s3-download-aws4.sh

#!/bin/sh -u

fileLocal="${1:-example-local-file.ext}"
bucket="${2:-example-bucket}"
remotePath="${3:-remote-path}"
region="${4:-}"
awsAccess="${5:-YOUR_AWS_ACCESS_KEY}"
awsSecret="${6:-YOUR_AWS_SECRET_KEY}"

m_openssl() {
  if [ -f /usr/local/opt/openssl@1.1/bin/openssl ]; then
    /usr/local/opt/openssl@1.1/bin/openssl "$@"
  elif [ -f /usr/local/opt/openssl/bin/openssl ]; then
    /usr/local/opt/openssl/bin/openssl "$@"
  else
    openssl "$@"
  fi
}

m_sed() {
  if which gsed > /dev/null 2>&1; then
    gsed "$@"
  else
    sed "$@"
  fi
}

awsStringSign4() {
  kSecret="AWS4$1"
  kDate=$(printf         '%s' "$2" | m_openssl dgst -sha256 -hex -mac HMAC -macopt "key:${kSecret}"     2>/dev/null | m_sed 's/^.* //')
  kRegion=$(printf       '%s' "$3" | m_openssl dgst -sha256 -hex -mac HMAC -macopt "hexkey:${kDate}"    2>/dev/null | m_sed 's/^.* //')
  kService=$(printf      '%s' "$4" | m_openssl dgst -sha256 -hex -mac HMAC -macopt "hexkey:${kRegion}"  2>/dev/null | m_sed 's/^.* //')
  kSigning=$(printf 'aws4_request' | m_openssl dgst -sha256 -hex -mac HMAC -macopt "hexkey:${kService}" 2>/dev/null | m_sed 's/^.* //')
  signedString=$(printf  '%s' "$5" | m_openssl dgst -sha256 -hex -mac HMAC -macopt "hexkey:${kSigning}" 2>/dev/null | m_sed 's/^.* //')
  printf '%s' "${signedString}"
}

iniGet() {
  # based on: https://stackoverflow.com/questions/22550265/read-certain-key-from-certain-section-of-ini-file-sed-awk#comment34321563_22550640
  printf '%s' "$(m_sed -n -E "/\[$2\]/,/\[.*\]/{/$3/s/(.*)=[ \\t]*(.*)/\2/p}" "$1")"
}


# Initialize defaults

fileRemote="${remotePath}/${fileLocal}"


echo "Downloading" "${fileLocal}" "->" "${bucket}" "${region}" 
echo "| $(uname) | $(m_openssl version) "

# Initialize helper variables

httpReq='GET'
authType='AWS4-HMAC-SHA256'
service="s3"
baseUrl=".${service}.amazonaws.com"
dateValueS=$(date -u +'%Y%m%d')
dateValueL=$(date -u +'%Y%m%dT%H%M%SZ')
if hash file 2>/dev/null; then
  contentType="$(file -b --mime-type "${fileLocal}")"
else
  contentType='application/octet-stream'
fi

# 0. Hash empty string

payloadHash=$(printf "" | m_openssl dgst -sha256 -hex 2>/dev/null | m_sed 's/^.* //')

# 1. Create canonical request

# NOTE: order significant in ${headerList} and ${canonicalRequest}

headerList='content-type;host;x-amz-content-sha256;x-amz-date'

canonicalRequest="\
${httpReq}
/${fileRemote}

content-type:${contentType}
host:${bucket}${baseUrl}
x-amz-content-sha256:${payloadHash}
x-amz-date:${dateValueL}

${headerList}
${payloadHash}"

# Hash it

canonicalRequestHash=$(printf '%s' "${canonicalRequest}" | m_openssl dgst -sha256 -hex 2>/dev/null | m_sed 's/^.* //')

# 2. Create string to sign

stringToSign="\
${authType}
${dateValueL}
${dateValueS}/${region}/${service}/aws4_request
${canonicalRequestHash}"

# 3. Sign the string

signature=$(awsStringSign4 "${awsSecret}" "${dateValueS}" "${region}" "${service}" "${stringToSign}")

# Upload
curl -s -L --proto-redir =https -X "${httpReq}" \
  -H "Content-Type: ${contentType}" \
  -H "Host: ${bucket}${baseUrl}" \
  -H "X-Amz-Content-SHA256: ${payloadHash}" \
  -H "X-Amz-Date: ${dateValueL}" \
  -H "Authorization: ${authType} Credential=${awsAccess}/${dateValueS}/${region}/${service}/aws4_request, SignedHeaders=${headerList}, Signature=${signature}" \
  "https://${bucket}${baseUrl}/${fileRemote}"  -o "${fileLocal}"

echo "https://${bucket}${baseUrl}/${fileRemote}"