Replace template file environment placeholders with values from the environment

replacer.sh

#!/bin/bash

if [ $# -ne 2 ]; then
    echo "Usage: $0 <input_file> <output_file>"
    exit 1
fi

input_file=$1
output_file=$2

template=$(cat "$input_file")

while IFS= read -r -d '' match; do
    variable=${match:1} # Remove the leading '$' from the match
    value=${!variable}  # Get the value of the environment variable
    template=${template//$match/$value}
done < <(echo "$template" | grep -o '\$[A-Za-z_][A-Za-z0-9_]*' | tr '\n' '\0')

echo "$template" > "$output_file"

Set the environment variables.

export TEST_ENV=env
export COLORTERM=red

Create a new file, input.txt, with contents.

test_env=$TEST_ENV
color=$COLORTERM

Run the script,

./replacer.sh input.txt output.txt

This will produce output.txt,

test_env=test
color=truecolor