Bash shell: Replace values of env file by actual variable values

replace_env_values.sh

#!/usr/bin/env bash
tmpFile="$(mktemp)"
inputFilePath="${1}"
#################################################################################
# This script will look up key=value in the ENV file                            #
# Replace the value by the value of key in the runtime environment              #
# If the value of the key is not existed, do nothing and keep the original one  #
# All other things will be kept to output                                       #
#################################################################################
# Input:                                                                        #
# 1: the ENV file path                                                          #
# structure: key=value or key="value" or key='value'                            #
# 2: the custom prefix key (default: none)                                      #
# Description: Look up "prefix_key" instead of "key" in the runtime environment #
#################################################################################
# Output: stdout (The result after replacing)                                   #
#################################################################################

# Check input file path
if [[ -z "${inputFilePath}" ]]; then
    echo "Please input the input file path" > /dev/stderr
    exit 1
fi

# Check input file path for existent
if [ ! -f "${inputFilePath}" ]; then
    echo "File \"${inputFilePath}\" does not exist" > /dev/stderr
    exit 2
fi

# Check input file for empty
if [ ! -s "${inputFilePath}" ]; then
    echo "File \"${inputFilePath}\" is empty" > /dev/stderr
    exit 3
fi

# Store all data of input file to temp file
cat "${inputFilePath}" > "${tmpFile}"
while IFS=\n read -r line; do
    arrLineIn=(${line//=/ })
    oriVarName="${arrLineIn[0]}"
    runTimeVarName="${oriVarName}"
    if [[ -n "${2}" ]]; then
        # if having the custom prefix
        runTimeVarName="${2}_${oriVarName}"
    fi
    
    valueOfRunTimeVarName="${!runTimeVarName}"
    # If the actual value of varName is not empty
    if [[ -n "${valueOfRunTimeVarName}" ]]; then
        replacedLine="${oriVarName}=\"${valueOfRunTimeVarName}\""
        sed -i "s/^${line}/${replacedLine}/g" "${tmpFile}"
    fi
done <<< "$(grep -o -P "^[a-zA-Z]*=[\"']*\w+[\"']*" "${tmpFile}")"
# Show result in stdout
cat "${tmpFile}" > /dev/stdout
# Remove tmpFile
rm "${tmpFile}" 

Get this file by

curl -s https://gist.githubusercontent.com/vleedev/dbe5d343e89b9fb2040a87defea735dc/raw/replace_env_values.sh\?_\=$\(uuidgen\) > replace_env_values.sh 

Execute:

bash replace_env_values.sh <.env file path> <Variable prefix if have>