Replace values by env vars (envsubst opposite)

values-subst.sh

#!/bin/bash

# 1. Let's assume we have a 'secrets.env' file that lists variables and their values.
#
# ENV_1=VALUE_1
# ENV_2=VALUE_2
# ENV_3=VALUE_3
#
# And we have a file that contains hard-coded values, e.g.
# my_content is VALUE_3
#
# We want this script to transform it into...
# my_content is ${ENV_3}
#
# Somehow, this is almost the opposite of the envsubst command.
# "envsubst" substitutes placeholders by env values.
# Here, we replace env values by placeholders.
# With special characters support, etc.

SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
while IFS= read -r line
do
    VAR=$(echo $line | cut -d "=" -f 1)
    VALUE=$(echo $line | cut -d "=" -f 2)
    echo $VALUE

    # Escape special characters for sed
    CLEANED_VALUE=$(sed -e 's/[&\\/]/\\&/g; s/$/\\/' -e '$s/\\$//' <<<"$VALUE")

    # Replace values by a reference to their env variable
    sed -i "s@$CLEANED_VALUE@\${$VAR}@g" my_file_to_update

done < "$SCRIPT_DIR/secrets.env"

Obviously, this would require some automated tests to verify all the cases.
I tested values with special chars like ? and &, but not with @. This gist is more a sticky note I created at the end of a busy day: it may be updated later.