Batch Rename Files with Sequential Numbering

#!/bin/bash

# This script renames files in the current directory with a common prefix
# and sequential numbering, preserving their original extensions.

# Usage: ./rename_files.sh "image" "jpg"
# Example: file1.jpg -> image-001.jpg, file2.jpg -> image-002.jpg

if [ "$#" -lt 1 ]; then
    echo "Usage: $0 <new_prefix> [extension_filter] [starting_number]"
    echo "  <new_prefix>: The new prefix for the filenames (e.g., 'photo')"
    echo "  [extension_filter]: Optional. Only rename files with this extension (e.g., 'jpg'). If empty, renames all."
    echo "  [starting_number]: Optional. The number to start counting from (default: 1)."
    exit 1
fi

NEW_PREFIX="$1"
EXTENSION_FILTER="${2:-}" # Default to empty string (no filter)
START_NUM="${3:-1}"       # Default starting number to 1

COUNTER=$START_NUM

# Use an array to store files to avoid issues with spaces in filenames
if [ -n "$EXTENSION_FILTER" ]; then
    # Filter by extension
    FILES_TO_RENAME=(*."$EXTENSION_FILTER")
else
    # No filter, rename all files
    FILES_TO_RENAME=(*)
fi

# Remove script itself from list if present and not intended for renaming
for i in "${!FILES_TO_RENAME[@]}"; do
    if [[ "${FILES_TO_RENAME[$i]}" == "$(basename "$0")" ]]; then
        unset 'FILES_TO_RENAME[i]'
    fi
}

echo "Preparing to rename files with prefix: '$NEW_PREFIX' (starting from $START_NUM)"
if [ -n "$EXTENSION_FILTER" ]; then
    echo "Filtering by extension: '$EXTENSION_FILTER'"
fi
echo "---"

for OLD_FILE in "${FILES_TO_RENAME[@]}"; do
    if [ -f "$OLD_FILE" ]; then
        # Extract extension
        EXTENSION="${OLD_FILE##*.}"
        FILENAME="${OLD_FILE%.*}"
        
        # Pad the counter with leading zeros (e.g., 001, 010, 100)
        # Use printf to format number with leading zeros (e.g., %03d for 3 digits)
        PADDED_NUM=$(printf "%03d" "$COUNTER")
        
        # Construct new filename
        if [ "$EXTENSION" != "$FILENAME" ]; then # Check if there was an extension
            NEW_FILE="${NEW_PREFIX}-${PADDED_NUM}.${EXTENSION}"
        else
            NEW_FILE="${NEW_PREFIX}-${PADDED_NUM}" # No extension, just prefix-num
        fi
        
        # Avoid overwriting existing files
        if [ -e "$NEW_FILE" ]; then
            echo "Warning: '$NEW_FILE' already exists. Skipping '$OLD_FILE'."
        else
            mv "$OLD_FILE" "$NEW_FILE"
            echo "Renamed '$OLD_FILE' to '$NEW_FILE'"
        fi
        
        COUNTER=$((COUNTER + 1))
    fi
done

echo "---"
echo "Batch rename complete."