Simple `sh` template substitution script

template.sh

#! /usr/bin/env bash

# Substitutes shell-like templates in input files.
#
# Usage:
#   template.sh [-x COMMAND] [-f FILE] [VAR=value]...
#
# Options:
#     -f FILE       Read template from this file [default: -]
#     -x COMMAND    Send output to this command [default: cat]
#
# Variables are also substituted from the current environment,
# which means that you can also pass arguments like this:
#
#   export VAR1=value
#   export VAR2=value
#   VAR3=value VAR4=value template.sh [-f FILE]

filename=
execute=cat

while getopts 'f:x:h' OPTION; do
    case $OPTION in
        x)	execute="$OPTARG" ;;
        f)	filename="$OPTARG" ;;
        *)  printf "Usage: %s: [-x command] [[-f] file] [variables ...]\n" $(basename "$0") >&2
			exit 2
    esac
done
shift $(($OPTIND - 1))

if [ "xx$filename" = "xx" ]; then
    if [ -e "$1" ]; then
        filename="$1"
        shift
    else
        filename=-
    fi
fi

random_suffix=`dd if=/dev/urandom bs=32 count=1 2> /dev/null | hexdump -e '"%04X"'`

env "$@" sh <<OUTER
$execute <<INNER$random_suffix
$(cat "$filename")
INNER$random_suffix
OUTER