simple bash template engine

bash_template_engine.sh

#!/usr/bin/env bash

if ! [[ ${BASH_VERSINFO[0]} -ge 4 && ${BASH_VERSINFO[1]} -ge 2 ]]; then
    echo "$(basename $0): bash version >= 4.2 required"
    exit 1
fi

#
# expands {{varname}} to varvalue
#
expand_variables ()
{
    local line="$(cat; echo -n a)"
    local end_offset=${#line}
	
    while [[ "${line:0:$end_offset}" =~ (.*)(\{\{([a-zA-Z_][a-zA-Z_0-9]*)\}\})(.*) ]] ; do
    	local pre="${BASH_REMATCH[1]}"
    	local post="${BASH_REMATCH[4]}${line:$end_offset:${#line}}"
    	local varname="${BASH_REMATCH[3]}"
    	eval 'local varval="$'$varname'"'
    	line="$pre$varval$post"
    	end_offset=${#pre}
    done
    echo -n "${line:0:-1}"
}

source deploy.rc.defaults
[ -f deploy.rc.local ] && source deploy.rc.local

expand_variables