halve_log.sh

#!/bin/bash

# Check if a file path was provided
if [ -z "$1" ]; then
    echo "Usage: $0 /path/to/your/logfile.log"
    exit 1
fi

FILE=$1

# Check if the file exists
if [ ! -f "$FILE" ]; then
    echo "Error: File '$FILE' not found."
    exit 1
fi

# Calculate lines
TOTAL_LINES=$(wc -l < "$FILE")
HALF_LINES=$((TOTAL_LINES / 2))

echo "Total lines: $TOTAL_LINES. Keeping the last $HALF_LINES lines..."

# Use tail to grab the bottom half and write to a temp file
# Using a temp file on the same disk makes the 'mv' operation atomic
tail -n "$HALF_LINES" "$FILE" > "$FILE.tmp" && mv "$FILE.tmp" "$FILE"

echo "Done. Log truncated by 50%."