download file from s3 using curl v4 authentication

s3-v4-file-download.sh

#!/bin/bash
set -e
# set -x

function log(){
    echo "$(date -u +'%Y%m%dT%H%M%SZ') - $*"
}

# variable declaration - start
bucket=<bucket>
access_key_id=<access_key_id>
secret_access_key=<secret_access_key>
amzFile=<file_to_download>
aws_region="us-east-1"
outputFile=file.tar.gz
dateValueS=$(date -u +'%Y%m%d')
dateValueL=$(date -u +'%Y%m%dT%H%M%SZ')
#emptySha=`echo -n ""|sha256sum|sed 's/\s*\*-//'`  #windows
emptySha=`echo -n ""|sha256sum|sed 's/\s*-//'`  #Linux
# variable declaration - end

log "downloading $amzFile"
# getting file form s3 - start
#creating a canonical request
log "creating canonical request"
echo GET > cform.txt
echo /${amzFile}>> cform.txt
echo >> cform.txt
echo "host:${bucket}.s3.amazonaws.com">> cform.txt
echo "x-amz-content-sha256:${emptySha}">> cform.txt
echo "x-amz-date:${dateValueL}">> cform.txt
echo >> cform.txt
echo "host;x-amz-content-sha256;x-amz-date" >> cform.txt
echo -n ${emptySha}>> cform.txt
#cp cform.txt req.txt
#taking hash of the canonical request
#canonicalRequestHash=`echo -n ${canonicalRequest}|sha256sum |sed 's/\s*\*-//'`
canonicalRequestHash=`sha256sum cform.txt|sed 's/\s.*$//'`

#creating string to sign
log "creating string to sign"
echo AWS4-HMAC-SHA256 > cform.txt
echo ${dateValueL} >> cform.txt
echo ${dateValueS}/us-east-1/s3/aws4_request >> cform.txt
echo -n ${canonicalRequestHash} >> cform.txt

function hmac_sha256 {
  key="$1"
  data="$2"
  echo -n "$data" | openssl dgst -sha256 -mac HMAC -macopt "$key" | sed 's/^.* //'
}

#creating an authorization string
log "creating auth string"
dateKey=$(hmac_sha256 key:"AWS4$secret_access_key" $dateValueS)
dateRegionKey=$(hmac_sha256 hexkey:$dateKey $aws_region)
dateRegionServiceKey=$(hmac_sha256 hexkey:$dateRegionKey s3)
signingKey=$(hmac_sha256 hexkey:$dateRegionServiceKey "aws4_request")
signature=`openssl dgst -sha256 -mac HMAC -macopt hexkey:${signingKey} cform.txt| sed 's/^.* //'`
rm -f cform.txt
#signature=$(awsStringSign4 "${secret_access_key}" "${dateValueS}" "${aws_region}" s3 "${stringToSign}")

#curl to s3 to get the file 
log "curling to https://${bucket}.s3.amazonaws.com/${amzFile}"                     
curl  -H "Authorization: AWS4-HMAC-SHA256 Credential=${access_key_id}/${dateValueS}/us-east-1/s3/aws4_request, SignedHeaders=host;x-amz-content-sha256;x-amz-date, Signature=${signature}" \
     -H "host: ${bucket}.s3.amazonaws.com" \
     -H "x-amz-content-sha256: $emptySha" \
     -H "x-amz-date: ${dateValueL}"\
     https://${bucket}.s3.amazonaws.com/${amzFile} -o $outputFile