s3-config.html#multipart_chunksize
#
# The Etag for an S3 object can be obtained from the command-line using:
# aws s3api head-object --bucket <bucket-name> --key <key-name> --query ETag --output text
# Note that the Etag may or may not correspond to the MD5 digest, see here for details:
# https://docs.aws.amazon.com/AmazonS3/latest/API/API_Object.html

# Adapted from: https://gist.github.com/emersonf/7413337
# Changes
# 7/23/2022
# - Parallelized hash calculation
# - Removed need for temporary files

# Script requires: dd, md5sum, xxd

set -euo pipefail

NUM_PARALLEL=$(nproc)

# Minimum filesize in bytes to switch to multipart uploads
# https://docs.aws.amazon.com/cli/latest/topic/s3-config.html#multipart-threshold
MULTIPART_MINSIZE=$((8*1024*1024))

if [[ $# -ne 2 ]]; then
   echo "Usage: $0 file partSizeInMb";
   exit 0;
fi
file="$1"
partSizeInMb=$2

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

# Calculate checksum for a specified file chunk
# inputs: file, partSizeInMb, chunk
# output: chunk md5sum
hash_chunk(){
   file="$1"
   partSizeInMb="$2"
   chunk="$3"
   skip=$((partSizeInMb * chunk))
   # output chunk + md5 (to allow sorting later)
   dd bs=1M count="$partSizeInMb" skip="$skip" if="$file" 2> /dev/null | echo -e "$chunk $(md5sum)"
}

# Integer quotient a/b after rounding up 
div_round_up(){
   echo $((($1 + $2 - 1)/$2))
}

partSizeInB=$((partSizeInMb * 1024 * 1024))
fileSizeInB=$(du -b "$file" | cut -f1 )
parts=$(div_round_up fileSizeInB partSizeInB)

if [[ $fileSizeInB -gt $MULTIPART_MINSIZE ]]; then
   export -f hash_chunk
   etag=$(seq 0 $((parts-1)) | \
       xargs -P ${NUM_PARALLEL} -I{} bash -c 'hash_chunk "$@"' -- "$file" "$partSizeInMb" {} | \
       sort -n -k1,1 |tr -s ' '|cut -f2,3 -d' '|xxd -r -p|md5sum|cut -f1 -d' ')"-$parts"
else
   etag=$(md5sum $file|cut -f1 -d' ')
fi

echo -e "${file}\t${etag}"