Run it: bash -x download_and_verify.sh

download_and_verify.sh

#!/bin/bash
#Mon Feb 26 13:01:50 PST 2018
#Darwin 16.7.0 x86_64
#GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin16)

#Download and verify a file's sha256sum
#Retry three times and then fail.
#Should check the sha256sum of the downloaded file before attempting to download.
#exit 0 on success else exit nonzero
#Usage:
#  download_and_verify /path/to/local/file <sha256 hash> <remote download location>
function download_and_verify() {
    if [ ! $# -eq 3 ]; then
        echo "ERROR: malformed function call: $0 $*" >&2
        echo 'EXPECTED 3 args: download_and_verify <local file> <sha256 hash> <remote download location>'
        return 1
    fi
    local lfile="${1}"
    local shasum="${2}"
    local rfile="${3}"
    local retries=3
    local verify_command=()
    if type -P sha256sum &> /dev/null; then
        verify_command+=( 'sha256sum' '-c' '-' )
    elif type -P shasum &> /dev/null; then
        verify_command+=( 'shasum' '-a' '256' '-c' '-')
    else
        echo 'ERROR: could not determine sha256sum command' >&2
        return 1
    fi
    if "${verify_command[@]}" <<< "${shasum}  ${lfile}"; then
      #file exists and matches checksum so return success
      return 0
    fi
    local current_try=0
    while [[ current_try -lt retries ]]; do
        if curl -fLo "${lfile}" "${rfile}"; then
            #download success
            break
        fi
        (( current_try++ ))
    done
    #return status code based on shasum
    "${verify_command[@]}" <<< "${shasum}  ${lfile}"
}

download_and_verify ~/Downloads/dumb-init \
057ecd4ac1d3c3be31f82fc0848bf77b1326a975b4f8423fe31607205a0fe945 \
https://github.com/Yelp/dumb-init/releases/download/v1.2.1/dumb-init_1.2.1_amd64