split_or_rejoin.sh

#!/bin/bash

set -e

if [ "$#" -ne 2 ]; then
    echo "Usage: $0 [split|join] <zip_file_or_chunk>"
    exit 1
fi

ACTION=$1
FILE=$2

if [ "$ACTION" = "split" ]; then
    if [ ! -f "$FILE" ]; then
        echo "File '$FILE' not found!"
        exit 1
    fi

    SIZE=$(stat -c%s "$FILE" 2>/dev/null || stat -f%z "$FILE")
    CHUNK_SIZE=$(( (SIZE + 3) / 4 ))

    split -b "$CHUNK_SIZE" -d -a 2 "$FILE" "$FILE.part"
    echo "Split done: $(ls "$FILE".part*)"

elif [ "$ACTION" = "join" ]; then
    BASENAME=$(basename "$FILE")
    DIRNAME=$(dirname "$FILE")
    TARGET="${DIRNAME}/${BASENAME%%.part*}.zip"

    cat "${FILE%%.part*}".part* > "$TARGET"
    echo "Join done: $TARGET"

else
    echo "Invalid action: $ACTION. Use 'split' or 'join'."
    exit 1
fi