Bash templating. {{VARIABLE}} renders to the env variable $VARIABLE

template.sh

#!/bin/bash

if [ -z "$1" ]; then
    echo "No file specified"
    exit 1
fi

if [ ! -f "$1" ]; then
    echo "File doesn't exist"
    exit 1
fi

#First character of a variable name must be a letter or an underscore
#Rest of variable can only contain letters, numbers, and underscores
for var in $(grep -oE '\{\{([A-Za-z_][A-Za-z0-9_]+)\}\}' "$1"); do
    #Remove { and } from variable name
    strippedVar=$(echo $var | tr -d '{}')
    #If variable is not set, ${var+x} evaluates to null: http://stackoverflow.com/a/13864829
    #${!var} explanation: http://stackoverflow.com/a/16553351
    if [ -z ${!strippedVar+x} ]; then
        echo "${strippedVar} not set, exiting"
        exit 1
    fi
    #Escape special characters in value for use in sed
    escapedValue=$(echo ${!strippedVar} | sed -e 's/[\/&]/\\&/g')
    sed -i "s/${var}/${escapedValue}/" "$1"
done