a demo script

download_file.sh

# This function downloads a file from a given URL using wget command.
# If the file already exists, it will be checked to see if it has been downloaded completely.
# If the file was previously partially downloaded, the function will continue the download from where it was left off.
# If the file does not exist, the function will download it from scratch.
# If the download fails, an error message is displayed and the script exits with a non-zero exit status.
#
# Args:
#   URL: the URL to download the file from.
#   OUT: optional output directory for the downloaded file. If not provided, the file will be downloaded to the current directory.
#
# Returns:
#   0 if the download is successful, 1 if it fails.

download_file() {
    local URL="$1"
    local OUT="${2:-.}" # Default to current directory if OUT is not specified

    # Check if wget is available
    if ! command -v wget &> /dev/null; then
        printf "Error: wget command not found\n"
        return 1
    fi

    # Extract the filename from the URL 
    local FILENAME=$(basename "$URL")

    # Remove trailing slash from OUT if it exists and append the output directory if specified
    if [ "$OUT" != "." ]; then
        FILENAME="${OUT%/}/$FILENAME"
    fi

    # Check if the file already exists and has been downloaded completely
    if [ -e "$FILENAME" ] && ! [ -e "$FILENAME.tmp" ]; then
        printf "File %s already exists.\n" "$FILENAME"
        return 0
    fi

    # Check if the partially downloaded file exists
    if [ -e "$FILENAME.tmp" ]; then
        # Continue downloading from where we left off
        wget --continue --show-progress -qO "$FILENAME.tmp" "$URL"
    else
        # Download the file from scratch
        wget --show-progress -qO "$FILENAME.tmp" "$URL"
    fi

    # Check if the download was successful
    if [ $? -eq 0 ]; then
        # Rename the tmp file to the original filename
        mv -q "$FILENAME.tmp" "$FILENAME"
        printf "Download successful: %s\n" "$FILENAME"
        return 0
    else
        printf "Download failed: %s\n" "$FILENAME"
        rm -f "$FILENAME.tmp" # Remove partial download
        return 1
    fi
}