envsubst in pure bash/sed

internal_envsubst.sh

#!/bin/bash
# this is a poor man's substitute for the envsubst command

function internal_envsubst() {
    tfile=$(mktemp)
    # use sed to build a sed input file specific to the current environment variables
    env | sed -E -e '
       # delete any special cases that make a mess of things
       /^_=/d; 
       # deal with variables that have backslashes in them: \\tsclient\home
       s=\\=\\\\=g; 
       # our output will use equal signs for separators, so escape any on the input
       s/=/\\=/g; 
       s/\\=/=/; 
       # transform each variable into a sed command for later execution
       s%^([^=]*)=(.*)$%s=\\$\\{?\1\\}?([^_A-Za-z0-9]|$)=\2=g;%
    ' > $tfile
    sed -E -f $tfile
    rm $tfile
}
internal_envsubst