Decodes and Encodes Allwinner env files

env_manager.sh

#!/bin/bash

# Path to the environment file
ENV_FILE="$1"

# Function to decode the environment file to text
decode_env() {
    # Skip the special header (assuming header length + null byte), convert null bytes to newlines, then strip trailing null bytes (represented by empty lines)
    tail -c +6 "$ENV_FILE" | tr '\0' '\n' | sed '/^$/d' # Remove empty lines which are the result of trailing null bytes
}

# Function to encode text back to the environment file format
encode_env() {
    SOURCE_FILE="$2"
    TARGET_SIZE="$3"
    if [[ -z "$TARGET_SIZE" || ! "$TARGET_SIZE" =~ ^[0-9]+$ ]]; then
        echo "Target size in bytes must be specified for encoding and must be a number."
        exit 1
    fi

    # Create a new file to avoid overwriting during tests
    NEW_ENV_FILE="${ENV_FILE}.new"
    # Add the special header including the null byte at the end
    echo -en '\xe1\xfa\x10\x62\0' > "$NEW_ENV_FILE"

    # Convert newlines in the source file to null bytes and append to the new file in one go
    tr '\n' '\0' < "$SOURCE_FILE" >> "$NEW_ENV_FILE"

    # Calculate current size and pad if necessary
    CURRENT_SIZE=$(stat -c "%s" "$NEW_ENV_FILE")
    PADDING_SIZE=$(( TARGET_SIZE - CURRENT_SIZE ))

    if [ "$PADDING_SIZE" -gt 0 ]; then
        # Creating and appending zero block in memory
        head -c "$PADDING_SIZE" /dev/zero >> "$NEW_ENV_FILE"
    fi
}

# Argument checks and action determination
if [ "$#" -lt 2 ]; then
    echo "Usage for decode: $0 <env_file> decode"
    echo "Usage for encode: $0 <env_file> encode <source_file_for_encode> <target_size_for_encode>"
    exit 1
fi

ACTION="$2"

case $ACTION in
    decode)
        decode_env
        ;;
    encode)
        if [ "$#" -ne 4 ]; then
            echo "Usage for encode: $0 <env_file> encode <source_file_for_encode> <target_size_for_encode>"
            exit 1
        fi
        encode_env "$ENV_FILE" "$3" "$4"
        ;;
    *)
        echo "Invalid action: $ACTION"
        echo "Valid actions are 'decode' or 'encode'"
        exit 1
        ;;
esac