envsubst alternative in pure bash - replace prefixed environment variables (eg. ${XX_MYVAR}) in a text file

envsubst.sh

#!/bin/bash

usage() {
    echo "Usage: $0 [OPTIONS] [file_path]"
    echo "OPTIONS:"
    echo "  -p, --prefix PREFIX      Specify the prefix for environment variables"
    echo "  -h, --help               Show this help message"
    echo "If no file_path is provided, the script reads from stdin."
    exit 1
}

# Function to replace environment variables with a prefix in a text file
replace_env_variables() {
    local content="$1"
    local prefix="$2"

    # Get environment variables with the specified prefix (only variable names)
    variables=$(env | grep "^${prefix}" | sed -e "s/${prefix}\([^=]*\)=.*/\1/")

    # Loop through variables and simulate replacement in the content
    while IFS= read -r variable; do
        key="${prefix}${variable}"
        value=$(printenv "${key}")
        if [ -n "${value}" ]; then
            content=$(echo "${content}" | sed "s|\${${key}}|${value}|g")
        fi
    done <<< "${variables}"

    echo "${content}"
}

# Initialize flags with default values
file_path=""
prefix=""

# Parse flags
while [[ $# -gt 0 ]]; do
    key="$1"

    case $key in
        -p | --prefix)
            prefix="$2"
            shift # past argument
            shift # past value
            ;;
        -h | --help)
            usage
            ;;
        *)
            file_path="$1"
            shift # past value
            ;;
    esac
done

# If no file path provided or stdin specified, read from stdin
if [ -z "${file_path}" ] || [ "${file_path}" == "-" ]; then
    content=$(cat -)
else
    # Check if file exists
    if [ ! -f "${file_path}" ]; then
        echo "File ${file_path} not found."
        exit 1
    fi
    content=$(< "${file_path}")
fi

replace_env_variables "${content}" "${prefix}"

The printenv command doesn't exist on OpenWRT. And env dumps function definitions as well as variables. Better to use declare.

Here's a version where the main loop is in pure bash:

envsubst() (
  shopt -s extglob
  local __envsubst_vars __envsubst_line __envsubst_var
  mapfile -t __envsubst_vars < <(declare | grep -E -o '^[^]([:space:]]+=' | cut -d = -f 1)
  while IFS= read -s -r __envsubst_line; do
    for __envsubst_var in "${__envsubst_vars[@]}"; do
      __envsubst_line="${__envsubst_line//\$${__envsubst_var}/${!__envsubst_var}}"
      __envsubst_line="${__envsubst_line//\$\{${__envsubst_var}\}/${!__envsubst_var}}"
    done
    __envsubst_line="${__envsubst_line//\$+([[:word:]])/}"
    __envsubst_line="${__envsubst_line//\$\{+([[:word:]])\}/}"
    printf '%s\n' "${__envsubst_line}"
  done
)

Which you can test with:

export content=""
mapfile -t vars < <(declare | grep -E -o '^[^]([:space:]]+=' | cut -d = -f 1 | sort)
for var in "${vars[@]}"; do
  content+="${var}=\$${var}"$'\n'
done
# shellcheck disable=SC2016
content+='notvars=$# $? $$'$'\n'
# shellcheck disable=SC2016
content+='undefined=$undefined'$'\n'
# shellcheck disable=SC2016
content+='undefined=${undefined}'$'\n'

envsubst <<<"${content}"