Make target file relative to script file

Since we expect them to be in the same directory

verify-script.template

@@ -1,10 +1,26 @@
 #!/bin/env sh
+# This script should have come along with two files:
+#   * The target file '{filename}'
+#   * The signature file '{filename}.sh.sig' 
+#     (or this file should have been signed in some other way)
+#
+# First, you should verify that this file came from the person you expect by
+# verifying the signature.
+#
+# Next, you should run this script which will check that the target file 
+# matches the hash that was calculated when the message below was written.
+# This will confirm that the file you have matches the file the message was
+# written about.
+#
+# Alternatively, if you don't want to run this script, you can verify the 
+# hash manually.
 
 MESSAGE="{message}"
 FILE="{filename}"
+FILE_PATH="$(dirname "$0")/$FILE"
 EXPECTED_HASH="{filehash}"
 
-if [ "$(sha256sum "$FILE" | awk '{print $1}')" = "$EXPECTED_HASH" ]; then
+if [ "$(sha256sum "$FILE_PATH" | awk '{print $1}')" = "$EXPECTED_HASH" ]; then
 	printf "File %s validated with message:\\n%s" "$FILE" "$MESSAGE"
 	exit 0
 else