Sign and verify a file using OpenSSL command line tool. It exports the digital signature in Base64 format.

sign.sh

#!/bin/bash
# Sign a file with a private key using OpenSSL
# Encode the signature in Base64 format
#
# Usage: sign <file> <private_key>
#
# NOTE: to generate a public/private key use the following commands:
#
# openssl genrsa -aes128 -passout pass:<passphrase> -out private.pem 2048
# openssl rsa -in private.pem -passin pass:<passphrase> -pubout -out public.pem
#
# where <passphrase> is the passphrase to be used.

filename=$1
privatekey=$2

if [[ $# -lt 2 ]] ; then
  echo "Usage: sign <file> <private_key>"
  exit 1
fi

openssl dgst -sha256 -sign $privatekey -out /tmp/$filename.sha256 $filename
openssl base64 -in /tmp/$filename.sha256 -out signature.sha256
rm /tmp/$filename.sha256

verify.sh

#!/bin/bash
# Verify a file with a public key using OpenSSL
# Decode the signature from Base64 format
#
# Usage: verify <file> <signature> <public_key>
#
# NOTE: to generate a public/private key use the following commands:
#
# openssl genrsa -aes128 -passout pass:<passphrase> -out private.pem 2048
# openssl rsa -in private.pem -passin pass:<passphrase> -pubout -out public.pem
#
# where <passphrase> is the passphrase to be used.

filename=$1
signature=$2
publickey=$3

if [[ $# -lt 3 ]] ; then
  echo "Usage: verify <file> <signature> <public_key>"
  exit 1
fi

openssl base64 -d -in $signature -out /tmp/$filename.sha256
openssl dgst -sha256 -verify $publickey -signature /tmp/$filename.sha256 $filename
rm /tmp/$filename.sha256

Congrats, Satoshi loves you, apparently. https://www.reddit.com/r/Buttcoin/comments/4hkpwe/inventor_of_bitcoin_proves_he_can_write_a_2line/