All other examples of rendering handlebars.js (double braces) templates in bash are way too complicated to put into a script

DEMO1-handlebar_templates_via_argument.sh

#!/bin/bash

# This function works with both GNU/Linux and macOS/BSD version of sed and grep
# Inspired by https://github.com/relaxdiego/renderest

render(){
    local template="$*"
    for varname in $(<<<"$template" grep -oE '\{\{([A-Za-z0-9_]+)\}\}' | sed -En 's/.*\{\{([A-Za-z0-9_]+)\}\}.*/\1/p' | sort | uniq); do
        template="$(<<<"$template" sed -E "s/\{\{$varname\}\}/$(sed 's_\\_\\\\\\\\_g;s_/_\\/_g' <<<"${!varname}")/g")"
    done
    printf "$template"
}

ipsum='//_M1_\\nnnn'
dolor='//_M2_\\tttt'
nascetur='//_M3_\\'
ridiculus='//_M4_\\'
quis='//_M5_\\'
consequat='//_M6_\\'
massa='//_M7_\\'
perd='//_M8_\\'
venenatis='//_M9_\\'

render 'Lorem {{ipsum}} {{dolor}} sit amet, consectetuer adipiscing elit. Aenean commodo ligula
eget {{dolor}}. Aenean massa. Cum sociis natoque penatibus et magnis dis parturient
montes, {{nascetur}}{{ridiculus}} mus. Donec quam felis, ultricies nec, pellentesque
eu, pretium {{quis}}, sem. Nulla {{consequat}} {{massa}} {{quis}} enim. Donec pede justo,
fringilla vel, aliquet nec, vulputate eget, arcu. In enim justo, rhoncus ut,
im{{perd}}iet a, {{venenatis}} vitae, justo. Nullam {{perd}}{{perd}} eu pede mollis pretium.'

echo "THE END"

DEMO2-handlebar_templates_via_pipe.sh

#!/bin/bash

# This function works with both GNU/Linux and macOS/BSD version of sed and grep
# Inspired by https://github.com/relaxdiego/renderest

render(){
    local template="$(cat)"
    for varname in $(<<<"$template" grep -oE '\{\{([A-Za-z0-9_]+)\}\}' | sed -En 's/.*\{\{([A-Za-z0-9_]+)\}\}.*/\1/p' | sort | uniq); do
        template="$(<<<"$template" sed -E "s/\{\{$varname\}\}/$(sed 's_\\_\\\\\\\\_g;s_/_\\/_g' <<<"${!varname}")/g")"
    done
    printf "$template"
}

ipsum='//_M1_\\nnnn'
dolor='//_M2_\\tttt'
nascetur='//_M3_\\'
ridiculus='//_M4_\\'
quis='//_M5_\\'
consequat='//_M6_\\'
massa='//_M7_\\'
perd='//_M8_\\'
venenatis='//_M9_\\'

render <<EOF
Lorem {{ipsum}} {{dolor}} sit amet, consectetuer adipiscing elit. Aenean commodo ligula
eget {{dolor}}. Aenean massa. Cum sociis natoque penatibus et magnis dis parturient
montes, {{nascetur}}{{ridiculus}} mus. Donec quam felis, ultricies nec, pellentesque
eu, pretium {{quis}}, sem. Nulla {{consequat}} {{massa}} {{quis}} enim. Donec pede justo,
fringilla vel, aliquet nec, vulputate eget, arcu. In enim justo, rhoncus ut,
im{{perd}}iet a, {{venenatis}} vitae, justo. Nullam {{perd}}{{perd}} eu pede mollis pretium.
EOF

echo "THE END"

output.txt

Lorem //_M1_\\nnnn //_M2_\\tttt sit amet, consectetuer adipiscing elit. Aenean commodo ligula
eget //_M2_\\tttt. Aenean massa. Cum sociis natoque penatibus et magnis dis parturient
montes, //_M3_\\//_M4_\\ mus. Donec quam felis, ultricies nec, pellentesque
eu, pretium //_M5_\\, sem. Nulla //_M6_\\ //_M7_\\ //_M5_\\ enim. Donec pede justo,
fringilla vel, aliquet nec, vulputate eget, arcu. In enim justo, rhoncus ut,
im//_M8_\\iet a, //_M9_\\ vitae, justo. Nullam //_M8_\\//_M8_\\ eu pede mollis pretium.THE END